1 
 
 
 A 
 
E.BIBL . RADCL. 
 
 
 
 THORNTO 
 Books 
 11 The 
 
THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
 
TWO ESSAYS, 
 
LONDON : 
 
 I'HINTLJ) TIY THOMAS DAMSON, WHITEFRIAUS. 
 
TWO ESSAYS: 
 
 ONE 
 
 UPON SINGLE VISION WITH TWO EYES; 
 
 THE OTHER 
 
 ON DEW. 
 A LETTER 
 
 TO THE 
 
 RIGHT HON. LLOYD, LORD KENYON 
 
 AND 
 
 AN ACCOUNT 
 
 OF 
 
 A FEMALE OF THE WHITE RACE OF MANKIND, 
 PART OF WHOSE SKIN RESEMBLES THAT OF A NEGRO; 
 
 WITH 
 
 . SOME OBSERVATIONS ON THE CAUSES OF THE DIFFERENCES IN 
 COLOUR AND FORM BETWEEN THE WHITE AND NEGRO 
 RACES OF MEN. 
 
 BY THE LATE WILLIAM CHARLES WELLS, 
 
 M.D. F.B.S. L. & E. 
 
 WITH 
 
 A MEMOIR OF HIS LIFE, 
 
 WRITTEN BY HIMSELF. 
 
 LONDON : 
 
 PRINTED FOR ARCHIBALD CONSTABLE AND CO. EDINBURGH, 
 
 LONGMAN, HURST, REES, ORME, AND BROWN, 
 AND HURST, ROBINSON, AND CO. LONDON. 
 
 1818. 
 
rr 
 
QII3 
 
 TO 
 
 MATTHEW BAILLIE, M.D. F.R.S. L.&E. 
 
 THIS VOLUME, 
 A MEMORIAL OF THEIR COMMON FRIEND, 
 
 IS INSCRIBED, 
 WITH MUCH RESPECT, ESTEEM, AND AFFECTION, 
 
 BY THE EDITOR. 
 

MEMOIR OF THE LIFE 
 
 OF 
 
 WILLIAM CHARLES WELLS, M.D. 
 
 WRITTEN BY HIMSELF. 
 
 Al&v dp&evsiv, xou vifeipoxpv 
 
MEMOIR. 
 
 I WAS born in Charles town, South Carolina, 
 in May, 1757, being the second son, but 
 fourth child, of Robert and Mary Wells, 
 both natives of Scotland. My mother bore 
 many children afterwards, none of whom 
 lived more than a few years, except one, 
 a daughter, who now resides in London ; 
 my brother died about twenty years ago; 
 my two eldest sisters survive. 
 
 My father and mother came to Carolina 
 in 1753 ; but a mercantile scheme which 
 he was then pursuing having failed, he 
 took to the business of a bookseller and 
 bookbinder, to which he had been bred 
 when a youth in Dumfries. He soon after- 
 wards added to these occupations, that of 
 
VH1 MEMOIR. 
 
 a printer of a newspaper, for which he was 
 well qualified from his previous educa- 
 tion, being a good Latin scholar, and par- 
 ticularly well read in history and the 
 belles lettres; he had, besides, studied his 
 own language grammatically, and wrote 
 it with great correctness and purity. In 
 these new employments, both character 
 and ease of circumstances were acquired 
 by him ; in consequence of the latter, he 
 was enabled to send my elder brother, 
 nearly five years older than myself, to a 
 considerable grammar-school at Dumfries, 
 which was then kept by a Mr. George 
 Chapman. 
 
 I was always my father's favourite, and 
 he, fearing that I should become tainted 
 with the disloyal principles which began 
 immediately after the peace of 1763 to 
 prevail throughout America, obliged me 
 to wear a tartan coat, and a blue Scotch 
 bonnet ; hoping, by these means, to make 
 
MEMOIR. IX 
 
 me consider myself a Scotchman. The 
 persecution I hence suffered produced this 
 effect completely. 
 
 This object was afterwards promoted 
 by sending me to Dumfries school before 
 I was eleven years old. I remained at it 
 nearly two years and a half, at the expira- 
 tion of which time, I had finished the 
 course of studies usually pursued there. 
 His correspondent in Scotland then sent 
 me to Edinburgh, in the autumn of 1770. 
 I attended there several of the lower classes 
 of the University, and went also to the 
 school of a drawing master; I mention 
 this latter circumstance, particularly, be- 
 cause it was in this school that I first 
 formed an acquaintance with two of my 
 present most intimate friends, Mr. David 
 Hume, and Mr. William Miller, now better 
 known by the title of Lord Glenlee. 
 
 I returned to Carolina in the summer of 
 1771, and a few months afterwards, was 
 
X MEMOIR. 
 
 placed as an apprentice with Dr. Alex- 
 ander Garden, the chief practitioner of 
 physic in Charlestown, and well known to 
 naturalists by his communications to the 
 Royal Society. My manners from my in- 
 fancy had always been rude and rough, 
 but after I went to Edinburgh, and fell 
 into the company of Mr. Hume and Mr. 
 Miller, and other } 7 oung men of superior 
 rank to myself, they became considerably 
 softened; but I had always from my ear- 
 liest boyhood a strong desire to act agree- 
 ably to truth. Dr. Garden had been ac- 
 customed to apprentices of a very different 
 character, and in consequence, frequently 
 suspected me of falsehood ; and upon one 
 of these occasions, he attempted to strike 
 me with his hand, but I eluded the blow. 
 From this time, however, I became in my 
 conduct towards him reserved and indig- 
 nant, and finding little or no entertain- 
 ment in the society of the young men of 
 
MEMOIR. XI 
 
 the place, I betook myself seriously to 
 study, and in the course of three years ac- 
 quired, perhaps, more knowledge, though 
 unassisted, than in any three subsequent 
 years of my life. When I had resided 
 with him somewhat more than three years, 
 the American rebellion first broke out in 
 New England. 
 
 My father, whose conduct as the printer 
 of a newspaper had become extremely 
 offensive to the people of Carolina from 
 his constantly maintaining the cause of 
 royalty, found it prudent to leave that 
 country and to return to Great Britain. 
 Soon after he went away, public matters 
 became worse, and I was desired with 
 others to sign a kind of state paper there, 
 " the association/' which as it appeared 
 to me to be an open act of rebellion, I 
 positively refused to do. I therefore de- 
 termined to leave the country also, but 
 my services were now of considerable 
 importance to my master, who was at the 
 
Xll MEMOIR. 
 
 same time one of my father's attornies; 
 my mother's brother was also one of his 
 attornies; and these two, along with my 
 elder brother, strongly resisted the execu- 
 tion of my design; but my mother who 
 was a third attorney, a woman of an en- 
 thusiastic turn of mind, declared, that the 
 first public act of my life should never 
 disgrace me; she, therefore, in spite of 
 the attempts of the others, sent me off to 
 England about three months after my 
 father had parted from her. I arrived 
 in this country in the autumn of the same 
 year, 1775, and was most kindly received 
 by my father, and applauded by him for 
 my conduct. 
 
 In the beginning of the winter of the 
 same year, I went to Edinburgh, and 
 commenced my regular medical educa- 
 tion ; strengthening at the same time, my 
 former friendships with Mr. Hume and 
 Mr. Miller, with whom indeed I had 
 kept up a correspondence whilst I was in 
 
MEMOIR. 
 
 Carolina. I studied three winters in Edin- 
 burgh, and in the course of that time 
 gained a third intimate friend, the present 
 Dr. Robertson Barclay. I passed my pre- 
 paratory trials for the degree of doctor in 
 medicine in the summer of 1778; but did 
 not at that time completely graduate. In 
 the autumn I returned to London, and 
 attended a course of Dr. William Hunter's 
 lectures, and took instructions in prac- 
 tical anatomy. 
 
 Having been about this time offered a 
 surgeoncy in a Scotch regiment, in the ser- 
 vice of Holland, to fit myself in some slight 
 degree for it, I became a surgeon's pupil 
 at St. Bartholomew's hospital for three 
 months. The only excuse I can offer for 
 my boldness in accepting this office, is, 
 that I was told by my friend who pro- 
 posed it to me, that the battalion to which 
 I should belong, did duty in the same gar- 
 rison with another battalion of the same 
 
XIV MEMOIR. 
 
 regiment, and that I should consequently 
 enjoy the assistance of the surgeon to it, 
 a man of considerable experience, but of 
 no school education. On this adventure 
 I embarked early in 1779, and for some 
 time felt myself pleasantly situated in the 
 regiment ; but the colonel, who had been 
 promoted to his command, merely from 
 being an officer in the Dutch guards, and 
 bearing a Scotch name, Hamilton, but 
 scarcely able to speak English, soon began 
 to find fault with my conduct, and in con- 
 sequence once confined me for two days, 
 in the main guard of the garrison, and #. 
 second time, for several days, in its pre- 
 vost, or military -prison. This behaviour, 
 it may well be supposed, could not be 
 borne by a high-spirited Englishman. I 
 therefore resigned my commission, and 
 upon the very day of receiving my dis- 
 missal from the service, I attacked him 
 openly in the street, and dared him to 
 
MEMOIR. XV 
 
 fight me ; he became furious, and ordered 
 a file of musqueteers to seize me, and 
 carry me to prison. Upon the same day 
 he dispatched an express to the Hague, 
 reporting to the Marshal Duke of Bruns- 
 wick, that he had been openly attacked 
 in the streets by one of the surgeons of 
 his regiment. This was held by the mar- 
 shal as so violent a breach of discipline, 
 that it was thought the least punishment 
 I should receive, would be confinement for 
 several years in a remote military prison. 
 Fortunately, colonel Hamilton was not the 
 senior officer of the garrison, and two 
 days after, there arrived, in the ordinary 
 course of business at the marshal's office 
 at the Hague, a report from the com- 
 manding officer, in which it was stated, 
 that surgeon Wells was no longer an officer 
 in the service. This completely altered 
 the state of affairs, and colonel Hamilton 
 was desired to seek redress for his injury, 
 
XVI MEMOIR. 
 
 in some other way than by complaining 
 to the marshal. There is no doubt that 
 the natural warmth of my temper urged 
 me to do, what might have appeared to 
 another person rash, but that I could not 
 have been much in fault is evident from 
 this, that, the present Dr. Storer of Not- 
 tingham, and the present Dr. Stewart of 
 Perth, both previous surgeons to the same 
 regiment, and the Rev. Mr. Pearson its 
 chaplain, now residing in the neighbour- 
 hood of Chelsea, had all abandoned their 
 situations in the regiment from the treat- 
 ment they had received from colonel Ha- 
 milton. 
 
 Immediately afterwards, that is in the 
 beginning of 1780, I went to Leyden, 
 where I remained for about three months, 
 chiefly occupied in preparing a Thesis 
 upon " Cold/' a paltry affair, and having no 
 other recommendation, than that its Latin 
 was altogether my own. From thence I 
 

 MEMOIR. 
 
 returned to Edinburgh, and in the autumn 
 of the same year, published my Thesis, 
 and received the honour of being made 
 Doctor in Medicine. While I was at 
 Edinburgh at this time, I formed a fourth 
 intimate friendship, namely, one with the 
 present Dr. Lister of London. 
 
 Carolina had now been lately conquered 
 by the king's troops; in consequence of 
 which, soon after my arrival in London, 
 in the autumn of that year, 1780, my 
 father requested me to go to that country, 
 to look after his affairs, which had been 
 greatly injured during the war; and as he 
 had not been at all satisfied with my bro- 
 ther's conduct of them, who was the only 
 one of the family that was now there. I 
 arrived in Carolina in the beginning of the 
 year 1781, and found his property there 
 much diminished from various causes; 
 amongst others, the burning of two valu- 
 able houses which had not been insured. 
 I still however was able to render him 
 
 b 
 
XV111 MEMOIR. 
 
 some service. Having accomplished this, 
 I next thought it would be doing a grateful 
 thing to my family, to produce a recon- 
 ciliation between my father and my bro- 
 ther. I therefore told the latter, that if 
 he would go to England to see my father, 
 I would remain in Carolina till his return, 
 for the purpose of managing his affairs 
 there. He readily accepted of my pro- 
 posal, and I in consequence became a 
 printer, a bookseller, and a merchant. I 
 had previously become an officer of vo- 
 lunteers, and had been entrusted by some 
 of my father's friends in England, with the 
 management of affairs in Carolina of con- 
 siderable importance. All these concerns 
 might have been supposed sufficient to 
 occupy the time and attention of a young 
 man of twenty-four (practically unac- 
 quainted with any other employment be- 
 sides that, which arose out of his parti- 
 cular profession). But some insubordina- 
 tion having been shown by a company 
 
MEMOIR. XIX 
 
 of volunteers which was composed of the 
 principal merchants of the place, it was 
 judged necessary that the chief delin- 
 quents should be tried by a general court 
 martial of militia officers, and I was ap- 
 plied to by the colonel commandant of 
 the militia, who was my particular friend* 
 to conduct the prosecution as Judge Ad- 
 vocate. I, foolishly enough perhaps, con- 
 sented, and had as my opponents the 
 two principal lawyers in the place, who 
 acted as counsel for the accused. In the 
 course of the trial I suffered considerable 
 obloquy in consequence of their violence ; 
 but the natural firmness of my mind, and 
 a consciousness of doing what appeared to 
 me to be right, enabled me to resist all 
 their attempts to browbeat me; and the 
 sentence given by the court martial was 
 altogether in conformity to my advice. 
 Not long after this, orders were received 
 from the commander-in-chief at New York, 
 to evacuate the garrison. Every thing was 
 
XX MEMOIR. 
 
 now to be performed in hurry and bustle, 
 and 1 immediately began to prepare for 
 my departure. 
 
 I embarked in December, 1782, for St. 
 Augustine, in East Florida, carrying with 
 me as much of my brother's moveable 
 property as I could ; amongst other things, 
 a printing press, and a considerable quan- 
 tity of printers' types. When I arrived at 
 St. Augustine, I determined to put up 
 the press there, and print a newspaper. 
 But here a considerable difficulty arose; 
 the press had been easily taken to pieces 
 in Carolina, and I naturally thought that 
 it might be readily put together again; 
 more especially as I had brought with me 
 a regular pressman: but to my surprise 
 he told me that he knew nothing of the 
 matter ; that he could work a press as well 
 as any person, when it was put together, 
 but that the putting it together consti- 
 tuted the particular business of a press- 
 joiner. In this dilemma I recollected that 
 
. 
 
 MEMOIR. XXI 
 
 there was amongst my brother's books, 
 one entitled " the Printer's Grammar/' con- 
 taining rude cuts of a printer's press. I 
 studied this book for several days with 
 the greatest diligence; and at length by 
 means of the information derived from it, 
 and with the assistance of a common 
 negro carpenter, completely succeeded in 
 my attempt to put the press in working 
 order. Immediately afterwards, I began 
 to publish a weekly newspaper in my bro- 
 ther's name; the first thing of the kind 
 ever attempted in that country. I still 
 however had much leisure upon rny hands ; 
 (for from the time that I left London, I 
 had scarcely ever read a book, and had 
 always resolved never to exercise my pro- 
 fession except in Great Britain ;) I there- 
 fore became a captain of volunteers, as 
 some threats had been made by the Ame- 
 ricans to attack East Florida, before ac- 
 counts had been received of the signing of 
 
XV111 MEMOIR. 
 
 some service. Having accomplished this, 
 I next thought it would be doing a grateful 
 thing to my family, to produce a recon- 
 ciliation between my father and my bro- 
 ther. I therefore told the latter, that if 
 he would go to England to see my father, 
 I would remain in Carolina till his return, 
 for the purpose of managing his affairs 
 there. He readily accepted of my pro- 
 posal, and I in consequence became a 
 printer, a bookseller, and a merchant. I 
 had previously become an officer of vo- 
 lunteers, and had been entrusted by some 
 of my father's friends in England, with the 
 management of affairs in Carolina of con- 
 siderable importance. All these concerns 
 might have been supposed sufficient to 
 occupy the time and attention of a young 
 man of twenty-four (practically unac- 
 quainted with any other employment be- 
 sides that, which arose out of his parti- 
 cular profession). But some insubordina- 
 tion having been shown by a company 
 
MEMOIR. XIX 
 
 of volunteers which was composed of the 
 principal merchants of the place, it was 
 judged necessary that the chief delin- 
 quents should be tried by a general court 
 martial of militia officers, and I was ap- 
 plied to by the colonel commandant of 
 the militia, who was my particular friend^ 
 to conduct the prosecution as Judge Ad- 
 vocate. I, foolishly enough perhaps, con- 
 sented, and had as my opponents the 
 two principal lawyers in the place, who 
 acted as counsel for the accused. In the 
 course of the trial I suffered considerable 
 obloquy in consequence of their violence ; 
 but the natural firmness of my mind, and 
 a consciousness of doing what appeared to 
 me to be right, enabled me to resist all 
 their attempts to browbeat me; and the 
 sentence given by the court martial was 
 altogether in conformity to my advice. 
 Not long after this, orders were received 
 from the commander-in-chief at New York, 
 to evacuate the garrison. Every thing was 
 
XX MEMOIR. 
 
 now to be performed in hurry and bustle, 
 and I immediately began to prepare for 
 my departure. 
 
 I embarked in December, 1782, for St. 
 Augustine, in East Florida, carrying with 
 me as much of my brother's moveable 
 property as I could ; amongst other things, 
 a printing press, and a considerable quan- 
 tity of printers' types. When I arrived at 
 St. Augustine, I determined to put up 
 the press there, and print a newspaper. 
 But here a considerable difficulty arose; 
 the press had been easily taken to pieces 
 in Carolina, and I naturally thought that 
 it might be readily put together again; 
 more especially as I had brought with me 
 a regular pressman : but to my surprise 
 he told me that he knew nothing of the 
 matter ; that he could work a press as well 
 as any person, when it was put together, 
 but that the putting it together consti- 
 tuted the particular business of a press- 
 joiner. In this dilemma I recollected that 
 
MEMOIR. XXI 
 
 there was amongst my brother's books, 
 one entitled " the Printer's Grammar," con- 
 taining rude cuts of a printer's press. I 
 studied this book for several days with 
 the greatest diligence; and at length by 
 means of the information derived from it, 
 and with the assistance of a common 
 negro carpenter, completely succeeded in 
 my attempt to put the press in working 
 order. Immediately afterwards, I began 
 to publish a weekly newspaper in my bro- 
 ther's name; the first thing of the kind 
 ever attempted in that country. I still 
 however had much leisure upon my hands ; 
 (for from the time that I left London, I 
 had scarcely ever read a book, and had 
 always resolved never to exercise my pro- 
 fession except in Great Britain ;) I there- 
 fore became a captain of volunteers, as 
 some threats had been made by the Ame- 
 ricans to attack East Florida, before ac- 
 counts had been received of the signing of 
 
XX11 MEMOIR. 
 
 the preliminary articles of peace; I also 
 accepted of the management of a com- 
 pany of young officers, who had agreed to 
 perform plays for the advantage of the 
 poorest loyal refugees from Carolina and 
 Georgia. In the course of my manage- 
 ment, a considerable difficulty arose, about 
 finding a person to undertake the cha- 
 racter of Lusignan, in Zara. As I had 
 once seen Garrick in this character, about 
 seventeen years before, (the only character 
 indeed, which I had ever seen him per- 
 form), I determined to attempt it myself. 
 My success was great. This induced me 
 soon afterwards to appear in the character 
 of Old Norval, in Douglas; and here my 
 success was still greater. I was foolish 
 enough not to stop, but afterwards ap- 
 peared in the part of Castalio, in the 
 Orphan. My exertions here were mere 
 common place ; and I failed still more, in 
 an attempt to appear in comedy. 
 
MEMOIR. XX111 
 
 About this time I received a letter from 
 my father, in which he requested me, now 
 that preliminaries of peace were signed, 
 to go again to Carolina, for the purpose 
 of looking after his affairs. I answered, 
 that I should immediately comply with 
 his desire, though I was confident it would 
 be at considerable risk in some way or 
 other. I accordingly went to Charlestown 
 about Midsummer, 1783, furnished with a 
 flag of truce from General Tonyn, which, 
 in the previous intercourse between the 
 two countries during the war, had always 
 been regarded as a sufficient security 
 against any attempt upon the liberty of 
 the person who bore it, though he might 
 be held indebted to any person in the 
 country to which he was going. Imme- 
 diately upon my arrival in Charlestown, I 
 was arrested upon a private suit, originat- 
 ing out of a transaction of my brother's. 
 I refused to give bail, on the ground that 
 
XXIV MEMOIR. 
 
 this would be deserting the security I had 
 obtained by the flag of truce received 
 from General Tonyn. My refusal proved 
 most fortunate. For the night after my 
 committal to prison, a numerous mob as- 
 sembled before the house of Mr. and Mrs. 
 Harleston, (who had insisted upon my 
 staying with them as their guest, whilst I 
 remained in Carolina,) demanding, that I 
 should be surrendered to them. I had 
 looked forward to an outrage of this kind, 
 and had therefore provided myself with 
 arms, being determined never to surrender 
 myself to a mob. A Mr. Simpson, a 
 young English barrister, the son of the 
 late attorney-general for Carolina, who 
 had never done any thing to excite the 
 animosity of the people of that country, 
 was, notwithstanding, upon the same night 
 seized in the house of a lady with whom 
 he was residing, and flung into the har- 
 bour in deep water, where he only escaped 
 
MEMOIR. XXV 
 
 death by being able to swim. I remained 
 in prison upwards of three months, during 
 which time I was robbed by another pri- 
 soner, and in consequence of my com- 
 plaining of this, was most grossly abused 
 by the jailor in the common news- 
 paper. Learning that an Irish gentle- 
 man, a Mr. .ZEdanus Bourke, was one of 
 the judges of that country, I immediately 
 complained to him of the jailor's usage, 
 which he directly put a stop to, by a 
 severe letter to him, a copy of which was 
 sent to me. Immediately upon my having 
 been confined, I wrote to General Tonyn, 
 acquainting him with my situation, and 
 saying, I should be ready to undergo any 
 suffering, rather than that his flag of truce 
 should be tarnished while in my posses- 
 sion. I received no positive answer, for 
 he was a very dilatory man, till upwards 
 of two months after my application had 
 been made to him. A government vessel 
 
XXVI MEMOIR. 
 
 then arrived at Charlestown, bringing a 
 Captain Wyllie as a commissioner from 
 General Tonyn, to demand my release ; 
 Captain Wyllie was, at the same time, pri- 
 vately instructed to inform me, that if this 
 could not be obtained in consequence of 
 his interference, I was completely at liberty 
 to regain my freedom, by paying the un- 
 just demand which had been made upon 
 me; this last measure it was at length 
 found necessary to have resort to. 
 
 As soon as this affair was terminated, I 
 embarked with Commissioner Wyllie in the 
 vessel which had brought him to Charles- 
 town, and proceeded towards St. Augus- 
 tine. The master of the vessel was king's 
 pilot for the harbour. This probably 
 made him fool-hardy; for in weather a 
 little windy, but not stormy, he ran his 
 vessel aground upon breakers which had 
 previously occasioned the loss of many 
 vessels. She immediately bulged, and lost 
 
MEMOIR. XXV11 
 
 her masts, and it was expected that her 
 deck would separate from her ribs, and 
 be carried out to sea, as the tide was now 
 falling. The wind, however, became mo- 
 derate, and the accident which we dreaded 
 did not happen. With some others of the 
 passengers, I had stripped myself com- 
 pletely naked, and lashed myself to the 
 capstan, in order that I might have some- 
 thing firm to abide by, and not be washed 
 away by the waves. Some hours after 
 this, the tide having begun to turn, and 
 set in towards the harbour, and the even- 
 ing becoming dusky, it was determined 
 by those who could swim, to make their 
 way through the breakers, as we saw boats 
 waiting for us in smooth water at their 
 edge. Commissioner Wyllie preceded me, 
 and when taken up, told an intimate friend 
 of mine, who had come down in his boat 
 to assist me, that I should certainly be 
 drowned, as I was unable to swim. Shortly 
 
XXV111 MEMOIR. 
 
 after he had left the wreck, I determined 
 upon making the same experiment myself, 
 and with the assistance of a stout sailor 
 got through the breakers, sometimes swim- 
 ming and sometimes wading. The weather 
 having become still more moderate in the 
 night, those who were left upon the wreck 
 were easily saved the next morning; but 
 in the course of a few hours afterwards, 
 the vessel went entirely to pieces. It may 
 be mentioned here, that the master of the 
 government vessel who had brought us 
 into this situation, was, a few months 
 afterwards, drowned among the same 
 breakers. 
 
 In the course of a few months, my 
 brother arrived from England ; in conse- 
 quence of which, I embarked at St. Au- 
 gustine for Great Britain, i. e. in the 
 month of May, 1784. 
 
 During the time I had been in America, 
 which was nearly four years, I completely 
 
MEMOIR. XXIX 
 
 gave up all study, not even reading the 
 magazines and pamphlets which had been 
 sent me by my father. I have already 
 said in what manner much of my time 
 had been employed ; I shall now add, that 
 another considerable portion of it was 
 spent in female society, which I had for- 
 merly much neglected. As soon as I re- 
 turned to London, I began to think se- 
 riously of studying my profession, to fit 
 myself for the exercise of it ; and in con- 
 sequence, cultivated the acquaintance of 
 medical persons. In this way I became 
 acquainted with the present Dr. Baillie, 
 and soon after contracted with him an in- 
 timate friendship, which now constituted 
 the fifth, and has been the last I have 
 ever formed. 
 
 The next spring I spent three months 
 in Paris, more with the view however of 
 seeing the place, than for improvement in 
 my profession. 
 
XXX MEMOIR. 
 
 About Midsummer 1785, I returned to 
 London, and in the autumn, had the name 
 of Dr. Wells affixed upon the door of a 
 lodging which I had hired. During the 
 war my father's affairs had prospered, and 
 at the end of it he regarded himself worth 
 about .20,000. But as soon as peace 
 took place, his principal correspondents 
 became dilatory in their remittances, but 
 were still urgent for additional supplies of 
 goods ; he was weak enough to comply ; 
 so that when 1 returned to this country 
 in 1784, I found him considerably em- 
 barrassed in his circumstances. He told 
 me, however, that he regarded this em- 
 barrassment as only temporary, and there- 
 fore urged, that I should exercise my 
 profession in London, expecting, that he 
 should hereafter be enabled to afford me 
 all necessary assistance, though at pre- 
 sent, he could give rne nothing more than 
 the use of his table. I was obliged, in 
 
MEMOIR. XXXI 
 
 consequence, to borrow 130 from one of 
 my friends, to enable me to commence my 
 career. 
 
 I soon found, after a very trifling ex- 
 perience, that I was a good deal unfit for 
 early success in my profession in London ; 
 for I entertained a very high notion of its 
 dignity, and felt a great contempt for 
 most of the apothecaries with whom I be- 
 came accidentally acquainted; in conse- 
 quence, I passed several years almost with- 
 out taking a single fee. I fortunately 
 then was chosen one of the physicians to 
 the Finsbury Dispensary ; for I now was 
 furnished with the means of studying me- 
 dicine practically, and received from the 
 institution a gratuity of 50 annually; 
 some few fees also were the consequence 
 of my appointment ; but I had resided in 
 London fully ten years, before my in- 
 come from every source amounted to 250 
 per annum. To supply the consequent 
 
XXX11 MEMOIR. 
 
 deficiency, I was frequently obliged to 
 make further loans from my friends, until 
 the whole of my debt amounted to about 
 600. I think it right in justice to myself 
 however, to mention, that these were my 
 only debts ; for I never allowed a trades- 
 man to call for money and go away with- 
 out it. 
 
 About 1795, my professional receipts 
 became equal to my expenditure; agree- 
 ably to the rigid, and almost sordid manner, 
 in which it was conducted. In the next 
 five years, I w r as enabled to pay off a little 
 of my debt. 
 
 At the expiration of that time, in 1800, 
 I was suddenly seized with a slight fit of 
 apoplexy. From this, however, I did not 
 recover so far as to be enabled to return 
 to the exercise of my profession, for several 
 months ; and I never afterwards regained 
 the complete possession of my memory. 
 I became, too, much more unfit for the 
 
MEMOIR. XXX111 
 
 pursuit of any difficult train of thought, 
 which was the production of another per- 
 son. I did not however, as well as I could 
 ascertain, become less equal than I had 
 been, for the pursuit of my own trains of 
 thought; in proof of which, I may perhaps 
 be allowed to say, that in the fourteen 
 years following this illness, I made more 
 literary efforts than I had done, during the 
 whole preceding period of my life. Dread- 
 ing however another attack of apoplexy, 
 or one of palsy, warnings of which I had 
 almost daily since that time received, I 
 determined to live most abstemiously, and 
 in consequence, took not more food when 
 I was at home (I dined there about four or 
 five times a week) than was sufficient for 
 a child of seven years old, and that con- 
 sisting of vegetable matter. I was the 
 more induced to adopt this manner of 
 proceeding, as my 'father and one of his 
 brothers had previously died of apoplexy ; 
 
XXXIV MEMOIR. 
 
 and a younger sister of my own had been 
 attacked with the same disease when in a 
 state of parturition. It was successful as 
 far as the disease of my head was con- 
 cerned; for I never suffered a second 
 attack of it; but my health became infirm 
 in other respects, and I was seized at dif- 
 ferent times with several dangerous dis- 
 eases, having no apparent relation to my 
 great ailment. 
 
 I had long meditated making some in- 
 quiry into the nature of Dew, which I 
 thought would not occupy me more than 
 a few nights, at the house of one of my 
 friends in the country. I commenced it 
 in the autumn of 1812, but soon found 
 that I had greatly miscalculated the time 
 which it would employ me. I determined 
 however to proceed from the natural 
 steadiness of my disposition, which would 
 never allow me to abandon any pursuit that 
 I had seriously undertaken.- I soon found 
 
MEMOIR. XXXV 
 
 that I was altogether unequal to it; for 
 each night's labour fatigued me so much, 
 that I could not undertake a second for 
 several days after. In the mean time my 
 ancles began to swell in the evening, which 
 I regarded as a mark of general weakness. 
 At length, I became so infirm about the 
 end of 1813, that I was absolutely obliged 
 to give up any further visits to the country. 
 In the beginning of 1814, a considerable 
 snow having fallen, I could not resist the 
 temptation of going for several evenings to 
 Lincoln's-Inn-Fields, during a very severe 
 frost, in order to repeat and extend some 
 of Mr. Wilson's experiments upon snow. 
 I soon however was obliged to desist. I 
 became breathless on slight motion ; and 
 was frequently attacked with palpitation 
 of my heart. My friend, Dr. Lister, be- 
 came alarmed at my situation, and strongly 
 urged my remaining quiet, as he thought 
 it improbable I should survive more than 
 
XXXV111 MEMOIR. 
 
 pened, I always found that I was lying 
 upon my right side, and when I placed 
 myself upon my back, the pains ceased. 
 This was the only situation I could as- 
 sume, for, ever since I had been affected 
 with palpitations and breathlessness, I 
 found it impossible to lie upon my left 
 side. 
 
 About the 10th or 12th of June, I was 
 seized at night with an attack of the 
 pains in my right side. As they did not 
 return, however, I went on the 14th on a 
 visit to Mr. Reid's, at Ewell. At dinner 
 on that day, I was as cheerful as usual, 
 and staid up as late as any of the family. 
 On the following day, I felt no disposition 
 to walk, but at dinner time it was re^ 
 marked, that I took more than ordinary 
 pains to entertain Mr. Reid and his com- 
 pany, which was a large one. Early in 
 the evening, however, 1 became languid 
 and drowsy, went to bed several hours 
 
MEMOIR. XXXIX 
 
 before the rest of the family, and slept 
 that night a much longer time than I had 
 been accustomed to do. In the morning I 
 was stupid and languid, but came to town 
 immediately after breakfast. On the same 
 day I informed Dr. Lister of my situation, 
 which he soon began to think required 
 the attendance of Dr. Baillie along with 
 his own. I shall not say any thing further 
 of my ailments, except, that at first I 
 never imagined that they would terminate 
 in hydro thorax. 
 
 I shall now attempt to give dates to 
 several events which occurred to me in 
 London. 
 
 I think it was in 1790, certainly not 
 later, probably twelve months earlier, that 
 I was appointed a physician to the Fins- 
 bury Dispensary ; I remained so till about 
 
xl 
 
 MEMOIR. 
 
 the year 1798*. In November, 1795, I 
 was elected assistant physician to St. Tho- 
 mas's hospital, and in 1800, I became one 
 of the physicians to it, in which situation 
 I still remain. In November, 1793, 1 was 
 admitted into the Royal Society of Lon- 
 don; in 1814, into that of Edinburgh. 
 About four years ago Dr. Baillie asked 
 me, in the name of the President of the 
 Royal College of Physicians of London, if I 
 had any desire to become a Fellow of it ; 
 to which I answered that I had none. 
 
 In 1792, my Essay on Vision was pub- 
 lished. In the Philosophical Transactions 
 for 1795, appeared my paper on the " In- 
 fluence which incites the muscles of animals 
 to contract, in Mr. Galvani's Experiments;" 
 in those for 1797, my " Experiments on 
 the Colour of the Blood ;" and lastly, in 
 
 * Dr. Wells was elected Physician to the Finsbury 
 Dispensary on the 3d of September, 1789, and resigned 
 the office on the llth of December, 1799- E. 
 
MEMOIR. Xli 
 
 * 
 
 those for 1811, some " Experiments and 
 Observations on Vision ." I have already 
 said that in 1814, my " Essay on Dew" 
 appeared. I formerly omitted to mention 
 that Dr. Darwin attacked in his Zoonomia, 
 what I had said upon giddiness*. I imme- 
 diately answered him in two letters sent 
 to the publisher of the Gentleman's Ma- 
 gazine. I have now referred to, every 
 thing which I have published on philoso- 
 phical subjects. In the second and third 
 volumes of the " Transactions of a Society 
 for the Promotion of Medical and Chi- 
 rurgical Knowledge/' almost every thing 
 that I have published upon medicine is 
 to be found ; but these are so numerous, 
 that I shall not particularize their dates. 
 In 1799, was printed by me, rather than 
 published, for it was never exposed to sale, 
 a " Letter to Lord Kenyon." These writ- 
 ings, as well as I can recollect, are the only 
 ones which ever were printed by me with 
 
 * In the Essay upon Single Vision with two Eyes. 
 
xlii MEMOIR. 
 
 my name affixed to them; but several 
 others have been given by me to the 
 world without this attendant. 
 
 The first of these, and indeed the first 
 thing that ever I wrote for the public, was an 
 account of Mr. Henry Laurens, some time 
 president of the American Congress, which 
 appeared in the form of a letter, under 
 the signature of " Marcus," to the printer 
 of the Public Advertiser, in September, 
 October, or November, 1780. In 1800, 
 I published, in the Gentleman's Maga- 
 zine, some account of the life of Mr. 
 Anthony Lambert, formerly of Calcutta. 
 In the course of the same year, I pub- 
 lished also some account of Mr. Wilson, 
 of Bedford-street, Covent Garden. In 
 the course of the next twelve years, ap- 
 peared in the same magazine, " Biogra- 
 phical Sketches," also written by me, of 
 the following physicians; Dr. George 
 Fordyce, Dr. David Pitcairn, and Dr. 
 Andrew Marshall. In Carolina, during 
 
" MEMOIR. xliii 
 
 the years 1780 and 1781, I published 
 many small political things, without at- 
 taching my name to them ; the principal 
 of which was written at the desire of the 
 commandant of the garrison of Charles- 
 town, the present General Nesbitt Balfour. 
 The cause was the following. Men of rank 
 in that country in the American service, 
 after having been taken prisoners, and sent 
 to their homes under their military paroles, 
 used to make no scruple whatsoever to 
 appear again in arms against the British 
 government. I therefore was desired to 
 show, by an appeal both to military usage, 
 and the nature of the thing itself, that such 
 conduct subjected them to the punishment 
 of death. This paper was held of that 
 importance by the commandant, that he 
 gave orders that its publication in the 
 public newspapers should be frequently 
 repeated ; and I think it highly probable, 
 that it was owing to this warning, that 
 General Balfour and Lord Moira thought 
 
xliv MEMOIR. 
 
 themselves justified in putting to death a 
 Colonel Haynes, the propriety of whose 
 fate was afterwards a subject of debate in 
 the British House of Commons. 
 
 I think it right to say something more 
 particular than I have hitherto done, re- 
 specting the clear profits of my profession, 
 the only source of revenue that I have 
 ever enjoyed in London. In 1801, the 
 sixteenth year after I had become a phy- 
 sician in London, they amounted only to 
 307, in which sum were included about 
 J60, which I had received in the form of 
 salary from St. Thomas's Hospital, and of 
 fees, for the attendance of medical pupils 
 there. The following year my total re- 
 ceipts diminished to 235. They remained 
 in this fluctuating state during the three 
 following years, that is, till 1806 included. 
 During the next six years, they fluctuated 
 between ^325 and ^455. In 1813, they 
 
MEMOIR, 
 
 xlv 
 
 were <457. In 1814, J441. In 1815, 
 ^764. In 1816 5 572 ; but in 1815, I 
 received at one time %10 for giving me- 
 dical evidence at Exeter. The smallness 
 of these receipts will perhaps appear the 
 more extraordinary, when I say, that dur- 
 ing a great part of this time, Dr. Pitcairn, 
 and during the whole of it, Dr. Baillie, 
 often sent patients to me ; and made every 
 exertion to promote my interest. But I 
 lived at a considerable distance from them, 
 and was unable, from the want of a car- 
 riage, and from various other circum- 
 stances, to appear properly as their repre* 
 sentative. In spite, however, of this small- 
 ness of my income, (which, during almost 
 the whole of the time spoken of, that is, 
 from 1801 to 1816, was rendered still 
 smaller, by my paying most rigidly the 
 income and property tax, and allowing an 
 annuity, for a good many years, of %Q to 
 a female relation,) so rigid was my eco- 
 nomy, that, during the few last years of 
 
xlvi 
 
 MEMOIR. 
 
 my life, I paid off the whole of the money 
 which I had borrowed, amounting, as was 
 formerly mentioned, to about ^600; and 
 when I was taken ill, about three months 
 ago, I had in my desk, for I never kept a 
 banker, nor ever invested any money in 
 the funds, about ^350. This sum consti- 
 tuted the greater part of my property. 
 For all my books, my little plate and fur- 
 niture, probably, though much more va- 
 luable to myself, will not be supposed by 
 others worth more than <200. In this 
 estimate, the value of my gold Rumford 
 medal is not included ; as the gold is quite 
 pure, it is held to be intrinsically worth 
 fifty guineas. 
 
 In the expectation that my life would 
 be prolonged, I had formed various literary 
 projects. One was, and this had often 
 passed rapidly through my mind during 
 the last forty years of my life, to show, 
 
MEMOIR. 
 
 xlvii 
 
 that there is a material difference in the 
 manner in which we acquire our ideas of 
 the primary and secondary qualities of 
 matter*. If, after a closer examination 
 of this subject than I had formerly given 
 it, I should have found, that my notions 
 respecting it were just, I should have at- 
 tempted in treating of it, to imitate, in 
 some slight degree, the inimitable manner 
 employed by Berkeley in his Treatise on 
 Vision. I should then have presented to 
 the Royal Society several papers on vision, 
 the chief of which would have treated of 
 those phenomena of light, which have been 
 denominated by authors coloured shadows, 
 ocular spectra, and by various other titles. 
 In the last place, I should have brought 
 together into one volume, all my publica- 
 
 * He made out, in his own hand- writing, during his 
 last illness, a short statement of his opinion upon this 
 subject, which, by his desire, has been put, since his 
 death, into the hands of a philosopher, whose great learn- 
 ing and profound researches into the human mind pecu- 
 liarly fit him for estimating it justly. E. 
 
xlviii MEMOIR. 
 
 tions upon vision ; which I would have 
 inscribed to the memory of Robert Wells 
 my father, in gratitude, for the great exer- 
 tions which he had made to give me the 
 education of a scholar and a gentleman, 
 when in narrow circumstances himself, 
 into which he had fallen, in consequence 
 of the American rebellion . 
 
 What I shall next say will no doubt be 
 held very ridiculous. I lived till I was 
 near eleven years old, close upon the har- 
 bour of a large sea-port in America, and 
 by this means associated much with black- 
 guard sailor boys. To this I attribute a 
 practice of swearing, of which I have from 
 the time of being a child, been frequently 
 guilty, when my feelings have been agi- 
 tated, and even sometimes when no excuse 
 of this kind has existed. 
 
MEMOIR. 
 
 xlix 
 
 My last declaration will relate to the 
 obligations under which I lie to my friends. 
 I have already spoken of my rare good 
 fortune, in having acquired, in the course 
 of my life, five most intimate friends. All 
 of these are still in being, and from all of 
 them I have received, throughout my ill* 
 ness, the warmest -proofs of attachment. 
 Two of them, however, have most espe- 
 cially afforded such proofs, Dr. Lister and 
 Dr. Baillie, partly from their residing in 
 London, and partly from the nature of 
 their profession. My obligations to Dn 
 Lister are extreme. During the whole of 
 my disease, he has visited me constantly 
 twice, and sometimes thrice a day ; and 
 during each of these visits, he has con- 
 ducted himself towards me, with fully as 
 much kindness, as if I had been his 
 brother. 
 
 I have likewise to express my very great 
 obligations to two other of my friends, Mr* 
 
 d 
 
1 J MEMOIR, 
 
 James Dunsmure, merchant, in Lothbury, 
 and Mr. Samuel Patrick , of Bar tlett's Build- 
 ings, surgeon ; since, in the whole course 
 of my illness, their attentions to me have 
 been most unremitted, and they have 
 also most generously promised to burthen 
 themselves with the care of my concerns, 
 after my death. 
 
 It must not be regarded as an instance 
 6f the weakness of an old man's mind, my 
 desiring, that my body may be deposited 
 in Lady Jersey's vault in St. Bride's 
 Church; immediately above that of my 
 mother, and in contact with it, as her's is 
 now placed with respect to that of my fa- 
 ther; for it has been my wish, for many 
 years past, that this should be done. I 
 have, indeed, never been desirous to con- 
 quer any natural feelings, when their in- 
 dulgence led to no harm ; on the contrary, 
 
MEMOIR. 
 
 ll 
 
 1 have always regarded such an indul- 
 gence, as highly conducive to the soften- 
 ing of the original hardness of my cha- 
 racter. 
 
 August 22d, 1817. 
 
 As I fancy that several parts of my cha- 
 racter, from various reasons, have been 
 a good deal misunderstood, even by my 
 most intimate friends ; I shall relate here, 
 with little regard to method or connection, 
 some circumstances which may tend to 
 illustrate it. 
 
 I began to show, even in my earliest 
 childhood, an invincible firmness of mind. 
 When my father, who was a passionate 
 man, beat me for a fault, which 1 was con- 
 scious 1 had committed, I used to entreat 
 mercy most piteously ; but if I believed, 
 that I was in the right, the utmost punish- 
 ment he could inflict would scarcely ever 
 
 a 2 
 
Hi MEMOIR. 
 
 force a tear from me. When I was at 
 Dumfries school, I had a playfellow, the 
 
 present Mr. , of Edinburgh. He one 
 
 day called me by some improper name, in 
 consequence of which I beat him, being 
 the stronger of the two. He complained 
 to Mr. Chapman our master, who ordered 
 me to promise, that I would never do the 
 like again. I answered that I could not, 
 for I would certainly beat him if he re- 
 peated the offence. Mr. Chapman tried 
 first the effect of corporal punishment 
 upon me ; but finding this of no avail, he 
 ordered me to retire to my room, for I 
 was one of his boarders, and forbade the 
 other boys to hold society with me. This 
 happened upon a Saturday, which was at 
 our school a half holiday . On the Monday 
 following, I was summoned to appear in 
 the school, as I thought, for the purpose 
 of being [finally] expelled from it ; for, I 
 had determined to submit'to this disgrace, 
 
MEMOIR. liii 
 
 rather than to swerve from my former de- 
 claration. To my astonishment, however, 
 I found that I was to receive from him the 
 highes^ commendation. On the Saturday 
 afternoon, my confinement not having been 
 strictly enforced, I was determined to break 
 through it, and to go into the neighbour- 
 ing country with some of my playfellows. 
 There was before the school a considera- 
 ble area, in which, while I was proceeding 
 to join my playfellows, I met a blind beg- 
 gar, who appeared to me to have lost his 
 way. The other toys had passed him 
 without attending to him. I went up to 
 him, and finding my conjecture to be 
 right, took him by the hand, and led him 
 to the house to which he was desirous of 
 going. My master was at his window, 
 and saw this. On the Monday he men^. 
 tioned it to the whole school, and received 
 me back into it, with great commendation 
 
Hv 
 
 MEMOIR. 
 
 of my conduct, without making any refer- 
 ence to my former expulsion. 
 
 My father was a man of great sobriety 
 himself, and restricted me, while I was a 
 boy, from drinking any thing but water ; 
 and I never, in any posterior part of my 
 life, have had the least desire to taste any 
 stronger liquor, except in compliance with 
 the ordinary customs of society. In 1782, 
 I became president of a club in Florida, 
 and agreeably to the custom of the coun- 
 try, thought it necessary to make my sub- 
 jects intoxicated. In this attempt, I ne- 
 cessarily became somewhat intoxicated 
 myself, but still in a less degree than the 
 others, from proceeding more cautiously. 
 During the other six days of the week, 
 though living constantly in society, I drank 
 nothing but water, nor did I ever after- 
 
MEMOIR. ly 
 
 wards, even before my health became in- 
 firm in 1800, desert this practice, except 
 I was in society. 
 
 My father, though naturally a passionate 
 man, in all the ordinary affairs of life con- 
 ducted himself with the greatest prudence, 
 except in the case already mentioned, when 
 he was induced, by too great ease of tem- 
 per, to swerve from it. My mother was 
 much his inferior in point of common sense, 
 and had a strong tendency to act a little 
 romantically. 
 
 I .resembled them both, not only in per- 
 son but disposition ; and, in consequence 
 of my resemblance to my mother in this 
 latter circumstance, began early to show 
 signs of a certain waywardness of dispo- 
 sition. 
 
 When I was a boy at Dumfries school, 
 1 used to wander on foot during the 
 
v MEMOIR. 
 
 autumnal holidays through the country, 
 without any fixed object. In one of these 
 rovings, being then in the twelfth year of 
 my age, I went to call upon a friend of 
 my father's, without any other clothes than 
 those which I had upon me. The follow- 
 ing morning, I thought my shirt looked 
 dirty, and therefore determined to wash it 
 myself. I chose, as a place fit for this 
 purpose, a little meadow on the side of the 
 river* Milk, which was sheltered by a high 
 bank behind me. Having done the busi- 
 ness in the best manner I could, without 
 any assistance from soap, I placed my 
 shirt upon the grass for the purpose of 
 drying it, and laid myself in the mean 
 time in the sunshine, upon another piece 
 of dry grass in the neighbourhood. When 
 my shirt was dry, I put it on, and returned 
 to my friends. In the course of the night, 
 
 * This, I ain told, is a rivulet rather than a river. E. 
 
MEMOIR. 
 
 Ivii 
 
 I was seized with a considerable degree of 
 fever, and in the morning my face, and 
 the parts of my body which had been ex- 
 posed to the sun, became considerably red 
 and swollen. 
 
 About a twelvemonth afterwards, I re- 
 ceived an invitation by letter from a school 
 chum, to visit him at his father's, who lived 
 in Galloway, about 31 miles from Dum- 
 fries. I showed this letter to Mr. Chap- 
 man, and requested money from him, to 
 enable me to make the visit. He most 
 properly refused to give any, upon the 
 ground, that the invitation had proceeded 
 only from a boy. I thought differently, 
 however, and, taking advantage of his ab- 
 sence, began my journey two days after, 
 without a halfpenny in my pocket, and 
 with no other clothes than I wore, as I had 
 determined to return to Dumfries the fol- 
 lowing day. My friend's father, whose 
 name was Macmurdo, had lived many years 
 
Iviii 
 
 MEMOIR. 
 
 in Virginia as a merchant, and when he 
 returned to Scotland, brought with him a 
 wife, who was a native of the former coun- 
 try. They received me most kindly, no 
 doubt somewhat influenced by iny having 
 been born in America, and retained me 
 as their guest for upwards of a month ; 
 supplying, amongst my other deficien- 
 cies, that of raiment. At the expiration 
 of the holidays, they sent me in a post- 
 chaise to Dumfries, with a part of their 
 own family. 
 
 In my journey to Mr. Macmurdo's house, 
 which I accomplished in eleven hours, I 
 had no food but hips and blackberries, 
 and a little milk, which a cottager would 
 sometimes give me when I asked for a 
 little water to drink. 
 
 My temper was naturally irritable, and 
 in small differences which have occurred 
 
MEMOIR. x 
 
 in society, particularly in my youth, 
 passionate and violent. But I must, in 
 justice to myself, say, in the first place, I 
 have not shown any considerable instance 
 of this kind for nearly twenty years ; and 
 in the second, that I did never show one, 
 even before that time, in any matter of 
 consequence, or when I had any respect 
 for the person with whom I differed. In 
 confirmation of both these remarks, I shall 
 mention, first, that I have never had the 
 smallest difference with any one of my five 
 most intimate friends ; and secondly, that 
 I have borne the grossest insult, when it 
 was unmanly to take immediate notice 
 of it. 
 
 From the time of the murder of the 
 princess of Lamballe, I foresaw the ruin 
 of all civilized society in France, and 
 dreaded a similar ruin of all civilized 
 
x MEMOIR. 
 
 society in Europe. I have never, there- 
 fore, been able to hear, with the least 
 patience, any serious defence of the con- 
 duct of the French ; and have always at- 
 tributed such a defence to incurable folly, 
 self-interest, or madness. In all points of 
 domestic 'politics, I have kept myself free 
 from personal influence, by never seeking 
 the acquaintance of any person of the least 
 influence in the country. By principle I 
 am a constitutional Tory; but my man- 
 ners, I should think, would lead most per- 
 sons to regard me a republican. 
 August 28, 1817. 
 
 DR. WELLS, from a very early period in 
 his illness, looked forward to a fatal ter- 
 mination of it, and employed himself in 
 arranging his affairs with the utmost self- 
 possession and diligence, until he had 
 
MEMOIR. x 
 
 settled, with great exactness, every thing 
 which he thought important. From the 
 8th of August, his physicians, as well as 
 himself, abandoned all hopes of his reco- 
 very. He died in the evening of the 18th 
 of September, 
 
ADVERTISEMENT 
 
 THE Memoir, with the omission of an 
 anecdote, which might have given pain to 
 a family with which the author had been 
 on terms of great intimacy, and of a name 
 and designation, which it was believed the 
 very respectable person referred to might 
 wish to be suppressed, and with a very 
 slight alteration in a very few expres- 
 sions, is precisely as it was left by the 
 author. He dictated it to his friend Mr. 
 Patrick at intervals during his illness, 
 after he had lost all hope of recovery, 
 and while he was uncertain whether he 
 should live to finish it, and when he w r as 
 too feeble to speak long, or to write 
 much. It must be considered a proof 
 of extraordinary composure and vigour of 
 mind in such circumstances. 
 
ADVERTISEMENT. 
 
 The writings of the author, which have 
 been selected for publication with this 
 Memoir, either as the most interesting in 
 themselves, or as affording the best exhi- 
 bition of his character and talents, are, an 
 Essay upon Single Vision with two Eyes, 
 and an Essay upon Dew ; a Letter to the 
 Right Honourable Lloyd, Lord Kenyon, 
 relative to some conduct of the College 
 of Physicians of London, posterior to the 
 decision of the Court of King's Bench, in 
 the case of Dr. Stanger, and containing 
 observations on a principal ground of that 
 decision; and an Account of a Female of 
 the White Race of Mankind, part of whose 
 Skin resembles that of a Negro ; with some 
 observations on the causes of the differences 
 in colour and form between the white and 
 negro races of men. The last of these 
 writings was read before the Royal Society 
 in 1813, but was never printed until now. 
 It was put by the author into the hands 
 of the editor, with an express permission 
 
Ixiv 
 
 ADVERTISEMENT. 
 
 to publish it, and no alteration has been 
 made in it, besides a very slight one of 
 expression, in a few places, which its being 
 presented to the public, instead of being 
 addressed to a philosophical society, ren- 
 dered necessary. 
 
 All his other works, whether philoso- 
 phical, literary, or medical, (excepting only 
 those of a political nature, which are men- 
 tioned in the Memoir, and to which no 
 more particular reference could be made 
 than what is made in it,) are enumerated 
 in the following list, in order that they 
 may be more generally known and more 
 easily referred to. 
 
 Two letters, in reply to Dr. Darwin's 
 remarks, in his " Zoonomia," upon what 
 Dr. Wells had written in his " Essay upon 
 single Vision with two Eyes/' on the ap- 
 parent rotation of bodies, which takes 
 place during the giddiness occasioned by 
 turning ourselves quickly and frequently 
 round. These were published in the 
 
ADVERTISEMENT. 
 
 Gentleman's Magazine for September and 
 October 1794. 
 
 Observations on the influence which in- 
 cites the muscles to contract in Mr. Gal- 
 vani's experiments. These were published 
 in the Philosophical Transactions in 1795. 
 
 Observations and experiments on the 
 colour of blood.. These were published in 
 the Philosophical Transactions in 1797. 
 
 Some account of the life of Mr. Anthony 
 Lambert, formerly of Calcutta ; and some 
 account of Mr. George Wilson, apothecary, 
 of Bedford-street, Covent Garden. Both 
 these appeared in the Gentleman's Maga- 
 zine for 1800. 
 
 A biographical sketch of Dr. George 
 Fordyce. This appeared in the Gentle- 
 man's Magazine for 1802. 
 
 A short account of Mr. John Savage, 
 formerly of Charlestown. This appeared 
 in the Gentleman's Magazine in 1804. 
 
 A biographical memoir of Dr. David 
 
ADVERTISEMENT. 
 
 Pitcairn. This appeared in the Gentle- 
 man's Magazine in 1809- 
 
 Observations and experiments on Vision. 
 These were published in the Philosophical 
 Transactions in 1811. 
 
 A biographical sketch of Dr. Andrew 
 Marshall. This was published in the Gen- 
 tleman's Magazine in 1813. 
 
 An answer to remarks in the Quarterly 
 Review, upon the Essay on Dew. An 
 answer to Mr. Prevost's queries respecting 
 the explanation of Mr. B. Prevost's expe- 
 riments on Dew. These appeared in Dr. 
 Thomson's Annals of Philosophy for 1815. 
 
 A short letter " on the Condensation of 
 Water upon Glass." This was published 
 in Dr. Thomson's Annals of Philosophy 
 for 1816. 
 
 The titles of his medical writings are, 
 
 1. Observations on Erysipelas. 
 
 2. An Instance of an entire want of Hair 
 in the Human Body. 
 
 
ADVERTISEMENT. 
 
 Ixvii 
 
 3. Observations on the Dropsy which 
 succeeds Scarlet Fever. 
 
 4. A Case of Aneurism of the Aorta, 
 attended with ulceration of the (Esopha- 
 gus and Windpipe. 
 
 5. A Case of Epilepsy and Hemiplegia, 
 apparently produced by a sharp projection 
 from the inner Table of the Skull 
 
 6. A Case of Tetanus, with Observa- 
 tions on the Disease. 
 
 7. A Case of Aneurism of the Aorta, 
 communicating with the Pulmonary Artery. 
 
 8. A Case of considerable enlargement 
 of the Caecum and Colon. 
 
 9. A Case of an extensive Gangrene 
 of the Cellular Membrane, between the 
 Muscles and Skin of the Neck and Chest. 
 
 10. On Rheumatism of the Heart. 
 
 11. On the presence of the Red Matter 
 and Serum of the Blood in the Urine of 
 Dropsy, which has not originated in Scar- 
 let Fever. 
 
Ixviii ADVERTISEMENT. 
 
 12. Observations on Pulmonary Con- 
 sumption and Intermittent Fever, chiefly 
 as Diseases opposed to each other ; with 
 an attempt to arrange several other dis- 
 eases, according to the alliance or opposi- 
 tion which exists between them and one 
 or other of the two former. 
 
 These were all published in the second 
 and third volumes of the " Transactions 
 of a Society for the promotion of Medical 
 and Chirurgical Knowledge/' 
 
 There is also a case of Aphonia Spasmo- 
 dica described by him, and communicated 
 by Dr. Carmichael Smith, in the second 
 volume of the " Medical Communications/' 
 
CONTENTS. 
 
 ESSAY UPON SINGLE VISION WITH TWO EYES. 
 
 PART I. 
 
 Of the different opinions concerning single vision with two 
 eyes j and principally of those of Dr. Smith and Dr. Reid 
 
 pagel 
 
 PART II. 
 
 Of a new theory respecting visible direction, and of a solu- 
 tion hence derived of the question, why objects are seen 
 single with two eyes . . . .28 
 
 PART III. 
 
 Of some consequences from the foregoing theory of objects 
 being seen single with two eyes ; together with the ex- 
 planation of several other phenomena of vision . 51 
 
 EXPERIMENTS AND OBSERVATIONS ON SEVERAL 
 SUBJECTS IN OPTICS. 
 
 ARTICLE I. 
 
 On visible position and visible motion . . 69 
 
Ixx CONTENTS. 
 
 ARTICLE II. 
 
 On a supposed consequence of the duration of impressions 
 upon the retina ; and the effects of accurate vision being 
 confined to a single point of that membrane . page 86 
 
 ARTICLE III. 
 
 On the connexion between the different refractive states of 
 the eyes, and the different inclinations of the optic axes to 
 each other . . . . , .94 
 
 ARTICLE IV. 
 
 On the limits of perfect and distinct vision . . 1O/ 
 
 ESSAY ON DEW 
 
 AND 
 
 SEVERAL APPEARANCES CONNECTED WITH IT ug 
 Introduction . . . . . 123 
 
 PART I. 
 
 OF THE PHENOMENA OF DEW. 
 
 SECT, i. Of circumstances which influence the production 
 of dew . . .. . . .127 
 
 SECT. ii. Of the cold connected with the formation of 
 dew . . ; :. . . 152 
 
 PART II. 
 
 OF THE THEORY OF DEW. 
 
 Former theories . . . . . ] 77 
 
 A NEW THEORY PROPOSED. 
 
 Deto is the production of a preceding cold in the substances 
 upon which it appears . . . .181 
 
CONTENTS. Ixxt 
 
 That cold precedes the formation of dew ascertained by ex- 
 periment '- 1 %* .... page 182 
 This fact applied to explain several natural appearances. 
 
 1. The variety in the quantities of dew on different bodies, 
 exposed to the air during the same time of the night, 
 but in different situations . . * 185 
 
 2. The cold connected with dew, not being always pro- 
 portional to the quantity of that fluid . . ib. 
 
 3. The production of heat by the formation of dew 186 
 
 4. The fact of more dew being acquired, in very calm 
 nights, by substances placed upon a raised board, than 
 by others of the same kind on the grass ; and that of 
 a slight agitation of the atmosphere, when very preg- 
 nant with moisture, increasing the quantity of dew 
 
 188 
 
 5. The fact of dew never being formed in temperate 
 climates upon the naked parts of a living and healthy 
 human body . . . L3lv;j ?'? 189 
 
 6. The fact of hygrometers, formed of animal and vegeta- 
 ble substances, when exposed to a clear sky at night, 
 marking a degree of moisture beyond what is actually 
 resident in the atmosphere . . .190 
 
 The cold which produces detv, is itself produced by the radia- 
 tion of heat, from those bodies, upon u>hich deiu is deposited 
 
 191 
 
 The cold produced by the radiation of heat from substances 
 upon the surface of the earth, is compensated or over- 
 balanced in the day-time by the heat from the sun, and 
 lessened at night by various causes . .196 
 
 The cold originating in the nightly radiation of heat from 
 bodies upon the surface of the earth, though lessened by 
 various causes, is often very considerable . .198 
 
 Some of the useful effects of the radiation of heat from the 
 earth at night . : ,'. . . . ^ ,, 201 
 
 Observations upon, or explanations of the under-mentioned 
 circumstances. 
 
Ixxii CONTENTS. 
 
 1 . The prevention, wholly or in part, of cold from radia- 
 tion, in substances on the ground, by the interposition 
 of any solid body between them and the sky page 203 
 
 2. The prevention, wholly or in part, of cold from radia- 
 tion, in substances on the ground, by the interposition 
 of clouds ", '';'- . . . . 205 
 
 3. The prevention, wholly or in part, of cold from radia- 
 tion, by fogs . ... 207 
 
 4. The prevention, wholly or in part, of cold from radia- 
 tion, by conduction from warmer substances in contact 
 with the radiating substance . . .211 
 
 5. The effect of wind in compensating the cold from ra- 
 diation, and sometimes in lessening, and sometimes in 
 increasing the production of dew . . 212 
 
 6. The cold from radiation, of a thermometer placed on a 
 board, being less diminished than that of one suspended 
 in the air ..... 213 
 
 7. The hurtful effects of cold occurring chiefly in hollow 
 places, according to a remark of Theophrastus 214 
 
 8. Frost being less severe upon hills, than in neighbouring 
 plains, in calm and serene nights . -<'.. 215 
 
 Reasons assigned for believing that air is actually heated by 
 the sunbeams which enter it, and that it not only absorbs, 
 but radiates heat .... >/;. 217 
 
 9. The leaves of trees often remaining dry throughout 
 the night, while those of grass are covered with dew 
 
 227 
 
 10. Bright metals exposed to a clear sky in a calm night 
 being less dewed on their upper surface, than other 
 solid bodies j and those metals which radiate heat most, 
 being most attractive of dew . . #?V 228 
 
 11. The difference between black and white bodies with 
 respect to radiation, when exposed to the sky at night 
 
 235 
 
 Whether dew is the product of vapour emitted during the 
 night by the earth and plants upon it . . 236 
 
CONTENTS. Ixxiii 
 
 PART III. 
 
 OP SEVERAL APPEARANCES CONNECTED WITH DEW. 
 
 1. Of the greater moisture, sometimes observed in winter 
 mornings upon the insides of the panes of glass in win- 
 dows covered with inside shutters, than upon those not 
 covered by them .... page 247 
 
 2. Of the greater sensation of cold, which is sometimes ex- 
 perienced upon exposure to the sky in a clear night, than 
 is to be explained by the temperature of the atmosphere 
 
 249 
 
 3. Of the effect of those means, employed by gardeners to 
 protect tender plants from cold during the night, which 
 screen them from the sky . . .252 
 
 4. Of the effect of a covering of snow, or of other matters, 
 during still and serene nights, in protecting vegetables 
 from cold ..... 257 
 
 5. Of the putrefaction which has been supposed to take 
 place in animal substances exposed to moonshine 258 
 
 6. Of the formation of ice, during the night in Bengal, 
 when the temperature of the air is above '32 200 
 
 Conclusion . .... 280 
 
 A LETTER 
 
 TO THE 
 
 RIGHT HON. LLOYD, LORD KENYON, 
 
 RELATIVE TO SOME CONDUCT OF THE COLLEGE OF PHYSICIANS 
 
 OF LONDON, POSTERIOR TO THE DECISION OF THE COURT 
 
 OF KING'S BENCH, IN THE CASE OF DR. STANGER; 
 
 AND CONTAINING 
 
 OBSERVATIONS ON A PRINCIPAL GROUND OF THAT DECISION 283 
 
Ixxiv CONTENTS. 
 
 AN ACCOUNT 
 
 OF 
 
 A FEMALE OF THE WHITE RACE OF MANKIND, 
 PART OF WHOSE SKIN RESEMBLES THAT OF A NEGRO ; 
 
 WITH SOME 
 
 OBSERVATIONS ON THE CAUSES OP THE DIFFERENCES, 
 IN COLOUR AND FORM, BETWEEN THE WHITE AND 
 
 NEGRO RACES OF MEN . page 423 
 
 - 
 
AN 
 
 ESSAY 
 
 UPON 
 
 SINGLE VISION WITH TWO EYES 
 
 TOGETHER WITH 
 
 EXPERIMENTS 
 
 AND 
 
 OBSERVATIONS 
 
 ON 
 
 SEVERAL SUBJECTS IN OPTICS. 
 

 
AN 
 
 ; .. ESSAY 't'' 
 
 UPON 
 
 SINGLE VISION WITH TWO EYES. 
 
 PART I. 
 
 Of the different Opinions concerning single Vision with 
 two Eyes ; and principally of those of Dr. Smith and 
 Dr. Reid. 
 
 1 HE end I have chiefly in view, in this Essay, 
 being to offer a new solution of the question, 
 why objects are perceived single with two eyes, 
 I think it incumbent upon me, in the first 
 place, to show, that none of the opinions I have 
 met with upon this subject, can be admitted as 
 just. 
 
 These opinions, or such of them, at least, as 
 have gained any considerable reputation, may 
 be reduced into two classes. The first com- 
 prehends those of Galen, Alhazen, Rohault, 
 Dr. Briggs, and Sir Isaac Newton, all of whom 
 have regarded the question I have mentioned 
 
^ AN ESSAY ON 
 
 as equivalent to the following one : Whence 
 comes it, that the mind should be affected with 
 only one perception from two impressions upon 
 the external organs of sight, since either of 
 those impressions is, of itself, sufficient to 
 produce a similar perception ? Their universal 
 answer has been : Because the two impressions 
 are united before they are communicated to 
 the mind. And the only difference among 
 these authors, has been with respect to the 
 manner in which such an union takes place. 
 To the second class are to be referred the 
 opinions of those, who hold it as certain, that 
 an object is seen single by both eyes, because 
 it is seen by each of them in the same external 
 place ; and who profess to point out some law, 
 or constant rule of vision, from which this 
 sameness of place is to be derived as a neces- 
 sary consequence. Aguilonius, I believe, first 
 gave this view of the question, which has since 
 been adopted by Dechales, Dr. Porterfield, 
 Dr. Smith of Cambridge, and Dr. Reid of 
 Glasgow. 
 
 In opposition to the opinions of the first 
 class, more especially as they have been re- 
 peatedly examined by others, I think I need 
 only say, that they must all be considered as 
 mere conjectures, founded upon certain sup- 
 posed changes in the brain and nerves, the 
 
SINGLE VISION. * 
 
 existence of which it is impossible, from the 
 nature of the parts, either to demonstrate, or 
 to refute by experiments ; and that no one of 
 them, though admitted to be true, is yet suf- 
 ficient to explain the phenomena on account 
 of which it was framed. 
 
 The opinions of the second class being built, 
 as their authors think, upon experiments and 
 observations, both allow and demand a more 
 accurate investigation. I shall proceed, there- 
 fore, to examine such of them as I am acquainted 
 with, beginning with that of Aguilonius ; and 
 what I shall observe concerning it will apply 
 also to those of Dechales and Dr. Porterfield, 
 who have done little more than copy what he 
 has said. 
 
 If a line be drawn through the point of the 
 mutual intersection of the optic axes, parallel 
 to the interval between the eyes, Aguilonius 
 calls it, from its office, the horopter-, and if 
 through this line, a plane be made to pass at 
 right angles to that of the optic axes, he names 
 it the plane of the horopter. After defining 
 these terms, he asserts, that, by a law of our 
 constitution, all bodies which we see with one 
 glance or look, whatever are their real places, 
 appear to each eye to be situated in this plane. 
 And if this be granted to him, he easily and 
 satisfactorily shows, why some should be seen 
 
 B 2 
 
AN ESSAY ON 
 
 single with two eyes, and others double. For 
 since, according to a second opinion maintained 
 by him, and not contradicted, I believe, by any 
 other writer upon vision, the two lines of direc- 
 tion, in which an object is seen when we em- 
 ploy-both eyes, can meet each other only in 
 one point, it follows, that all bodies which are 
 really situated in the plane of the horopter, 
 must necessarily appear single, as the lines of 
 direction in which any one of them is perceived 
 by the two eyes, coincide in that plane, and no 
 where else ; and that all bodies, which are not 
 situated in the plane of the horopter, must as 
 necessarily appear double, since, in this case, 
 the lines of their visible directions intersect 
 each other, either before or after they pass 
 through it*. 
 
 Against the truth of this explanation, only 
 one argument need be offered. Were the visi- 
 ble places of all bodies to be contained in the 
 plane of the horopter, these would appear of 
 magnitudes proportional to the angles which 
 they subtend at the eye. A finger, for instance, 
 held near to the face, would seem as large as 
 the part of a remote building it might conceal 
 from the sight. But as this is contrary to ex- 
 perience, the principle from which it is derived, 
 
 * Aguilonii Optica, p. 110, 148, 331, 344. 
 
SINGLE VISION. 
 
 must be rejected, together with all its conse- 
 quences. To Aguilonius, however, the merit 
 is due, of being the first who so far generalised 
 the phenomena of single and double vision, as 
 to observe, that those objects alone are seen 
 single, which are really situated in the plane of 
 the horopter. 
 
 The opinion of Dr. Smith is the next in the 
 order of time. * " If it be asked (says that 
 author) why, in seeing with both eyes, we do 
 not always see double, because of a double sensa- 
 tion, I think it is sufficient to say, that in the 
 ordinary use of our eyes, in which the pictures 
 of an object are constantly painted upon | corre- 
 sponding places of the retinas, the predominant 
 sense of feeling has originally and constantly 
 informed us that the object is single. By this 
 
 * Complete System of Optics. Vol. I. p. 48. 
 
 f Dr. Smith gives the following definition of corresponding 
 points. " When the optic axes are parallel, or meet in a 
 point, the two middle points of the retinas, or any points 
 which are equally distant from them, and lie on the same 
 sides of them, either towards the right hand or left hand, 
 or upwards or downwards, or in any oblique direction, are 
 called corresponding points." Vol. I. p. 46. According to 
 this definition, points correspond which have a certain agree- 
 ment in situation. No contradiction is, therefore, implied 
 in this system, by saying, that an object may appear single, 
 though its pictures should fall upon points which do not 
 correspond. Dr. Reid's definition of the same term is very 
 different. 
 
6 AN ESSAY ON 
 
 means our idea of its outward place is connected 
 with both those sensations, as is manifest by its 
 appearing in two places when its pictures are 
 not painted upon corresponding places of the 
 retinas; which is only a direct consequence 
 arising from our general habit of seeing." 
 Should any one now inquire whence it is, that, 
 to produce single vision, all men agree in di- 
 recting their eyes toward the object in such a 
 manner as to receive its pictures upon corre- 
 sponding points of the retinas, since custom 
 might have connected the sensations of any 
 other two points with the information of its 
 unity from feeling* : This answer maybe given 
 in the words of Dr. Smith t : " When we view 
 an object steadily, we have acquired a habit 
 of directing the optic axes to the point in 
 view; because its pictures falling upon the 
 middle points of the retinas, are then distincter 
 than if they fell upon any other places ; and 
 since the pictures of the whole object are equal 
 to one another, and are both inverted with re- 
 spect to the optic axes, it follows that the pic- 
 tures of any collateral point are painted upon 
 corresponding points of the retinas." 
 
 * This objection is made to Dr. Smith's theory by Dr. 
 Reid, who seems to have overlooked the answer. Reid's 
 Inquiry into the Human Mind, 8vo. p. 332. 
 
 t Vol. I. p. 46. 
 
SINGLE VISION. ' 
 
 Such is the solution which Dr. Smith has 
 given of this celebrated question, and such the 
 reply which his general account of vision fur- 
 nishes to one objection against it. But there 
 are others which, in my opinion, cannot be so 
 easily repelled. Before I offer these, however, 
 I beg leave to remark, that although it were 
 proved, as I think it maybe, that he is mistaken 
 in the fact of objects appearing single, when 
 their pictures fall upon the middle or other 
 corresponding points of the retinas, still the 
 truth of what is peculiar to him * of the solu- 
 tion he gives, might remain unshaken. Ob- 
 jects, it may be said, are constantly seen single 
 when we direct our eyes to them in a particular 
 manner. Their pictures must, consequently, 
 in every such case, fall upon the same places of 
 the retinas ; and whether these be correspond- 
 ing or not, the unity of the visible appearances 
 will be owing to the connexion, which has uni- 
 formly been observed between the sensations of 
 
 * Dr. Reid attributes to Bishop Berkeley the opinion, 
 that objects appear single to two eyes, from an experienced 
 connexion between particular sensations of sight, and the 
 informations of touch. But I no where find it mentioned in 
 the works of that author j and I even think it probable, that 
 he purposely avoided treating of the question, as he found 
 that the solution of it, which naturally flowed from his prin- 
 ciples of vision, was with difficulty to be reconciled to other 
 conclusions he had derived from the same source. 
 
AN ESSAY ON 
 
 those places, and the information from feeling, 
 that the objects which cause them are single. 
 What I shall say, therefore, upon his opinion, 
 will tend to show, that, admitting the fact re- 
 specting corresponding points to be true, his 
 explanation of it ought, however, to be rejected. 
 For, first, it may be observed, that, if we are 
 taught by feeling to see objects single, notwith- 
 standing a sensation in each eye, the informa- 
 tions of the former sense ought to be uniform, 
 or else one set of visual appearances would be 
 associated with different reports from feeling, 
 and no certain mark afforded us which of them 
 we should trust. Now Dr. Smith himself is 
 obliged to confess, that we sometimes feel dou- 
 ble, " as in the dark, when a button is pressed 
 with two opposite sides of two contiguous fin- 
 gers laid across ; for this reason, that those op- 
 posite sides of the fingers have never been used 
 to feel one but always' two things at a time*." 
 He adds, " We have learned, therefore, by ex- 
 perience of both senses compared together, to 
 make their informations consistent with each 
 other." Here, then, we find him to allow, that 
 
 * Vol. I. p. 48. Dr. Smith, however, has, from the in- 
 fluence of system, I suppose, mistaken this fact; for the 
 button is felt double when pressed in the manner above 
 mentioned, though we should not be in the dark, and should 
 even see it to be single. 
 
SINGLE VISION. 9 
 
 feeling is not always the predominant, but 
 sometimes the inferior sense ; that its informa- 
 tions are not constant and original, but change- 
 ful and derived ; positions directly contrary to 
 those he had immediately before maintained. 
 But, in the first instance of difference between 
 the informations of the two senses, what rule 
 had we for determining which .was the most 
 worthy of credit ? How does a blind man cor- 
 rect his errors of touch ? If the button be felt 
 double, because pressed by two parts not ac- 
 customed to feel the same thing at the same 
 time, there must have been a period in the life 
 of every person, when a body pressed by any 
 two parts would have been felt double, by three 
 parts triple, and so on. Nor could sight have 
 corrected those deceptions, if they can be called 
 such ; for every thing, by the same hypothesis, 
 must then have also been seen double. How 
 came we, therefore, both to feel and see things 
 single ? Surely not by comparing the informa- 
 tions of the two senses together. 
 
 But, secondly, were we to grant that the 
 sense of touch has originally and constantly 
 informed us that objects are single, it would 
 not follow, that we are thence taught to see 
 them also single. For, since the place which 
 an object seems to either eye to possess, mani- 
 festly depends both upon its apparent distance 
 
10 AN ESSAY ON 
 
 and its apparent direction from that eye, if 
 visible place be, in the language of Dr. Smith, 
 only an idea of real or tangible place, visible 
 direction must bear the same relation to tangi- 
 ble direction ; a consequence of which is, that 
 \ve can never have a more accurate knowledge 
 of the direction, in which an object may lie 
 from any part. of our bodies, by sight than by 
 touch. Facts, however, prove the contrary. 
 Let any person, for instance, taking a pin in 
 his hand, endeavour, without looking, to bring 
 its head upon a level with either of his eyes ; 
 and there are many chances to one but he will 
 fail in the attempt, of which sight will inform 
 him, when he turns his eye to the object. This 
 to me is a convincing argument, that external 
 bodies are not seen in certain directions, be- 
 cause they have been previously felt in them ; 
 and, consequently, that visible place, of which 
 visible direction is a component part, is not 
 merely a representative of the place perceived 
 by touch. But if the place, in which an object 
 appears to each eye separately, does not entirely 
 depend upon any lesson from feeling, the in- 
 ference is, that when an object appears in one 
 and the same place to both eyes together, nei- 
 ther is this effect to be attributed solely to the 
 informations of that sense. 
 
 Thirdly, in whatever direction an object may 
 
SINGLE VISION. 11 
 
 % 
 
 appear to either eye, it certainly cannot be seen 
 in the same place by both, except at some point 
 common to the two directions. Dr. Smith ac- 
 knowledges this, and says*, that when an ob- 
 ject is perceived single with both eyes, it is 
 seen at the mutual intersection of the two visual 
 rays ; the visible direction of any object coin- 
 ciding, according to him, with the visual ray, 
 or the principal ray of the pencil which flows 
 from it to the eye. Should we then even 
 allow, that all we know by sight of the places 
 of bodies has been borrowed from feeling, it 
 will still be easy to show, that the rule of vision 
 for each eye, which he has derived from such 
 experience, 'that of our seeing objects in the 
 directions of their visual rays, is inconsistent 
 with many of the phenomena of sight with two 
 eyes ; and, consequently, that he has left un- 
 removed the chief difficulty of his subject, which 
 was to explain the single appearance of objects 
 to both eyes, from those laws, or rules of vision, 
 which affect each of them singly. For it is a 
 well-known fact, that if two bodies of the same 
 shape, size, and colour, be placed, one in each 
 optic axis, they appear but as one body, pro- 
 vided they be at equal distances from the eyes. 
 Agreeably to the theory of our seeing objects 
 
 * Vol. II. Remarks, p. 86. 
 
12 AN ESSAY ON 
 
 in the direction of their visual rays, this cannot 
 happen, except the united body appear at the 
 intersection of the optic axes. Dr. Smith, ac- 
 cordingly*, maintains that it does. Now, in 
 the first place, I appeal to experiment for a 
 direct proof that it does not ; and, in the se- 
 cond, I observe, that, as the two bodies in the 
 optic axes appear as one, whether they be 
 situated within or beyond the concurrence of 
 those lines, and as a right line joining the 
 bodies, and extended both ways, appears at the 
 same time to the sight as a right line, it follows, 
 upon admitting the fact which I have denied, 
 that all objects in the plane of the optic axes 
 which are seen in one position and" state of'the 
 eyes, however near to us, or however remote 
 they may in reality be, must appear to be 
 equally distant, or rather in a line drawn through 
 the concourse of the optic axes, parallel to the 
 interval between the eyes, and named by opti- 
 cians the horopter. Again, if a right line be 
 made to pass through any part of the plane of 
 the optic axes, at right angles to it, the por- 
 tions above and below this plane are perceived 
 to be in the same right line with the point which 
 is situated in it, and the whole appears perpen- 
 dicular to the plane. But the point in the plane 
 
 * Vol. II. Remarks, p. 86. 
 
SINGLE VISION. 13 
 
 is seen, by the last article or proposition, in the 
 horopter ; the whole, therefore, of the perpen- 
 dicular line must be seen in a plane passing 
 through the horopter at right angles to that of 
 the optic axes ; or in other words, in the plane of 
 the horopter, in which consequently all bodies 
 will have their visible places. But this was the 
 very opinion of Aguilonius, to which he was 
 probably led by a similar train of reasoning ; 
 though, as a teacher, he might choose rather 
 to ground it immediately upon an original law 
 of our constitution. 
 
 It is probable, however, that Dr. Smith did 
 not perceive the conclusions which might be 
 drawn from' his doctrine of objects being seen 
 in the directions of their visual rays, since he 
 has no where spoken of them. At any rate, it 
 is manifest he did not admit them, as he has 
 mentioned the following circumstance as a 
 fact*, to which they cannot be reconciled; 
 that, when an object is seen double, both its 
 apparent places are situated between its real 
 place, and the mark at which we look. For if 
 this were just, together with what he has else- 
 where advanced, phenomena ought in many 
 cases to be observed, very different from those 
 which are in truth found to exist. Thus, for 
 
 * Vol. I. p. 48. 
 
14 AN ESSAY ON 
 
 example, if a right line be any where placed in 
 the plane of the optic axes, it follows, from 
 what he has said in one part of his book, that 
 those points of it, through which the axes pass, 
 must be seen united at the mark we look at, 
 the axes crossing each other there ; and from 
 what I have just quoted, that every other point 
 must be seen by each eye between its real place 
 and that mark. The appearances, therefore, of 
 all the points, if they do not lie disjoined, but 
 are connected together in some orderly manner, 
 will be arranged in the forms, either of two 
 curves, both passing through the intersection of 
 the optic axes, or of four right lines meeting 
 one another at that point. If the right line be 
 placed nearer to the face than the mark we 
 look at, the apparent lines, whether curved or 
 straight, will approach toward us from their 
 common point, but recede from us, if the real 
 line be situated beyond the mark. Such are 
 the phenomena which ought to follow upon the 
 admission of these two parts of Dr. Smith's 
 theory of vision with two eyes, but which are 
 not found to exist in nature. Aguilonius was 
 at least consistent when he maintained, that all 
 objects are seen in the plane of the horopter ; 
 while Dr. Smith, by deserting that opinion in 
 part, seems only to have involved himself the 
 more deeply in error. 
 
SINGLE VISION. 15 
 
 Having now said what, I hope, will be 
 thought sufficient to show, that the reason given 
 by Dr. Smith, for our seeing objects single with 
 both eyes, is neither grounded on well-attested 
 facts, nor adequate to the explanation of the 
 phenomena observed, I pass to the examination 
 of the opinion of Dr. Reid. 
 
 As this neither rests upon nor includes any 
 new fact in vision, I need only mention, in 
 order to give an account of it*, that its author 
 maintains with Dr. Smith, that an object is seen 
 in the same place with both eyes, and conse- 
 quently single, when its pictures fall upon the 
 centres of the retinas, or upon points in them, 
 which are similarly situated with respect to the 
 centres ; but differs from him in this, that he 
 makes the property to be original, by which 
 any two places in those membranes exhibit only 
 one object, while Dr. Smith derives it altoge- 
 ther from custom f. 
 
 In my examination of the opinion of Dr. 
 
 * Inquiry into the Human Mind, c. vi. sect. 13. 
 
 f They differ also with respect to the meaning of a term -, 
 Dr. Smith calling corresponding points, such as have the posi- 
 tion just mentioned, whether they represent objects single or 
 not j whereas Dr. Reid says, that those points correspond, 
 whatever their position may be, which represent objects 
 single 5 and he appears to me not always to attend to the 
 double use of the same term, when he speaks of the opinions 
 of Dr. Smith. 
 
AN ESSAY ON 
 
 Smith, I took occasion to remark, that the truth 
 of what distinguished it from all others might re- 
 main unshaken, though it were proved, that ob- 
 jects do not appear single, when their pictures 
 occupy any of the corresponding points of the 
 two retinas, since custom might have associated 
 the perceptions of touch, with the sensations of 
 any other parts whatsoever of those membranes. 
 The same observation will not apply with equal 
 justice to the opinion of Dr. Reid. On the 
 contrary, could it be shown, that the places of 
 the two retinas, which represent an object single 
 when each receives its picture, are not the cen- 
 tres, or such others as are similarly situated, an 
 obvious inference would be, that the single ap- 
 pearance of the object is not occasioned by a 
 property in those places, bestowed upon them 
 for this special purpose by nature ; it being 
 reasonable to expect, that such a property 
 should be found, if any where, in those parts of 
 the retinas which are the most like to each 
 other. I have, therefore, reserved till now, the 
 observations which have occurred to me upon 
 this subject, and which, when stated, must at 
 least, raise some doubt concerning what has 
 been regarded as true by Dr. Smith and Dr. 
 Reid, and by almost every other writer on 
 vision since the time of Kepler. 
 
 Anatomists have commonly taught, that the 
 
SINGLE VISION. 17 
 
 centres of the spheres, to which the cornea, the 
 ball of the eye, and the two portions of the 
 crystalline belong, are all placed in the same 
 right line, hence called the optic axis, and that 
 this being produced both ways, passes through 
 the centres of the cornea and retina, considered 
 as surfaces. Opticians, on their part, observe, 
 that an object appears single to both eyes, when 
 the axis of each is accurately directed to it; 
 from which they infer, that the centres of the 
 retinas agree in suggesting but one object, 
 though each receives its picture. Again ; since 
 it is known by experience, that, while any object 
 is seen single, to which the optic axes are turned, 
 others at the same distance from the eyes like- 
 wise appear so ; and since the pictures of these 
 lateral objects fall upon points in the two retinas, 
 equidistant from their centres, and both upon 
 the same side, that is, both to the right or left 
 of the centres, or both above or below them, 
 opticians conclude, that every two places of the 
 retinas, which are similarly situated with respect 
 to the centres, must also agree in exhibiting 
 but one object, though pictures are received by 
 both. 
 
 But the whole of this reasoning is built upon 
 a circumstance in the fabric of the eye, which 
 has been shown by some of the most eminent 
 
 c 
 
IS AN ESSAY ON 
 
 anatomists not to have place. For Varolius* 
 long ago observed, that the crystalline is not 
 situated in the middle of the eye, but more in- 
 wardly ; and the accurate Zinnt has more lately 
 mentioned, that if the eye be divided into a 
 right and left half, the centre of the crystalline 
 will be found in the inner portion. HallerJ 
 confirms this fact ; and Winslow's || observation, 
 that the centres of the pupil and iris do not 
 coincide, but that the former is 'nearer to the 
 nose than the latter, is connected with it ; since 
 both Zinn and Haller agree, that the centre of 
 the pupil is placed in the axis of the crystalline, 
 while that of the iris is evidently in the common 
 axis of the cornea and globe. Now, a conse- 
 quence of this position of the crystalline is, that, 
 contrary to what I believe is universally main- 
 tained, no ray of light whatsoever can pass un- 
 bent to the retina from the atmosphere, or any 
 other medium differing in refractive power from 
 the aqueous humour. If, then, the line joining 
 the centres of the cornea and globe of the eye 
 be what is called the optic axis, and if it be 
 true, that objects appear single when we direct 
 
 * Varolii Anatomia, 12mo. p. 16. 
 
 f De Oculo, 4to. p. 127. 
 
 + Elementa Physiologiae, tozn. v. p. 403. 
 
 J| Winslow's Anatomy, vol. ii. p. 379, English edition* 8vo. 
 
SINGLE VISION. 19 
 
 both these axes to them, it must be evident, to 
 such as are acquainted with the common rules 
 of optics, that the pictures of those objects do 
 not fall upon the centres of the retinas, but 
 more internally ; and, therefore, that the centres 
 and all the other points of those membranes, 
 which by the present system are supposed to 
 represent objects single, do in fact exhibit them 
 double. 
 
 It will be said here, perhaps, that the line* 
 passing from each eye, which we turn to objects 
 when we see them single, is not a production of 
 the common axis of the cornea and globe, but 
 some other, disposed in such a manner, that the 
 pictures of those objects are received by the 
 centres of the retinas. I answer; I readily 
 grant the possibility of the thing, but I assert, 
 at the same time, that we have no proof of it, 
 
 - 
 
 * I am of opinion, that this line, or at least the line which 
 we turn to objects when we see them most distinctly with 
 one eye, is not the common axis of the globe and cornea. 
 For I find, that, when I plac j the flame of a candle between 
 either of my eyes, and a plane mirror, in such a manner that 
 it may conceal its own image in the mirror from the sight of 
 that eye, or rather that it may be a little below this image, 
 but in the same vertical plane with it, the image of the flame, 
 seen by reflection from the cornea, does not appear upon the 
 middle point of this coat, but upon that point of it which is 
 opposite to the centre of the pupil. 
 
 C 2 
 
20 AN ESSAY ON 
 
 which is a sufficient reason for rejecting every 
 conclusion that depends upon its truth. 
 
 Admitting, however, that objects are repre- 
 sented single, when their pictures fall upon the 
 centres of the retinas, or upon any other two 
 points which are equally distant from the cen- 
 tres, and both upon the same side, it appears to 
 me, notwithstanding, to be in violation of all 
 analogy, to ascribe this effect, with respect to 
 the points, at least, on the right and left sides 
 of the centres, to any peculiar property which 
 they possess from nature. For when anatomists 
 find, in a new species of animals, organs similar 
 in structure to those of others they are already 
 acquainted with, they immediately conclude, 
 that they are also similar in regard to their use. 
 In animals of the same species, they believe 
 with certainty, that the organs they see in one 
 have the same properties, as the corresponding 
 organs of another ; and, if it be possible, they 
 attribute with greater certainty the same pro- 
 perties to two organs of the like kind, which 
 are found in the same individual. Such is the 
 influence of the rule, that resemblance of pro- 
 perty is implied by resemblance of structure. 
 Now it is an universal fact, that if an animal 
 be divided into a right and left half, the cor- 
 responding parts of those organs, which exist in 
 pairs, are found at equal distances from the plaae 
 
SINGLE VISION. 21 
 
 of partition. Thus, for instance, in respect to 
 the eyes, the two optic nerves penetrate their 
 outward coat at the same distance from this 
 plane. Their muscles, blood-vessels, and every 
 other of their component parts and appendages, 
 are arranged in the like manner ; those nearest 
 to the dividing plane, or the innermost, in the 
 one, being similar in structure to the innermost 
 in the other, the outermost to the outermost, 
 and the intermediate to the intermediate. It 
 is surely, therefore, natural to expect, that such 
 parts should also be similar in their properties ; 
 and we in fact find this similarity to exist, where- 
 ever it can be clearly ascertained what the pro- 
 perties are. Every person, for example, admits, 
 that the internal straight muscle of the right 
 eye performs the same office, with respect to 
 that eye, as the other internal straight muscle 
 does with respect to the left eye. What judg- 
 ment are we then to form of the opinion of Dr. 
 Reid, which attributes the same original pro- 
 perties, or rather the joint possession of one ori- 
 ginal property, to places in the retinas situated 
 at unequal distances from the general plane of 
 partition ; which makes an external point in one 
 to correspond, in use, with an internal point in 
 the other, and this too by a principle implanted 
 by nature ? If such things exist, they may,, at 
 
22 AN ESSAY ON 
 
 least, be said to stand opposed to a most ex- 
 tensive analogy. 
 
 To these arguments, a priori, against the 
 opinion of Dr. Reid, I shall now add others, 
 which are derived from a consideration of its 
 consequences. 
 
 First ; Since visible place, as was formerly 
 observed, includes in it visible distance, it is 
 evident that, if both eyes, by virtue of an ori- 
 ginal property, see an object in the same place, 
 distance must also be originally perceivable by 
 sight. Dr. Reid*, however, has himself so 
 ably shown, that we should never have ac- 
 quired, by means of our eyes, any knowledge of 
 distance, unless they had been assisted by the 
 sense of feeling, that I forbear to say any thing 
 more upon this head, than that the existence of 
 no property can be admitted, which leads to the 
 conclusion I have stated. 
 
 Secondly ; If distance be not immediately 
 perceivable by sight, the only manner, in which 
 an original property of the eyes can affect the 
 visible places of bodies, is by occasioning them 
 to appear in certain directions. Now Dr. Reid 
 maintains t, that every external point is seen in 
 
 * Inquiry into the Human Mind, chap. vi. sect. 3 & 20. 
 f Ibid. chap. vi. sect. 12. 
 
SINGLE VISION. 23 
 
 the direction of a line passing from its picture 
 on the retina, through the centre of the eye. 
 If, therefore, this direction be the same as that 
 suggested by the original property so often men- 
 tioned, the latter law is merely another ex- 
 pression for the former, and ought to be rejected 
 as superfluous. If it be different, and should 
 the two laws exist together, objects seen with 
 both eyes might sometimes appear quadruple, 
 sometimes triple, but never single. Were they 
 to exist successively, one when we employ one 
 eye, the other when both, an object, though at 
 rest, should always appear to move when viewed 
 alternately by one and by both eyes ; neither of 
 which conclusions is agreeable to experience. 
 
 Thirdly ; To show in a different way, and one 
 perhaps more easily understood, that the opinion 
 of Dr. Reid is not consistent with the pheno- 
 mena of vision it ought to explain, I shall sup- 
 pose an experiment to be made upon a person 
 who squints. But I must premise, that it ap- 
 pears, from the observations of Dr. Jurin* and 
 himselff, that all such persons have one eye of 
 a weaker sight than the other ; that when both 
 eyes are open, the weaker is turned away from 
 objects, which are attentively viewed \ but that 
 
 * Smith's Optics, Vol. II. Remarks, p. 3O 
 f Inquiry, chap. vi. sect. 10. 
 
24 AN ESSAY ON 
 
 when the strong eye is closed, the weaker is 
 pointed to objects,, exactly as the former would 
 be in the same situation ; and that it likewise 
 perceives them in similar directions. Let now 
 the ordinary position of the person's eyes, upon 
 whom the experiment is made, be such, that 
 the optic axes intersect each other about an 
 inch or two from the face ; and while the other 
 is closed, let the flame of a candle be placed in 
 the axis of the weak eye, which I shall call the 
 left, at the distance of some feet from it, and on 
 the right side of the body. The flame will con- 
 sequently appear in the same direction, as if his 
 eye had no fault, and will be seen on his right, 
 where it is in reality situated. Both eyes re- 
 taining the same position with respect to his 
 head and each other, let the weak eye be after- 
 ward shut, and the right opened, and let another 
 object be placed in the axis of the latter, an 
 opake body being at the same time so disposed, 
 as to hide from it the candle which is in the 
 axis of the left eye. This object in the right 
 axis will consequently appear on the left side* 
 Now, since the two objects, which have been 
 thus viewed separately, are situated, one to the 
 right, and one to the left ; and since they have 
 been also seen in those positions, their visible 
 places must be two, as well as their tangible, 
 and must be remote from each other. How 
 
SINGLE VISION. 25 
 
 then should these objects appear, if, instead of 
 being viewed alternately, each by the eye in 
 the axis of which it is placed, they were seen 
 by the two together ; the positions and internal 
 states of the eyes being in both cases the same ? 
 Dr. Reid must answer ; They will possess but 
 one visible place, since their pictures fall upon 
 the centres of the two retinas, points endowed 
 with the original property of representing ob- 
 jects single. But where is this one place to be 
 found ? In the axis of the right eye, or in that 
 of the left, or between the two ? In any of these 
 cases, or in any other that can be imagined, the 
 law of visible direction, so much insisted upon 
 by Dr. Reid, that objects appear in the per- 
 pendiculars to their pictures upon the retina, 
 and in truth every other law of visible direction 
 hitherto published, must be suspended with re- 
 spect to one or both eyes ; unless, indeed, the 
 united object be referred to the intersection of 
 the optic axes, about an inch or two from the 
 face. This, I believe, Dr. Reid would not 
 readily admit ; but if he should, another case 
 of squinting may be imagined, in which the 
 optic axes recede from each other, and where 
 the same reasoning will apply without the pos- 
 sibility of its force being thus eluded. It now 
 remains for me to mention, that the experiment 
 here stated by the way of supposition, in which 
 
26 AN ESSAY ON- 
 
 the optic axes cross each other near to the fare, 
 was actually made by Dr.. Reid, with this result, 
 that the two objects appeared in different places, 
 when seen by both eyes together ; and that the 
 other experiment, in which the optic axes are 
 supposed to diverge, was made by myself, with 
 a similar event. Dr. Reid, however, instead of 
 being led, by the termination of his experiment, 
 to impute a fault to the principle from which 
 he had expected a different one, concluded from 
 it, that there was something unnatural, beside 
 the squinting, in the person's eyes, upon whom 
 it was made ; though it had been previously 
 ascertained, that objects appeared in the ordi- 
 nary manner to each of them, when separately 
 employed. 
 
 My examination of the second class of opi- 
 nions, respecting the cause of the single appear- 
 ance of objects to two eyes, being finished, some 
 person, perhaps, will now say ; Granting that no 
 error can, at first sight, be shown in your argu- 
 ments against those of Dr. Smith and Dr. Reid, 
 is it not a sufficient reason for believing them 
 fallacious, that they prove too much ? If objects 
 appear single neither from custom, nor an ori- 
 ginal property of the eyes, have we not an effect 
 without a cause, and must there not be some- 
 thing wrong in the facts or reasoning which lead 
 to such a conclusion ? The answer I make is 
 
SINGLE VISION. 27 
 
 as follows : Since visible place contains in it 
 both visible distance and visible direction, it is 
 not necessary that the single appearance of an 
 object, to both eyes, should depend altogether 
 either upon custom, or an original principle of 
 our constitution ; for its visible distance to each 
 eye may be learned from feeling, and its visible 
 direction be given by nature ; in which case, 
 the unity of its place to the two eyes, will be 
 owing to neither of those causes singly, but to 
 a combination of both ; and this I regard as a 
 sufficient reply. 
 

 PART II. 
 
 Of a new Theory respecting Visible Direction, and of' a 
 Solution hence derived of the Question, why Objects are 
 seen single with two Eyes. 
 
 1 NOW proceed to offer a new opinion, why 
 objects are seen single with two eyes ; or in 
 other words, why they appear in the same place 
 to both, this being the light in which I view the 
 fact to be explained. 
 
 In every part of natural philosophy, accidents 
 often lead to discoveries, which reason alone 
 might not easily have reached. Under this 
 cover I hope to shelter myself from the charge 
 of presumption, in venturing to give the solu- 
 tion of a problem, upon which the talents of 
 many persons of great learning and "genius have 
 been unsuccessfully employed ; for should I 
 prove more fortunate than such men have been, 
 this must be attributed to the knowledge of a 
 circumstance I observed by chance, in repeating 
 some very common experiments. 
 
 The visible place of an object being com- 
 posed, as I have already several times remarked, 
 of its visible distance and visible direction, to 
 show how it may appear the same to both eyes, 
 it will be necessary to explain, in what manner 
 
AN ESSAY O1ST SINGLE VISION. 29 
 
 th distance and direction, which are perceived 
 by one eye, may coincide with those which are 
 perceived by the other : and first with respect 
 to the distance. 
 
 In judging of distance by sight, we frequently 
 make considerable mistakes, even when the ob- 
 jects are not very remote ; but no person, I 
 believe, has ever observed, that while an object 
 seemed to one of his eyes at a certain distance, 
 it has appeared to the other to be at a different 
 distance, and from this circumstance alone has 
 been seen double ; or, to express the same thing 
 in another way, that while the visible appear- 
 ance of an object to one eye, covered the visible 
 appearance of the same object to the other eye, 
 the two appearances did not seem entirely to 
 coincide, and make one, but were seen separate 
 by the two eyes. I do not stop to give the rea- 
 son of this fact, which must be plain to those who 
 are acquainted with Bishop Berkeley's theory of 
 visible distance ; but proceed to mention, that 
 the difficulty in finding a true and sufficient 
 cause for the union of the two visible places of 
 one or two objects to two eyes, must therefore 
 consist altogether in showing, in what manner 
 the two apparent directions may coincide, con- 
 sistently with the attending phenomena. 
 
 Since Kepler's great discovery of the seat and 
 manner of vision, there have been, as far as I 
 
30 AN ESSAY ON 
 
 know, only two theories offered respecting the 
 apparent directions of objects. One is, that 
 they are perceived in the direction of lines pass- 
 ing from their pictures on the retina, through 
 the centre of the eye ; the other, that their 
 apparent directions coincide with their visual 
 rays *. But both of these theories are inconsist- 
 ent with the phenomena of single vision with 
 two eyes. For according to neither of them can 
 an object, placed at the concourse of the optic 
 axes, be seen single, unless we have a most ac- 
 curate knowledge of its distance ; nor will either 
 admit two objects to be seen as one, which are 
 situated in the optic axes, whether on this side, 
 or beyond where they meet, unless the united 
 object be referred by sight to their very point 
 of intersection ; both of which conclusions are 
 contradicted by experience. It is evident, 
 therefore, that some other theory of visible di- 
 rection is required, which shall not be liable to 
 
 * Mr. D'Alembert has said (Opuscules Mathematiques, 
 Tom. I. p. 265) that all optical writers before him had re- 
 garded it as an axiom, that every visual point is seen in the 
 direction of its visual ray. But the assertion is not well 
 founded. For Kepler long ago taught (Paralipomena in 
 Vitellionem, p. 173), that objects are perceived in lines pass- 
 ing from their pictures upon the retina, through the centre of 
 the eye j in which he was followed by Dechales and Doctor 
 Porterfield ; to the latter of whom Dr. Reid improperly at- 
 tributes the discovery of the same supposed law. 
 
SINGLE VISION. 31 
 
 "these objections ; and such a theory, I hope, I 
 shall bring forward in the following proposi- 
 tions, after mentioning the meanings which I 
 &ffix to several terms I shall frequently employ. 
 
 EXPLANATION OF TERMS. 
 
 I. When a small object is so placed with re- 
 spect to either eye, as to be seen more distinctly 
 than in any other situation, I say it is then in 
 the optic axis, or the axis of that eye ; and if 
 another small body be interposed between the 
 former and the eye, so as to conceal it, and if 
 a line joining the two be produced till it falls 
 upon the cornea, I call this line the optic axis> 
 or the axis of the eye ; leaving for future de- 
 termination the precise point of the cornea it 
 falls upon, or what part of the retina receives 
 the picture of an object which is placed in it. 
 
 II. When the two optic axes are directed to 
 a small object not very distant, they may be 
 conceived to form two sides of a triangle, the 
 base of which is the interval between the points 
 of the corneas, where the axes enter the eyes ; 
 but if the object be very distant, then they may 
 be supposed to be two sides of a parallelogram, 
 whose base is the same interval. To avoid cir- 
 cumlocution, I shall call this interval the visual 
 base. 
 
 III. If there be drawn a line from the middle 
 
32 AN ESSAY ON 
 
 of the visual base, through the point of inter- 
 section of the optic axes, or parallel to them, if 
 they be parallel to each other, I name it the 
 common axis*. This term, I believe, was in- 
 vented by Alhazen ; but with him it signified a 
 line drawn from the centre of the junction of 
 the optic nerves, through the middle of the in- 
 terval between the centres of the retinas. Such 
 a line was consequently immoveable. As the 
 term, however, is not in modern use, no mis- 
 take can arise from confounding the two mean- 
 ings, and the reason will soon be seen, why I 
 employ it in the sense I have mentioned. Those 
 who are acquainted with the writings of the 
 older opticians, will perceive, that I give it 
 nearly the same signification as they did to their 
 common radius. 
 
 * It may be said, perhaps, that as I do not define the 
 points of the corneas, upon which the optic axes fall, I can- 
 not, with propriety, desire the line which connects them to be 
 divided. To this I answer, that it is not necessary for the 
 purpose I have mentioned, that they should be defined ; if it 
 be granted to me, and I think it cannot be refused, that upon 
 whatever point of the right cornea the right axis falls, the 
 left axis will fall upon a similarly situated point of the left 
 cornea ; that is, if this point of the right cornea be at any 
 given distance from its middle, and upon the inside of it, the 
 corresponding point of the left cornea will be at the same 
 distance from the middle of this, and also upon its inside. 
 Whatever extent, therefore, the line connecting these places 
 of the corneas may have, its middle point will be the same. 
 
SINGLE VISION. 
 
 PROPOSITION I. 
 
 Objects situated in the Optic Axis, do not appear to be in 
 that Line, but in the Common Axis. 
 
 EVERY person knows, that, if an object be 
 viewed through two small holes, one applied to 
 each eye, the two holes appear but as one. The 
 theories hitherto invented afford two explana- 
 tions of this fact. According to Aguilonius, 
 Dechales, Dr. Porterfield and Dr. Smith, the 
 two holes, or rather their borders, will be seen 
 in the same place as the object viewed through 
 them, and will consequently appear united, for 
 the same reason, that the object itself is seen 
 single. But whoever makes the experiment 
 will distinctly perceive, that the united hole is 
 much nearer to him than the object ; not to 
 mention, that any fallacy on this head might be 
 corrected by the information from the sense of 
 touch, that the card, or other substance, in 
 which the holes have been made, is within an 
 inch or less of our face. The other explana- 
 tion is that furnished by the theory of Dr. Reid. 
 According to it, the centres of the retinas, which 
 in this experiment receive the pictures of the 
 holes, will, by an original property, represent 
 but one. This theory, however, though it 
 
 D 
 
34 AN ESSAY ON 
 
 makes the two holes to appear one, does not 
 determine where this one is to be seen. It 
 cannot be seen in only one of the perpendicu- 
 lars to the images upon the retinas, for no reason 
 can be given why this law of visible direction, 
 which Dr. Reid thinks established beyond dis- 
 pute, if it operates at all, should not operate 
 upon both eyes at the same time ; and if it be 
 seen by both eyes in such lines, it must appear 
 where those lines cross each other, that is, in 
 the same place with the object viewed through 
 the holes, which, as I have already mentioned, 
 is contrary to experience. Nor is it seen in any 
 direction, the consequence of a law affecting 
 both eyes considered as one organ, but sus- 
 pended when each eye is used separately. For 
 when the two holes appear one, if we pay at- 
 tention to its situation, and then close one eye, 
 the truly single hole will be seen by the eye re- 
 maining open, in exactly the same direction as 
 the apparently single hole was by both eyes. 
 
 Hitherto I have supposed the holes almost 
 touching the face. But they have the same 
 unity of appearance, in whatever parts of the 
 optic axes they are placed ; whether both be 
 at the same distance from the eyes, or one be 
 close to the eye in the axis of which it is, and 
 the other almost contiguous to the object seen 
 through them. If a line, therefore, be drawn 
 
SINGLE VISION. 35 
 
 from the object to one of the eyes, it will re- 
 present all the real or tangible positions of the 
 hole, which allow the object to be seen by that 
 eye, and the whole of it will coincide with the 
 optic axis. Let a similar line be drawn to the 
 other eye, and the two must appear but as one 
 line ; for if they do not, the two holes in the 
 optic axes will not, at every distance, appear 
 one, whereas experiments prove that they do. 
 This united line will, therefore r represent the 
 visible direction of every object situated in 
 either of the optic axes. But the end of it, 
 which is toward the face, is seen by the right 
 eye to the left, and by the left eye as much to 
 the right. It must be seen then in the middle 
 between the two, and, consequently, in the 
 common axis. And as its other extremity coin- 
 cides with the point where the optic axes inter- 
 sect each other, the whole of it must lie in the 
 common axis. Hence the truth of the proposi- 
 tion is evident, that objects, situated in the optic 
 axis, do not appear to be in that line, but in the 
 common axis. 
 
 Many other experiments might be mentioned 
 which demonstrate the same thing. If, for ex- 
 ample, the head of a pin, or of a needle, be 
 interposed between each eye, and any small 
 object to which both the optic axes are directed, 
 
 D 2 
 
36 AN ESSAY ON 
 
 the heads of the two pins or needles will con~ 
 stantly appear as one in the common axis. 
 When the heads, however, are near to the eyes, 
 this experiment is not so satisfactory as the 
 former, since, in these positions, they seem as 
 broad transparent shadows, for reasons known 
 to every person a little conversant in optics; 
 whereas the holes appear well defined, though 
 almost touching us. Again ; if we hold two 
 thin rulers in such a manner, that their sharp 
 edges shall be in the optic axes, one in each, or 
 rather a little below them, the two edges will 
 be seen united in the common axis, and this 
 apparent edge will seem of the same length with 
 that of either of the real edges, when seen alone 
 by the eye in the axis of which it is placed. If 
 instead of two rulers we employ two strings of 
 different colours, as red and green, the like unity 
 of appearance will be observed. But in this 
 experiment it frequently happens, that, contrary 
 to what we might naturally expect, only one of 
 the strings is seen at a time. When, however, 
 only one is seen, its apparent situation is exactly 
 the same as that of the string, compounded, if 
 I may so express myself, of the two when seen 
 together; and hence we have a convincing 
 proof, if any were wanted, that the single ap- 
 pearances of objects must depend upon some 
 
SINGLE VISION. 37 
 
 law of visible direction affecting each eye, when 
 employed by itself, in the same manner as when 
 it is used conjointly with the other*. 
 
 * Du Tour expected, that if two objects of different colours 
 were seen in the same place by both eyes, which however he 
 says, he was never able to observe, the colour of the ap- 
 parently united object would be compounded of those of the 
 two really single objects. Memoires des Savans Etrangers, 
 torn. iv. p. 500. And Dr. Reid mentions expressly that it is 
 so compounded. Inquiry, p. 2Q3. But in all my experi- 
 ments upon this subject I have remarked, that, when the two 
 objects appeared united, each was seen, notwithstanding, in 
 its proper colour ; the red, for example, appearing as it were 
 through a transparent green, and the green, in the same ex- 
 periment, as through a transparent red. Nor is there any 
 thing in this inconsistent with the received doctrine of the 
 composition of colours. For in every instance of the pro- 
 duction of a new colour, from rays of different colours being 
 at the same time sent to the eye, these rays fall upon the 
 same sentient extremities of the same nerve. But, in the 
 case before us, the differently-coloured rays fall upon the 
 sentient extremities of two different nerves, which have no 
 communication with each other, except through the medium 
 of the brain. We have greater reason, therefore, for expect- 
 ing, that the colours impressed upon the two eyes, should be 
 perceived uncompounded, than there is for two colours being 
 perceived separately, which are impressed upon two different 
 parts of the same eye. 
 
 From the fact of the two colours being thus perceived di- 
 stinct from each other, I would infer, by analogy, a mode of 
 argument indeed often fallacious, that if it were possible for 
 <as to hear any one sound with one ear only, and another 
 sound with the other ear only, such sounds would in no case 
 
38 AN ESSAY ON 
 
 PROPOSITION II. 
 
 Objects, situated in the Common Axis, do not appear to 
 be in that Line, but in the Axis of the Eye, by which 
 they are not seen. 
 
 THE facts which demonstrate the truth of 
 this proposition, are both numerous and com- 
 mon. If a piece of wire, or any other sub- 
 stance, representing a physical line, be placed 
 in the common axis, with one of its extremities 
 near to the visual base, and if both the optic 
 axes be directed to its farther or distant ex- 
 tremity, instead of one, two wires will be seen, 
 meeting each other at their farther ends, and 
 gradually diverging as they approach the face, 
 till they apparently terminate at the eyes. If 
 the right eye be closed, the wire which seemed 
 
 coalesce either wholly or in part, as two sounds frequently do, 
 when heard at the same time by one ear 5 that consequently, 
 if the sounds of one musical instrument were to be heard by 
 one ear only, and those of another, by the other ear only, we 
 could have little or no perception of harmony from such 
 sounds -j and that, if in any succession of sounds emitted by 
 one instrument, we were to hear the 1st, 3d, 5th, and so on, 
 by one ear only, and the 2d, 4th, 6th, and so on, by the other 
 ear only, we should be deprived, in a considerable degree, of 
 the melody of such sounds, as this seems to depend in a great 
 measure upon a new impression being made upon the audi- 
 tory nerve by one sound, before the impression of the sound 
 immediately preceding has passed away. 
 
SINGLE VISION. 39 
 
 to terminate at the left eye, disappears ; and if 
 the left eye be closed, then the other wire dis- 
 appears, whose termination was at the right 
 eye. The real wire, therefore, in the common 
 axis, appears to the right eye to be situated in 
 the axis of the left, and to the left eye to be 
 situated in the axis of the right, agreeably to 
 what the proposition asserts. 
 
 The following experiments will illustrate and 
 confirm both this arid the preceding proposition. 
 Through a piece of card, or pasteboard, let two 
 small holes be made, the interval between which 
 is such, that while a very remote object is seen 
 through one of them by the right eye, the same 
 object may be seen through the other by the 
 left eye. Make afterward another hole in the 
 card, or pasteboard, exactly in the middle be- 
 tween the two former ; and let the object be 
 viewed through them as before. These, or 
 the outer holes, will now appear one, precisely 
 where the sense of feeling indicates the middle 
 hole to be ; while the middle hole will appear 
 as two, which seemingly occupy the places of 
 the real outer ones. The two appearances of 
 the middle hole, which is placed by construction 
 in the common axis, are therefore seen in the 
 optic axes ; and as the left is not seen when the 
 right eye is shut, nor the right when the left 
 eye is shut, each appearance is observed in the 
 
40 AN ESSAY ON 
 
 axis of the eye, by which it is not seen. As I 
 have supposed the distance between the outer 
 holes to be adapted to the interval of the eyes 
 when they are directed to a very remote object, 
 the optic axes may, in this case, be regarded as 
 parallel to each other. The object, therefore, 
 will still be seen through those holes, though 
 the distance of the card from the eyes be con- 
 siderably varied ; and at all the different di- 
 stances, the same appearances will be observed, 
 as those which have been mentioned. 
 
 Again ; take three strings of different colours, 
 as red, yellow, and'green, and fasten, by means 
 of a pin, one end of each to the same point of a 
 table. Place now their loose ends in such a 
 manner, that when you look at the pin with 
 both eyes, the visual base being parallel to the 
 edge of the table, the red string may lie in the 
 axis of the right eye, the green in that of the 
 left, and the yellow in the common axis. When 
 things are thus disposed, and both eyes are di- 
 rected to the pin, the red and green strings, in- 
 stead of appearing separate, each in one of the 
 optic axes, and inclined to the visual base or 
 edge of the table, will now be seen occupying 
 but one place, either together or successively, 
 as was formerly mentioned, and at right angles 
 to the visual base, or edge of the table ; in short, 
 exactly in the situation, which the yellow string 
 
SINGLE VISION. 41 
 
 in reality possesses ; and the yellow string, in- 
 stead of appearing single in the common axis, 
 and perpendicular to the visual base, will now 
 be seen as two, each inclined to the base ; that 
 seen by the right eye, apparently occupying the 
 place in reality possessed by the green string, 
 and that seen by the left eye, the place of the 
 red string. 
 
 PROPOSITION III. 
 
 Objects, situated in any Line drawn through the mutual 
 Intersection of the Optic Axes to the Visual Base, do 
 not appear to be in that Line, but in another, drawn 
 through the same Intersection, to a Point in the Visual 
 Base distant half this Base from the similar Extremity 
 of the former Line, towards the left, if the Objects be 
 seen by the Right Eye, but towards the right, if seen 
 by the Left Eye. 
 
 Two cases of this proposition have already 
 been proved. For it has been shown by the 
 first proposition, that objects, placed in the axis 
 of either eye, appear to it to be situated in the 
 common axis. But the common axis is a line 
 drawn through the mutual intersection of the 
 optic axes to the visual base, and its termina- 
 tion there is distant, by construction, half that 
 base, from the similar terminations of the axes 
 
42 AN ESSAY ON 
 
 of both eyes, to the left of the right axis, and to 
 the right of the left. Again, it has been shown 
 by the second proposition, that objects, placed 
 in the common axis, appear to each eye to be 
 situated in the axis of the other ; and the ter- 
 minations of both optic axes, at the visual base, 
 are distant half this base, from the similar ter- 
 mination of the common axis, the left being to 
 its right, and the right to its left. 
 
 Let it now be supposed that two objects, one 
 placed in the axis of either eye, the right for 
 instance, and the other in the common axis, be 
 viewed at the same lime by that eye, it is evi- 
 dent that the visible directions of both will be 
 equally removed to the left, from their real posi- 
 tions. But such an alteration of visible direc- 
 tion, from real position, cannot be imagined to 
 happen, with respect to objects placed in the 
 optic and common axes, unless a similar effect 
 be, at the same time, produced upon such as are 
 situated any where between those lines, or in 
 their vicinity. Facts confirm this : If a line, for 
 example, be drawn through the intersection of 
 the optic axes to a point in the visual base, 
 exactly in the middle between the terminations 
 there of the right and common axes, its ap- 
 parent situation, to the right eye, will be found 
 to have the same relation to the apparent situa- 
 tions of lines placed in the right and common 
 
SINGLE VISION. 43 
 
 axes, as its real situation has to the real situa- 
 tions of such lines. And the like will be found, 
 by observation, to be true of every other line, 
 which may be drawn through the point of inter- 
 section of the optic axes to the visual base. 
 
 The whole of what has here been said may be 
 illustrated and confirmed, by having again re- 
 course to the experiments with strings of dif- 
 ferent colours. In formerly describing those 
 experiments, I did not mention all the appear- 
 ances which occurred upon making them, hut 
 only such, as had immediate reference to the 
 points then under consideration. When, for 
 instance, a red string was placed in the axis of 
 the right eye, and a green one in that of the 
 left, I said that they both appeared in the com- 
 rnon axis. But this is not the only phenomenon 
 to be observed with respect to their apparent 
 number in this experiment. For as the red 
 string is also seen by the left eye, and the green 
 by the right, two other strings become visible, 
 beside that in the common axis, the apparent 
 positions of both of which will be found to be 
 the same with those, which ought to follow from 
 the present proposition. iShould now a yellow 
 string be placed between the two former, as in 
 the proof of the second proposition, its appear- 
 ance to the right eye will bisect the space be- 
 tween the appearances of the red and green 
 
44 AN ESSAY ON 
 
 strings to that eye ; and the like will be true 
 with respect to the appearances of the three 
 strings to the left eye, agreeably to what the 
 same proposition teaches us to expect. 
 
 I believe I need scarcely remark, that, al- 
 though in most of the proofs and illustrations 
 of these propositions, I have confined myself to 
 the visible appearances of lines between the in- 
 tersection of the optic axes and the visual base, 
 the same things, however, must be equally true 
 of those lines, when they are produced beyond 
 the intersection, with this difference only, that, 
 while the portions within, seem, to the right eye, 
 to be farther situated to the left than they really 
 are, but to the left eye farther to the right, the 
 portions beyond the intersection will seem to 
 the right eye to the right of their real positions, 
 but to the left eye to the left of them. For it is 
 manifest, that, if a line be seen by one eye in a 
 certain direction, a prolongation of it must be 
 seen in the same direction ; and that, if a line 
 be made to turn upon any point in itself, the 
 two extremities must move contrary ways. 
 
 Should the optic axes be parallel to each 
 other, the same proofs and illustrations will still 
 apply, since we may here suppose them to meet 
 at an infinite distance from the visual base. In 
 this case, the visible appearances of lines, drawn 
 from this supposed point of intersection to the 
 
SINGLE VISION. 45 
 
 visual base, will be parallel to the real lines, and 
 distant half this base from them, through their 
 whole extent. 
 
 As I have thus, I think, sufficiently proved, 
 that the apparent directions of objects are go- 
 verned by a law, different from any which has 
 hitherto been thought to exist, I shall now pro- 
 ceed to state, in a few words, in what manner 
 the phenomena of single and double vision with 
 two eyes are dependant upon it. 
 
 I formerly mentioned, that, since an object is 
 never seen double, merely from its being seen 
 at different distances by the two eyes, the only 
 difficulty in explaining its single appearance 
 consists in showing how its two visible direc- 
 tions may coincide, consistently with the attend- 
 ing phenomena. But we are enabled to do this, 
 with the utmost ease, by the theory 1 have en- 
 deavoured to establish. For, if the question be 
 concerning an object at the concourse of the 
 optic axes, I say it is seen single, because its two 
 similar appearances, in regard to size, shape, 
 and colour, are seen by both eyes in one and 
 the same direction, or, if you will, in two direc- 
 tions, which coincide with each other through 
 the whole of their extent. It therefore matters 
 
46 AN ESSAY ON 
 
 not, whether the distance be truly or falsely 
 estimated ; whether the object be thought to 
 touch our eyes, or to be infinitely remote. And 
 hence we have a reason, which no other theory 
 of visible direction affords, why objects appeared 
 single to the young gentleman mentioned by 
 Mr. Cheselden, immediately after his being 
 couched, and before he could have learned to 
 judge of distance by sight. 
 
 When two similar objects are placed in the 
 optic axes, one in each, at equal distances from 
 the eyes, they will appear in the same place, 
 and therefore one, for the same reason that a 
 truly single object, in the concourse of the optic 
 axes, is seen single. Here again, as the two 
 visible directions coincide in every point, it is 
 not necessary that the united appearance should 
 be judged to be at any particular distance ; that 
 it should be referred, for instance, to the con- 
 course of the optic axes, where the two other 
 theories of visible direction are obliged to place 
 it, in opposition to the plainest observations. 
 
 Objects, any where in the horopter, will be 
 seen single, because their apparent directions to 
 the two eyes will then completely coincide. 
 And for a contrary reason, those placed in any 
 other part of the plane of the optic axes will 
 appear double. To make these things evident, 
 let a line pass through the point of intersection 
 
SINGLE VISION. 47 
 
 of the optic axes and any given object, to the 
 visual base, which is to be produced, if neces- 
 sary ; and let it be called the line of the ob- 
 ject's real position. Take afterward, in the 
 visual base, or its production, two points, one 
 on each side of the line of real position, and 
 both distant from its termination there, half 
 the visual base. Lines drawn from these points, 
 through the point of intersection of the optic 
 axes, must consequently contain the two visible 
 positions of the object. But when this is 
 situated in the horopter, the line of real posi- 
 tion will coincide with the horopter, and will 
 not therefore reach the visual base, unless at 
 an infinite distance from the eyes. For which 
 reason, the two lines, containing the visible po- 
 sitions of the object, must fall upon the visual 
 base at a like distance, and must consequently 
 be regarded as coinciding with each other. 
 When the object is not in the horopter, the two 
 lines of visible direction will be found, by the 
 same means, not to coincide. 
 
 That I might simplify a matter, which under 
 my management, must, I fear, still be of dif- 
 ficult apprehension, I have, in expressing the 
 law of vision, so frequently mentioned, pur- 
 posely confined it to objects situated in the 
 plane of the optic axes. But in persons who 
 do not squint, or whose eyes are not distorted 
 
48 AN ESSAY ON 
 
 by external violence, the two appearances of 
 an object, seen double, are always, either in 
 that plane, or in some one parallel to it; so 
 that, if the visual base be parallel to the ho- 
 rizon, a line joining the two appearances will, 
 in every case, be also parallel to the horizon. 
 Whoever then is able to explain, why objects 
 in the plane of the optic axes appear either 
 single or double, may readily give a reason for 
 the like appearances of such as are placed any 
 where else. Not to spend much time, there- 
 fore, upon this part of the subject, I shall 
 shortly observe, that if planes be supposed to 
 pass through the two optic and common axes, 
 perpendicular to that in which they all lie, and 
 if two lines be drawn from any point of the 
 common intersection of the former planes to 
 the visual base, one along each of the perpen- 
 dicular planes which pass through the optic 
 axes, these two lines will appear as one, in the 
 perpendicular plane of the common axis ; the 
 single visible line, however, possessing the same 
 elevation, in regard to the horizon, as the two 
 real lines : And again, that, if a line be drawn 
 from any point of the same intersection to the 
 visual base, along the perpendicular plane of 
 the common axis, it will appear as two, one in 
 each of the planes which pass through the optic 
 axes ; the two visible lines having the same in- 
 
SINGLE VISION. 49 
 
 clination to the horizon in their progress to the 
 visual base, as the real single one. In this man- 
 ner, every thing may be shown to be true, with 
 respect to the single and double appearances of 
 objects without the plane of the optic axes, 
 which has already been done with regard to 
 those placed in it. But farther ; since any 
 point, taken at pleasure, in the common inter- 
 section of the three perpendicular planes, ap- 
 pears single, the whole of the line of intersec- 
 tion must appear so, and likewise every point 
 of a plane made to pass through it, parallel to 
 the visual base. Such a plane necessarily in- 
 cludes the horopter, and is the same as that, 
 which is called by Aguilonius the plane of the 
 horopter. 
 
 To exemplify the principal property of this 
 plane, I shall mention an experiment, which at 
 first I did not understand, though the result 
 was a direct consequence of my own principles. 
 I suspended a fine chord at right angles to the 
 horizon, and retreating a step or two, I looked 
 steadily at a point in it, which was upon a level 
 with my eyes. The chord, in these circum- 
 vstances, appeared single ; but whenever I di- 
 rected my eyes to any other point of it, either 
 above or below the former, two chords would 
 appear, crossing each other at the part, to which 
 the eyes were directed. In the first case, the 
 
50 AN ESSAY ON SINGLE VISION. 
 
 whole chord was in the plane of the horopter, 
 but in every other, only that point of it to which 
 both eyes happened to be turned. A conclusion 
 from this experiment is, that no object, which 
 is truly perpendicular to the horizon, will ap- 
 pear to be so, while our bodies are erect, unless 
 we direct our eyes to a point in it exactly upon 
 a level with themselves. 
 
 It was once my intention to subjoin here 
 several instances, from the most approved 
 authors, of inaccurate descriptions of the single 
 and double appearances of objects ; in order to 
 show, that the theory of visible direction, which 
 I have advanced, is not only consistent with the 
 universally received facts, but that it also dis- 
 covers to us, some minute errors, which un- 
 guided sense has committed upon this subject ; 
 it being, perhaps, one of the surest tests of the 
 soundness, as well as one of the chief uses, of 
 theories in philosophy, that they lead to the 
 knowledge of what, otherwise, might have re- 
 mained for ever hidden. But fearing I have 
 already proved tiresome, I give up this design, 
 and hasten to the consideration of some conse- 
 quences from my theory, which seem to me 
 both curious and important, and which, when 
 first mentioned, may appear to carry with them 
 their own refutation. 
 
PART III. 
 
 Of some Consequences from tfie foregoing Theory of 
 Objects being 1 seen single with two Eyes, together with 
 the Explanation of several other Phenomena of Vision, 
 
 IT has hitherto, I believe, been thought by 
 opticians, that, if the position of the eye be un- 
 changed, the visible direction of an object will be 
 the same, as long as its picture occupies any one 
 point of the retina ; and that, in every different 
 position of the eye, a picture, which continues to 
 occupy the same point of the retina, will repre- 
 sent its object in a different direction. But if 
 the theory be just, which I have advanced in 
 the preceding part of this Essay, neither of 
 those opinions can be universally true. For it 
 follows, from what was there mentioned, that 
 if one of the optic axes be kept fixed, and the 
 other be at different time's variously bent to- 
 ward it, objects, though situated in the fixed 
 axis, will nevertheless change their visible di- 
 rections, with every variation of the moveable 
 axis j since they must always appear in the 
 common axis, which alters its position with 
 every change of the moveable axis : And again, 
 that>, if the two optic axes should vary their 
 
 E 2 
 
52 AN ESSAY ON 
 
 inclinations to each other in such a manner, that 
 the common axis, may, notwithstanding, remain 
 fixed, an object placed in either optic axis, and 
 following it in every motion, will possess but 
 one visible direction,, in all this variety of real 
 positions. That these conclusions from my 
 theory, or rather parts of it, are true in fact, 
 I can assert upon the authority of observations, 
 and I shall now attempt to trace them both to 
 a common principle, by means of some experi- 
 ments, which were instituted with a very dif- 
 ferent view. 
 
 When we have looked steadily for some time 
 at the flame of a candle, or any other luminous 
 body, a coloured spot will appear upon every 
 object, to which we shortly after direct our 
 eyes, accompanying them in all their motions, 
 and exactly covering the point, which we desire 
 to see the most accurately. Whatever there- 
 fore can be proved concerning the apparent 
 direction of such a spot, in any given position 
 of the eyes, must likewise be true in the same 
 position of the eyes, with regard to the appa- 
 rent direction of an object, situated at the con- 
 currence of the optic axes ; as its pictures must 
 occupy, in this case, the very -parts of the re- 
 tinas, upon the affections of which the illusion of 
 the spot depends. This being premised, I shall 
 now relate one or two observations, respecting 
 
SINGLE VISION. 
 
 the apparent directions of the spot, and conse- 
 quently upon those of external objects, which, 
 as far as 1 know, have not been mentioned by 
 any other person. 
 
 1. The spot is always seen single, whether 
 the surface, upon which it is projected, be 
 touching the face, or at the greatest distance 
 from us j and the reason is plain. For the 
 parts of the retinas, by whose affections from 
 the luminous body it is occasioned, are those 
 likewise which receive the pictures of objects, 
 placed at the intersection of the optic axes; 
 and as such objects always appear single, so 
 must also the spot. The fact indeed is so open 
 to observation, and its cause so easily shown, 
 that I should scarcely have thought of men- 
 tioning it, had not Dr. Darwin* lately told us, 
 that the spot is seen double, as often as the 
 eyes are directed to an object more or less 
 distant than the luminous body which gave rise 
 to it. With respect to our different assertions 
 upon this point, I shall only say, that I have 
 made the experiment, I believe, upward of an 
 hundred times, uniformly with the same result; 
 and that, if the spot ever appears double, this 
 must be from some cause very wide of a change 
 
 * Philosoph. Transact, for 1786, p. 318. Dr. Darwin 
 indeed, says, p. 341, that Buffon had observed the same 
 fact ; but it is evident he has mistaken that author's meaning, 
 
AN ESSAY ON 
 
 in the mutual inclination of the optic axes, to 
 which he attributes it*. 
 
 2. The spot not only appears single in every 
 ordinary position of the optic axes, but cannot 
 even be made to appear double, by any means 
 whatsoever. If it be projected, for example, 
 upon a piece of white paper, whoever makes 
 the trial will find, that, although, on pressing 
 one eye upward or downward, or to either side, 
 the paper will be seen double, yet the spot will 
 always appear single, and to possess its former 
 place on the paper, as se6n by the eye, which 
 is not disturbed. Before I knew the result of 
 this experiment, I had imagined, that, the po- 
 sition of one eye being forcibly altered, the 
 
 * The only way, in which I think it possible for the spot 
 to appear double, consistently with the universally acknow- 
 ledged fact, that an object at the intersection of the optic 
 axes is always seen single,, is this, that, when the intersec- 
 tion is near to the face, an object placed in it shall not send 
 its pictures to the same points of the two retinas, as it does, 
 when the intersection is more remote. And such I once 
 hoped to find to be the case; for I had formed, upon the 
 supposition of its truth, a more plausible account of the 
 manner in which the eyes are fitted to receive, successively, 
 pictures equally distinct from objects at different distances, 
 than any I had met with. But, after many experiments to 
 ascertain the matter, I was obliged to return to the common 
 opinion, that the picture of an object in the optic axis, what- 
 ever be its distance from the eye, is always received upon 
 the same point of the retina. 
 
SINGLE VISION. 55 
 
 external situation of the spot, which was sug- 
 gested by the affection of that eye, would like- 
 wise be altered, and the spot by consequence 
 be seen double. As the event, however, was 
 contrary to my expectation, I began to suspect 
 some cause of fallacy had been overlooked, 
 which at length I thought might be this, that 
 the spot had been seen by that eye only whose 
 position was not disturbed, the violence, suf- 
 fered by the other, interrupting the due exer- 
 cise of its functions. To determine, therefore, 
 whether my conjecture was well founded or 
 not, I made another experiment, which is men- 
 tioned in the following article : 
 
 3. Having looked steadily for some time at 
 the flame of a candle, with one eye only, I 
 directed afterward, with both eyes open, my 
 attention to the middle of a sheet of paper, a 
 few feet distant; the consequence of which 
 was, that a spot appeared upon it in the same 
 manner, as if I had viewed the flame with both 
 eyes, though somewhat fainter. My attention 
 remaining fixed upon the sheet, I now pushed 
 the eye, by which the spot was seen, succes- 
 sively upward and downward, to the right and 
 to the left, and in every oblique direction ; the 
 spot however never altered its position, but 
 kept constantly upon the middle of the appear- 
 ance of the paper, perceived by the undistorted 
 
AN ESSAY ON 
 
 eye, though the appearance of the paper to the 
 distorted eye, was always separate from the 
 former, and the sheet consequently seen double. 
 My conjecture, therefore, was proved to be ill 
 grounded, and all suspicion of fallacy in the 
 former experiment ceased. 
 
 Now it is evident, from these two last experi- 
 ments, that the situation of the spot does not 
 depend upon the bare position of the eyes, or 
 else, in the former of them, it would have ap- 
 peared double, and in the latter, it would have 
 been moved from the middle of the paper, 
 when the only eye by which it was seen was 
 pushed from its place. Neither can it depend 
 upon the bare position of the muscles of the 
 eye, as these were also moved in the same ex- 
 periments ; nor upon any affection whatever of 
 the optic nerve. For since this last substance 
 is altogether passive, even in those motions of 
 the eyes which do occasion a change of the 
 spot's situation, every alteration, induced upon 
 the nerve by those motions, must be ultimately 
 ascribed to a change of its position ; and we 
 have seen, that similar changes of its position 
 have been produced by external violence, with- 
 out any alteration of the spot's situation. The 
 apparent situation of the spot being, therefore, 
 dependant upon none of these circumstances, 
 and being at the same time affected by the 
 
SINGLE VISION. 
 
 voluntary motions of the eye, it must, I think, 
 be necessarily owing to the action of the mus- 
 cles, by which these motions are performed. 
 Assuming then as true, that the apparent di- 
 rection of an object, which sends its picture to 
 any given point of the retina, depends upon the 
 state of action existing at the same time in the 
 muscles of the eye, and consequently that it 
 cannot be altered, except by a change in the 
 state of that action, I shall proceed to trace 
 to this principle, several phenomena of vision, 
 particularly the uniform singleness of the spot 
 already described, and the two facts respecting 
 the visible directions of objects in the optic 
 axis, which were mentioned in the beginning 
 of this part of my Essay. 
 
 The thing itself is universally acknowledged, 
 though a dispute has arisen whether custom or 
 an original property be the cause, that every 
 voluntary motion of one eye, in persons who 
 do not squint, is attended with a corresponding 
 motion in the other. Now as all voluntary 
 motions are produced by muscular action, it 
 follows, that every state of action, in the 
 muscles of one eye, has its corresponding state 
 in those of the other, and that the two are 
 constantly conjoined. When, therefore, the 
 spot appears single to both eyes in their free 
 positions, the states of action in the muscles 
 
58 AN ESSAY ON 
 
 must be such, that the direction, in which it is 
 seen by one eye, coincides with that in which 
 it is seen by the other. But, if we push one 
 eye from its place, no change is hereby made 
 in the action of its muscles ; for the state of 
 action in those of the free eye is confessedly 
 the same as it was ; and it will be attended 
 with a corresponding state in those of the dis- 
 torted eye ; in proof of which it may be ob- 
 served, that, whenever the pressure is removed, 
 the distorted eye immediately returns to its 
 former position, without the aid of any new 
 muscular effort. The conclusion then is, that, 
 since there has been no alteration in the action 
 of its muscles, neither ought there to be any in 
 the direction of the spot seen by it, which is 
 the fact to be explained. 
 
 Hence also is to be derived the true reason, 
 why objects appear double, when one eye is 
 pushed from its place. For as their pictures 
 must fall upon points of the retina in this eye, 
 different from what they formerly possessed ; 
 and as no change is made, by the distortion, 
 upon the visible direction, suggested by any 
 part of the retina, the objects will be seen by 
 the pressed eye, exactly in the same directions 
 as they would have been, before it was pressed, 
 had the pictures then fallen upon the points of 
 the retina, which they now occupy. They must 
 
SINGLE VISION. 59 
 
 therefore be now seen in different directions by 
 the two eyes, and consequently double. An 
 experiment with a contrary event will confirm 
 this explanation, and likewise show more 
 clearly, in what I differ from those who have 
 endeavoured to account for the same fact. 
 Both eyes being open, let one of them be 
 pushed from its situation, and let two similar 
 objects, such as two pieces of money of the 
 same metal and stamp, be afterward so placed, 
 that one shall He in each optic axis ; these two 
 objects will now appear to be one, and the 
 object so compounded will be seen in the place, 
 to which the undisturbed eye refers the truly 
 single object lying in its axis. 
 
 Another inference from this doctrine is, that, 
 if the eyes are in any very unusual position 
 with respect to each other from the action of 
 their own muscles, as in persons who squint, 
 two objects placed in the optic axes, one in 
 each, will not appear as one object ; for each 
 will be seen in the direction, which is de- 
 termined by the state of action in the muscles 
 of the eye, upon whose retina its picture falls ; 
 and as this state, in one eye, does not corre- 
 spond with that in the other, the directions 
 cannot coincide. This conclusion is verified 
 by the result of an experiment of Dr. Reid 
 upon a person, affected with strabismus, and 
 
60 AN ESSAY ON 
 
 by that of another, made by myself, both of 
 which have been already related. 
 
 To explain, therefore, why an object in the 
 optic axis appears at different times in different 
 directions, though the axis be kept fixed, it 
 is only necessary to show, that, whenever this 
 happens, a change, notwithstanding, occurs in 
 the actions of the muscles which move the eye. 
 With this view, I observe, that the motions of 
 that organ maybe divided into two sets; the 
 first, consisting of those, by which one eye 
 is carried along with the other, upward and 
 downward, to the right and to the left, and in 
 every oblique direction, the interval between 
 the pupils remaining constantly the same ; the 
 second, of the motions of the pupils, or the an- 
 terior parts of the eyes, to and from each other. 
 Supposing now, that both the optic axes are 
 perpendicular to the visual base ; should the 
 left axis be afterward inclined to the right side, 
 the natural tendency of the right axis is to in- 
 cline equally to the same side, so as to preserve 
 its former parallelism to the left. This ten- 
 dency, however, in the right axis to follow the 
 left, may be counteracted by an effort of the 
 muscles, which regulate the interval of the 
 pupils, until the two axes intersect each other 
 within two or three inches of the face. But it 
 is evident, that the same degree of muscular 
 
SINGLE VISION. 61 
 
 force will be required to retain the right eye in 
 its original position, as is necessary to give to 
 the left eye its motion toward the right ; and 
 hence, that, in every different inclination of 
 the left axis to the right, an object placed in 
 the latter, though its real position be un- 
 changed, will, nevertheless, appear in a dif- 
 ferent direction, in consequence of the dif- 
 ferent state of action in the muscles of the right 
 eye, which accompanies every new degree of 
 inclination of the axes to each other. As the 
 object must always appear in the common axis, 
 the alteration, in this example, of its visible di- 
 rection, from an increase of the mutual inclina- 
 tions of the optic axes, will be from left to 
 right ; but when the inclination decreases, 
 from right to left. If the right axis be the one 
 which is moved, and the left fixed, the altera- 
 tions of visible direction in an object placed 
 in the latter, from similar changes in their in- 
 clinations, will be contrary to those which have 
 just been mentioned. 
 
 The reason also can now be made to appear, 
 why an object, preserving constantly its place 
 in the optic axis, may, in a considerable variety 
 of its real positions, possess but one visible 
 direction. For, in such cases, the change of 
 its visible direction, which might be expected 
 to accompany the motion of the eye in the axis 
 
62 AN ESSAY ON 
 
 of which it is situated, is prevented from oc- 
 curring, by a tendency to a change of its visible 
 direction the contrary way, produced by the 
 muscular actions which regulate the mutual 
 distance of the pupils. To know how this 
 happens, suppose the two optic axes to be pa- 
 rallel to each other, and perpendicular to the 
 visual base ; and let a physical line be placed 
 in either of them, so as entirely to coincide 
 with it. This line will, therefore, not only be 
 in reality perpendicular to the visual base, but 
 will, in the present state of things, likewise 
 appear so. Incline afterward both the axes 
 equally to the left side, and it is manifest that 
 the line coinciding, say, with the right axis, 
 must appear equally inclined. Let now the 
 right axis be kept fixed, and the left be carried 
 back again, and its motion continued, until it 
 be as much inclined toward the right side, as 
 itself was just before, and as the right axis is 
 still to the left side ; the consequence will be, 
 that the line in the right axis must again be 
 seen perpendicular to the visual base; for such 
 is the present position of the common axis. 
 Here then we have had two opposite causes of 
 change of apparent direction acting in succes- 
 sion. The muscular actions, producing the joint 
 motions of the eyes, first bent the visible posi- 
 tion of a line, in the right optic axis, from a 
 
SINGLE VISION. 63 
 
 perpendicular to the visual base toward the left; 
 and the muscular actions, which regulate the 
 mutual distances of the pupils, by increasing 
 the inclinations of the axes to each other, 
 moved it afterward, from the left to the right, 
 back again to a perpendicular to the visual base. 
 Let these two causes act together, and it is 
 plain, that no observable effect will be pro- 
 duced by either, as long as they are thus pro- 
 portioned. When they are not so, only the 
 difference of their forces will be exhibited by 
 the phenomena. 
 
 But farther ; to show the extent of this theory 
 of visible direction being dependant upon the 
 actions of the muscles of the eyes, I shall now 
 apply it to the explanation of an instance of 
 apparent motion, which at first may be thought 
 to furnish an argument against it. Look with 
 one eye, the other being closed, at any remote 
 object through a small hole in a card. If you 
 should afterward suddenly attempt to view the 
 hole itself accurately, with the same eye, you 
 will observe both it and the distant object, par- 
 ticularly the latter, to move from left to right, 
 if the right eye be used ; but; if the left eye be 
 the one employed, then from right to left. 
 Shift now your attention as suddenly back from 
 the hole to the object seen through it, and 
 both will return to the places they formerly 
 
AN ESSAY ON 
 
 occupied. In this experiment, no real change 
 can be supposed to have occurred in the posi- 
 tion of the distant object ; and had any hap- 
 pened with respect to either the eye or the hole, 
 the object would not have been seen through 
 the latter. No other fallacy, therefore, exists 
 here, than that things, which are truly at rest, 
 appear, notwithstanding, to be in motion. 
 
 The argument, which I have mentioned may 
 hence be derived against my theory, is this : 
 The visible directions of objects, in the optic- 
 axis which remained fixed, were formerly said 
 to be altered, because a new state of muscular 
 exertion was required to keep it so, in every 
 different degree of the inclination to it of the 
 moveable axis. But in the last experiment, 
 there seems no good reason for supposing any 
 change in the inclination of the moveable axis 
 to the other ; for, as one eye is closed, the 
 obvious intention of directing the two axes to 
 the same object, which is, that we may see it 
 single, no longer exists. If then an apparent 
 lateral motion be, in one instance, observed in 
 objects truly at rest, without any change of the 
 interval of the pupils, may not every other 
 motion of the like kind be also independent 
 of the muscular actions, which regulate that 
 interval ? 
 
 Itisevident, that this argument rests altogether 
 
SINGLE VISION. 5 
 
 upon the supposition, that in the experiment 
 just mentioned, no alteration occurs in the in- 
 terval of the pupils. Now, we iriay be easily 
 convinced, that some alteration does occur, by 
 applying a finger to the closed eye, which will, 
 by this means, be felt to move toward the nose, 
 when we endeavour to view the hole accurately, 
 and from the nose, when we carry our attention 
 back again to the remote object. Were, in- 
 deed, the opinion of Aguilonius * just, that the 
 mind perceives only those objects distinctly, 
 which are situated at the concourse of the optic 
 axes, whether they are seen with one or with 
 two eyes, both the necessity and the degree of 
 the alteration would be clearly ascertained. 
 But as this opinion is not just, which I mean 
 to prove from experiments in a succeeding part 
 of this work, I shall proceed to give another 
 reason, and I think the true one, why the in- 
 terval of the pupils should be as much altered, 
 when we look with one eye at objects succes- 
 sively, which are placed at different distances, 
 as if we were to view them with both. 
 
 It is a fact, for which I have the authority of 
 experiments almost without number, though I 
 do not recollect to have seen it mentioned by 
 any author beside Dr. Porterfield, that every 
 
 * Optica, p. 84. 
 F 
 
66 AN ESSAY ON 
 
 change of the mutual positions of the optic axes 
 is conjoined, in persons who do not squint, 
 with a change of the power, in both eyes, to 
 refract the rays of light which fall upon them. 
 When the axes are parallel to each other, the 
 eyes are in their lowest refracting state ; but 
 in their highest, when the axes are mutually 
 intersected within two or three inches of the 
 face ; every intermediate inclination being also 
 conjoined with an intermediate degree of re- 
 fracting power. Now, since those objects are 
 seen most distinctly, the radious pencils from 
 which are accurately brought to points in the 
 retina, it follows, that, although we employ one 
 eye only, the same reason exists for adjusting 
 its refractive power to their distances, as if we 
 saw with both. When, therefore* we view a 
 remote object with one eye, we use it in its 
 lowest refracting state, which, I have observed, 
 is conjoined with the widest interval of the 
 pupils. Should we afterward attempt to see 
 accurately a very near object, the eye will 
 assume its highest refractive state, and the in- 
 terval of the pupils be lessened ; the conse- 
 quence of which must be, that both the objects 
 lying in the optic axis will appear to move in 
 the manner already related. 
 
 To finish this part of my subject, it seems 
 
SINGLE VISION. 67 
 
 only necessary to determine, whether the de- 
 pendance of visible direction upon the actions 
 of the muscles of the eyes be established by 
 nature, or by custom. But facts are here 
 wanting. As far as they go, however, they 
 serve to prove, that it arises from an original 
 principle of our constitution. For Mr. Chesel- 
 den's patient saw objects single, and conse- 
 quently in the same directions with both eyes, 
 immediately after he was couched; and persons 
 affected with squinting from their earliest in- 
 fancy, see objects in the same directions with 
 the eye they have never been accustomed to em- 
 ploy, as they do with the other they have con- 
 stantly used. 
 
 Having thus shown in what directions ex- 
 ternal bodies are seen, when their situation 
 with respect to the eye is given, and upon what 
 circumstance the various directions depend, in 
 which a picture upon any one place of the 
 retina can exhibit the object producing it ; I 
 should render the theory of visible direction 
 complete, were I now to point out the relative 
 positions of the two lines of direction, in which 
 any two different parts of the retina represent 
 their objects. To ascertain this, the first step 
 must be, to find the place of the retina which 
 receives the picture 'of an object, whose situa- 
 tion with respect to the external eye is known; 
 
 F2 
 
68 AN ESSAY ON SINGLE VISION. 
 
 and if two such points of the retina were deter- 
 mined, I think the chief difficulty in this matter 
 would then be overcome. But as it appears to 
 me, that the structure of the eye has not yet 
 been sufficiently explained, to enable any person 
 to take this first step, I forbear saying any thing 
 more upon the subject* 
 
.EXPERIMENTS AND OBSERVATIONS 
 
 ON 
 
 SEVERAL SUBJECTS IN OPTICS. 
 
 ARTICLE I. 
 
 On Visible Position, and Visible Motion. 
 
 IN the estimates we make by sight of the 
 situation of external objects, we have always 
 some secret reference to the position of our 
 own bodies, with respect to the plane of the 
 horizon ; and from this cause, we often judge 
 such to be at rest, whose relative places to us 
 are continually changing ; and others to be in 
 motion, though they may constantly preserve, 
 in regard to us, the same distance and direc- 
 tion. To give an instance, let us suppose our 
 eyes first directed to a star near to the horizon ; 
 should we afterward, by a mere motion of the 
 head, point them to another, some degrees 
 above the former, this second star will appear 
 higher than the first did. Were we now, while 
 the eyes are kept fixed in relation to the head, 
 
70 EXPERIMENTS, &c. 
 
 and the head in relation to the shoulders, to in^ 
 cline the trunk of the body backward, until we 
 bring the optic axes to a third star, this will ap- 
 pear still higher than the second was perceived 
 to be. If instead of directing the eyes succes- 
 ^sively to different objects, the same object be 
 suffered to remain at the concurrence of the 
 optic axes in all these different positions of the 
 body, it is evident, that it must be seen to move, 
 during the change from one position to another. 
 The facts I have mentioned are so obvious, 
 that I should not have spoken of them, had I 
 not intended they should introduce the follow- 
 ing question : What is there within us, to in- 
 dicate these positions of the body ? To me it 
 appears evident, that since they are occasioned 
 and preserved by combinations of the actions of 
 various voluntary muscles, some feeling must 
 attend every such combination, which suggests, 
 from experience perhaps, the particular posi- 
 tion produced by it. But in almost all the posi- 
 tions of the body, the chief part of our muscular 
 efforts is directed toward sustaining it against 
 the influence of its own gravity. Each posi- 
 tion, therefore, in which this takes place, must 
 be attended with a feeling, which serves to in- 
 dicate its relation to the horizontal plane of the 
 earth ; and consequently, if aur bodies pos- 
 sessed no gravity, or, if the thing were possible, 
 
IN OPTICS. 71 
 
 had we been created unembodied spirits, but 
 with the same faculties of perception as we 
 enjoy at present, we could no more have judged 
 one line to be perpendicular, and another to 
 be parallel to the horizon, than we can at pre- 
 sent determine, without some external aid, 
 which is the eastern, and which the western 
 point of the heavens. I shall now draw from 
 these principles, the explanation of a fact, 
 which was first mentioned by one of the most 
 ingenious authors that have written upon vision, 
 but left by him still to be justly accounted for. 
 " I have frequently" (says Mr. Melvill)* 
 <c observed, when at sea, that, though I pressed 
 my body and head firmly to a corner of the 
 cabin, so as to be at rest in respect to every 
 object about me, the different irregular mo- 
 tions of the ship, in rolling and pitching, were 
 still discernible by sight. How is this fact 
 to be reconciled to optical principles? Shall 
 we conclude that the eye, by the sudden 
 motions of the vessel, is rolled out of its due 
 position? Ov, if it retains a fixed situation 
 in the head, is the perception of the ship's 
 motion, owing to a vertigo in the brain, a de- 
 ception of the imagination, or to what other 
 cause ?" 
 
 * Edinburgh Physical Essays, vol. ii. p. 8O. 
 
72 EXPERIMENTS, &c. 
 
 I need not, I believe, offer to show, that the 
 fact here spoken of, is not owing to any of the 
 causes Mr. Melvill has specified. I shall there- 
 fore, in a few words, point out its dependance 
 upon the principles which have just been men- 
 tioned. 
 
 It is generally known, I suppose, that when 
 a vessel at sea, in the language of sailors, is 
 said to pitch, its two extremities turn upon its 
 shorter axis, and that the term of rolling is con- 
 fined by them to its motions upon the longer 
 axis. In both pitching and rolling then, the 
 relative position of a vessel to a horizontal 
 plane is necessarily changed. Consequently, 
 though, in the abovementioned experiment, 
 Mr. Melvill's body and head were at rest with 
 respect to every object about him, still a dif- 
 ferent degree of muscular effort was required 
 to ]keep them so, in every such different posi- 
 tion of the vessel. But each degree of mus- 
 cular effort, to sustain his body against the 
 operation of its gravity, would suggest to him 
 its concomitant position with regard to the 
 plane of the horizon ; each deviation, there- 
 fere, of the vessel from its former situation, re- 
 latively to the same plane, would be perceived, 
 and the vessel itself be seen to move. In short, 
 nothing more takes place in this, than in the 
 following experiment: Let a pole be placed 
 
IN OPTICS. 73 
 
 upon firm ground, at right angles to the horizon. 
 If, while we are standing erect, it be inclined 
 upon its lower extremity, successively backward 
 and forward, to the right and to the left, these 
 motions must, without contradiction, be per- 
 ceived. Suppose now, our bodies to be simi- 
 larly inclined with the pole, during its different 
 positions, so as to be constantly parallel to it ; 
 it is evident, that its motions will be as readily 
 perceived in this case, as they were, when our 
 bodies were erect ; and this is all that happens 
 in the experiment of Mr. Melvill. 
 
 Should the necessity of supporting the body 
 against its gravity, by the actions of our volun- 
 tary muscles, be suspended in whole, or in part, 
 our judgments of the situation of objects, with 
 respect to the horizon, must become irregular 
 and uncertain, notwithstanding any general 
 habit we may have acquired from experience. 
 An instance of this, I think, I have observed; 
 for I have frequently remarked during a sea 
 voyage, that, when the wind blew so strongly, 
 and in such a direction, as to occasion the vessel 
 to heel, or lean much to one side, chords freely 
 suspended from the roof of the cabin, and kept 
 Stretched by heavy bodies attached to them, 
 have appeared to me, as long as I lay in bed, 
 though they were necessarily perpendicular to 
 the horizon, to decline considerably from that 
 
74 
 
 EXPERIMENTS, &c. 
 
 position ; while the sides of the cabin seemed, 
 if not perpendicular, at least much less inclined 
 to the horizon than they were in reality. My 
 body being here supported by the bed, I was 
 consequently without those feelings, which in- 
 dicate its position with respect to the horizon. 
 Objects therefore appeared to me in those situa- 
 tions, in which I had been accustomed to see 
 them. In confirmation of which I may men- 
 tion, that, when I got up, and stood upon the 
 floor of the cabin, the chords seemed perpendi- 
 cular, or nearly so, and the sides of the cabin 
 inclined ; for I was now obliged to exert a pro- 
 per degree of muscular force, to keep myself 
 upright. What I here say, however, is from 
 the recollection of things observed some years 
 ago, when I had no thought of making the use 
 of them I now do ; for which reason, I may 
 possibly have committed some trifling error in 
 stating them ; but none, I believe, sufficient to 
 affect the theory they are brought to support. 
 
 It. being my intention to treat, in the present 
 article, of several facts relative to visible posi- 
 tion and motion, which seem to me to need 
 explanation, without regarding whether or not 
 they depend 5 upon any common cause; I pass 
 to the consideration of the apparent rotation of 
 objects, when we have become giddy, by turn- 
 ing ourselves quickly and frequently round. 
 
IN OPTICS. 75 
 
 Some of the older writers upon optics ima- 
 gined the visive spirits to be contained in the 
 head, as water is in a vessel, which therefore, 
 when once put in motion by the rotation of our 
 bodies, must continue in it for some time after 
 this has ceased ; and to this real circular move- 
 ment of the visive spirits, while the body is at 
 rest, they attributed the apparent motions of 
 objects in giddiness. Dechales* saw the weak- 
 ness of this hypothesis, and conjectured, that 
 the phenomenon might be owing to a real 
 movement of the eyes, but produced no fact in 
 proof of his opinion. Dr. Porterfieldf, on the 
 contrary, supposed the difficulty of explaining 
 it to consist in showing, why objects at rest 
 appear in motion to an eye which is also at 
 rest. The solution he offered of this repre- 
 sentation of the phenomenon, is not only ex- 
 tremely ingenious, but is, I believe, the only 
 probable one which can be given. It does not 
 apply, however, to the fact which truly exists ; 
 for I shall immediately show, that the eye is 
 not at rest, as he imagined. The last author, 
 I know of, who has touched upon this subject, 
 is Dr. Darwin t. His words are, " When any 
 one turns round rapidly on one foot till he 
 
 * Cursus Mathemat. Tom. ii. p. 422. 
 t Treatise on the Eye, Vol. ii. p. 426. 
 I Philosoph, Transact. Vol. Ixxvi. p. 315. 
 
76 EXPERIMENTS, &c. 
 
 becomes dizzy, and falls upon the ground, the 
 spectra of the ambient objects continue to pre- 
 sent themselves in rotation, or appear to librate, 
 and he seems to behold them for some time in 
 motion." I do not indeed pretend to under- 
 stand his opinion fully; but this much seems 
 clear, that, if such an apparent motion of the 
 surrounding objects depends, in any way, upon 
 their spectra, or the illusive representations of 
 those objects, occasioned by their former im- 
 pressions upon the retinas, no similar motion 
 would be observed, were we to turn ourselves 
 round with our eyes shut, and not to open them 
 till we became giddy ; for in this case, as the 
 surrounding objects could not send their pic- 
 tures to the retinas, there would, consequently, 
 be no spectra to present themselves afterward 
 in notation. But whoever will make the expe- 
 riment, will find, that objects about him appear 
 to be equally in motion, when he has become 
 giddy by turning 'himself round, whether this 
 has been done with his eyes open or shut. I 
 shall now venture to propose my own opinion 
 upon this subject. 
 
 If the eye be at rest, we judge an object to 
 be in motion when its picture falls in succeed- 
 ing times upon different parts of the retina ; 
 and if the eye be in motion, we judge an object 
 to be at rest, as long as the change in the place 
 
IN OPTICS. 77 
 
 of its picture upon the retina, holds a certain 
 correspondence with the change of the eye's 
 position. Let us now suppose the eye to be in 
 motion, while, from some disorder in the system 
 of sensation, we are either without those feel- 
 ings, which indicate the various positions of the 
 eye, or are not able to attend to them. It is 
 evident, that, in such a state of things, an 
 object at rest must appear to be in motion, 
 since it sends in succeeding times its picture to 
 different parts of the retina. And this seems 
 to be what happens in giddiness. I was first 
 led to think so from observing, that, during a 
 slight fit of giddiness 1 was accidentally seized 
 with, a coloured spot, occasioned by looking 
 steadily at a luminous body, and upon which I 
 happened at that moment to be making an ex- 
 periment, was moved in a manner altogether 
 independent of the positions I conceived my 
 eyes to possess. To determine this point, I 
 again produced the spot, by looking some time 
 at the ftame of a candle ; then turning myself 
 round till I became giddy, I suddenly discon- 
 tinued this mption, and directed my eyes to the 
 middle of a sheet of paper, fixed upon the wall 
 of my chamber. The spot now appeared upon 
 the paper, but only for a moment 5 for it im- 
 mediately after seemed to move to one side, 
 and the paper to the other, notwithstanding I 
 
7 H EXPERIMENTS, &c. 
 
 conceived the position of my eyes to be in the 
 mean while unchanged. To go on with the 
 experiment, when the paper and spot had pro- 
 ceeded to a certain distance from each other, 
 they suddenly came together again ; and this 
 separation and conjunction were alternately re- 
 peated a number of times ; the limits of the 
 separation gradually becoming less, till, at 
 length, the paper and spot both appeared to 
 be at rest, and the latter to be projected upon 
 the middle of the former. I found also, upon 
 repeating and varying the experiment a little, 
 that when I had turned myself from left to 
 right, the paper moved from right to left, 
 and the spot consequently the contrary way : 
 but that when I had turned from right to 
 left, the paper would then move from left to 
 right. These were the appearances observed 
 while I stood erect. When I inclined, how- 
 ever, my head in such a manner, as to bring 
 the side of my face parallel to the horizon, the 
 spot and paper would then move from each 
 other, one upward and the other downward. 
 But all these phenomena demonstrate, that 
 there was a real motion in my eyes at the time 
 I imagined them to be at rest ; for the apparent 
 situation of the spot, with respect to the paper, 
 could not possibly have been altered, without 
 a real change of the position of those organs. 
 
IN OPTICS. 79 
 
 To have the same thing proved in another way, 
 I desired a person to turn quickly round, till he 
 became very giddy ; then to stop himself and 
 look stedfastly at me. He did so, and I could 
 plainly see, that, although he thought his eyes 
 were fixed, they were in reality moving in their 
 sockets, first toward one side, and then toward 
 the other. 
 
 The last instance of visible motion I shall 
 notice, is one which has been mentioned by 
 Mr. Le Cat, in the following words * : " Place 
 a lighted candle at a moderate distance from a 
 polished body of considerable convexity, so that 
 the image of the flame, which is seen by reflec- 
 tion from it, may appear as a small luminous 
 point. The experiment will succeed better, if 
 the direct rays of the flame be intercepted from? 
 the sight. Close, after this, one eye, and view 
 the luminous point in a careless way, (en revanf) 
 that is to say, with the eye in a relaxed or dilated 
 state. The point will then be seen enlarged and 
 radiated. If you bring now your finger to the 
 right of the eye which i& open, and gradually 
 move it toward the left, in order to conceal the 
 luminous point from this eye, you will distinctly 
 perceive the shadow of your finger to proceed 
 from left to right, and to pass over the point in 
 
 * Trait6 des Sens. p. 419- 
 
80 EXPERIMENTS, &c. 
 
 a direction, contrary to that which you gave it- 
 Should you, afterward, move your finger back 
 from right to left, and in like manner, if your 
 finger be moved from above downward, or from 
 below upward,- the shadow will always proceed 
 the contrary way. It is therefore manifest, 
 that the soul must here see objects inverted, as 
 their images in the eye truly are ; and that it- 
 refers impressions to those parts of the eye 
 where it feels them, and not to the places from 
 which the rays are emitted, as it does when it 
 possesses the means of rectifying its judgment. 
 Whence does this happen ? Doubtless, because 
 the luminous point has neither a high nor a 
 low, neither a right nor left side, nor any well- 
 enlightened object in its vicinity, to awaken 
 the attention of the soul ; in short, nothing 
 which can determine its judgment." 
 
 I should scarcely have mentioned this experi- 
 ment, from any respect for the authority of its 
 author in optics ; but as-Haller* seems to assent 
 to the conclusion he draws from it, that the 
 soul sometimes sees objects inverted; and as 
 the Abbot Derochonf, a member of that learned 
 body, the Academy of Sciences of Paris, has 
 lately, but in my opinion unsuccessfully, at- 
 tempted to reconcile it to the commonly received 
 
 * Eleraenta Physiologiae, Tom. v. p. 479- 
 f Memoires de Physique, p. 66. 
 
IN OPTICS. 81 
 
 principles of vision, I think it worth while to 
 show, in a few words, that it is a direct conse- 
 quence of the very doctrine Mr. Le Cat means 
 to overthrow by its means. 
 
 It would be proper, indeed, to mention before- 
 hand, the opinion of the Abbot Derochon ; but 
 this I must, notwithstanding, omit doing, as it 
 could not be understood without the figure by 
 which he has illustrated it. I shall observe, 
 however, respecting it, first, that it requires the 
 side of the ringer next to the eye, to be without 
 the least illumination ; whereas the experiment 
 will succeed, whether it be illuminated or not : 
 secondly, that, according to it, the experiment 
 ought to succeed equally well, whether the 
 image of the flame in the mirror be seen as a 
 point, or as a surface; though, in truth, it 
 never does succeed, except in the latter cases : 
 thirdly, that the apparent shadow of the finger 
 is always much larger than it ought to be, were 
 it seen by reflection, as the Abbot thinks : 
 fourthly, that, while the eye, mirror, flame, and 
 finger, remain in the same positions, the shadow 
 seems at one time larger than at another, owing 
 to the different degrees of relaxation in the eye ; 
 but that this, for the reason just mentioned, 
 ought never to happen, according to his theory : 
 fifthly, that agreeably to his own reasoning, the 
 
82 EXPERIMENTS, &c. 
 
 shadow ought to move in the same direction 
 with the finger, which is the very reverse of 
 the fact to be explained. But as arguments 
 against error may be infinitely extended, and 
 as only one solution of a phenomenon can be 
 true, the readiest way of exposing the insuf- 
 ficiency of others, is to exhibit that which is 
 just. 
 
 This, in the present case, seems to lie upon 
 the very surface of optical knowledge, and has 
 already been given by others, of various forms 
 of the same fact. When the image of the flame 
 is seen in the mirror as a point, its rays must be 
 accurately collected to a focus in the retina ; 
 but when seen as a surface, this must necessarily 
 be attributed to their focus being either before 
 or behind it ; in either of which cases, they will 
 occupy a place upon that membrane of some 
 assignable dimensions. In the present instance, 
 their diffusion over a part of the retina, depends 
 on the focus being behind it; for the eye is 
 now, from a condition of the experiment, in a 
 more relaxed state than it was just before, when 
 the rays of the same object were brought there 
 accurately to a point. The rays, therefore, 
 which go to the right side of the enlightened 
 surface of the retina, or picture as I shall call 
 it, are those which enter the eye at the right 
 
IN OPTICS. 83 
 
 side of the pupil, and its left side is formed of 
 the rays entering at the left side of the pupil j 
 and the like must be true of its upper and lower 
 parts. Should we then begin to move a finger 
 from right to left across the eye, the rays form- 
 ing the right side of the picture must be first 
 intercepted. But from the known fact, that 
 the points of an external object are always in 
 an inverted position, with respect to the parts 
 of the retina, by the affections of which they 
 are suggested, when the right side of the picture 
 there is effaced, the left side of the external 
 object it suggests must disappear. And for 
 the same reason, if the motion of the finger be 
 continued from right to left across the eye, the 
 other parts of the luminous surface in the mirror 
 will successively vanish from left to right, and 
 thereby furnish the appearance of a shadow 
 passing over it in that direction. In like man- 
 ner, it maybe shown, that if the finger proceeds 
 from left to right, from above downward, or 
 from below upward, the shadow must move the 
 opposite way. 
 
 That this is the true explanation of Mr. Le 
 Cat's experiment, is, I think, plain, both from 
 its intrinsic evidence, and the following consi- 
 derations : If the mirror be brought within 
 four or five inches of the eye, and the candle 
 
84 
 
 EXPERIMENTS, &c. 
 
 be so placed, that the image of the flame must, 
 from the laws of reflection, be regarded as a 
 mere point; though we should now view it 
 with the utmost care, and though there should 
 be in its neighbourhood some well-enlightened 
 object to awaken the attention of the soul, as 
 Mr. Le Cat expresses it, still the seeming sha- 
 dow will move in a direction contrary to the 
 finger. For the image is now so near to the 
 eye, that no exertion we can make is sufficient 
 to bring its rays to a point upon the retina f the 
 picture, therefore, upon that membrane will 
 be formed of rays passing to a focus behind it, 
 which is the only condition necessary for the 
 success of the experiment. Again, if a short- 
 sighted person should place the mirror at the 
 distance of some feet from him, complying in 
 other respects with Mr. Le Cat's instructions, 
 he will constantly observe the shadow to move 
 in the same direction with the finger. For, in 
 his eye, the rays of the image, when at such a 
 distance, must meet before they fall upon the 
 retina. The right side, therefore, of the pic- 
 ture upon that membrane, must be composed, 
 in this case, of rays which enter the eye at the 
 left side of the pupil. Consequently, when these 
 are cut off, the left side of the apparent lu- 
 minous surface must disappear, and the shadow 
 
IN OPTICS. 85 
 
 be seen to move the same way as the finger, 
 when this successively intercepts the rays pro- 
 ceeding from the image to the eye*. 
 
 * Scheiner observed a fact of the like kind (Fundamentum 
 Opticum, p. 33) namely, that, if a small hole, made in any 
 substance, be held near to the eye, and an opaque body be 
 passed between them, from right to left, the left side of the 
 hole will first disappear. Mr. Grey afterward took notice 
 (Philosoph. Transact. Vol. xix. p. 286) that a needle he em- 
 ployed in this experiment was seen inverted ; from which he 
 supposed that the hole, or something in it, produced the 
 effect of a concave speculum. Mr. Harris, however, says 
 (Treatise of Optics, p. 141 ) that it is not the needle, but its 
 shadow on the other side, which is seen, and is the cause of 
 the inverted appearance. But the truth is, that the hole is 
 to be regarded as a luminous point, the rays of which fall 
 upon the retina before they are collected to a focus ; and 
 hence that the same appearances must be here observed as 
 in the experiment of Mr. Le Cat. In proof of this it may 
 be mentioned, that if the hole be placed at such a distance, 
 that the eye may refract its rays accurately to a point on the 
 retina, ; :no shadow or image of the needle will be seen j that 
 if the hole be still farther removed, and the eye be adapted 
 to a less distance, the shadow or image will again appear, 
 but its position will now be upright, and its motion the 
 same way as that of the needle itself $ and lastly, that, at 
 one given distance of the hole, either no shadow will appear, 
 or it will be seen upright, or it will be seen inverted, accord- 
 ing as the eye may be made to assume different states with 
 respect to its power of refraction, 
 
EXPERIMENTS, &c. 
 
 ARTICLE II. 
 
 On a supposed Consequence of the Duration of Impres- 
 sions upon the Retina; and the Effects of accurate 
 Vision being confined to a single Point ofiliat Mem- 
 brane, 
 
 FEW things, at first, appear more incredible 
 to a person, not conversant in optics, than that 
 he does not, at any one time, see distinctly a 
 surface larger than the head of a pin. After 
 he is convinced, by proper trials, of the truth 
 of this, he naturally asks, Whence comes it 
 then, that, in ordinary vision, I seem to view 
 distinctly so many objects at once ? I go into 
 a crowded street, and I fancy I have an accu- 
 rate perception by sight, of men, houses, car- 
 riages, and many other things, all at the same 
 time 5 whence proceeds this illusion ? 
 
 Only one answer, as far as I know, has been 
 given to this question. The impressions made 
 upon the retina by external objects, do not, it 
 is said, immediately cease, along with the re- 
 ception of the rays which flow from them ; and, 
 as in the ordinary mode of vision, the eye is 
 continually passing from object to object, the 
 impression left by a former one may be still 
 vivid, though the eye be directed to another j 
 
IN OPTICS. 87 
 
 and hence we may imagine we see both of them 
 distinctly, though the picture of only one occu- 
 pies that place of the retina, which alone fur- 
 nishes us with accurate vision. 
 
 There are, however, objections to this answer, 
 which seem to me insurmountable. For, in the 
 Jirst place, as the duration of impressions on the 
 retina must be greater or less, according to the 
 vivacity of the pictures which occasion them, 
 it follows, that, were this answer just, the ap- 
 parent field of our distinct vision ought to be 
 in proportion to the quantity of light admitted 
 by the eye ; that it should be contracted, there- 
 fore, by every cloud which passes over us, and 
 be enlarged by every burst of sunshine ; that, 
 at mid-day, it should possess its greatest extent, 
 and ought from that time gradually to decrease 
 till the evening, when its limits should be nearly 
 tjie same with those of the real field of accu- 
 rate vision. Secondly, since the coloured spot, 
 which is produced by looking steadily for some 
 time at aluminous body, appears projected upon 
 every object to which we direct our eyes, dur- 
 ing its continuance, and as such a spot is neces* 
 sarily the sign and effect of the duration of an 
 impression upon the retina ; every other visible 
 appearance from the same cause ought, in like 
 manner, to have its situation determined by 
 the position of the eye, as far as this may be 
 
88 EXPERIMENTS, &c. 
 
 occasioned by the action of its muscles. No 
 object, therefore, ought to appear separate and 
 distinct from others, if the answer were true 
 which I am combating ; but, on the contrary, 
 all those to which we successively direct our 
 eyes during the limits of the duration of an 
 impression upon the retina, should seem 
 crowded into one place; and, consequently, 
 none of them should be perceived with any 
 tolerable accuracy. Such are the conclusions 
 from the truth of this answer. I need scarcely 
 mention, that they are contradicted by expe- 
 rience. 
 
 There is another form of the same fact, to 
 which, it may be thought, an explanation taken 
 from the duration of impressions on the retina 
 will better apply ; I mean the appearance of a 
 fiery circle, when any red-hot body is moved 
 quickly round. But it seems, to me, that such 
 an explanation cannot even here be admitted. 
 For, if the circle depended upon the cause I 
 have mentioned, it could only be observed as 
 long as the impressions upon the retina were 
 also disposed in the form of a circle. Were this 
 broken upon, which it must be by every move- 
 ment of the eye, the appearance suggested by 
 the last impression would no longer be so ar- 
 ranged, with respect to the appearance suggested 
 by the present impression, as to lie with it in the 
 
IN OPTICS. oy 
 
 circumference of a circle; and hence some 
 very different figure would be observed. Every 
 person, however, may easily convince himself, 
 that the circular form of the fiery appearance 
 is equally perceived, whether the eye be at rest, 
 or be moved in the most irregular manner. 
 
 If these arguments be thought sufficient for 
 the purpose I had in view, it must also follow 
 from them, since the fact still remains to be ex- 
 plained, why we apparently see so many objects 
 with equal distinctness at once, that past im- 
 pressions upon the retina are perceived as pre- 
 sent, by means of some higher faculty than that 
 of sight. This faculty cannot, with propriety, 
 be named memory y as it is essential to a thing's 
 being remembered, that it be perceived #s past. 
 Nor can it be called imagination, since we be- 
 lieve in the present existence of what it per- 
 ceives. In one point of view it may seem 
 rather a defect in our natures, that we should 
 not be able to distinguish between things past 
 and present. However this may be, I am in- 
 clined to be of opinion, that many other pheno- 
 mena, both of thought and external sense, are 
 partly to be resolved into the same general fact. 
 From the present instance of it, we learn, that 
 several muscular actions may be performed, in 
 succession, during the least perceptible portion 
 of time. 
 
EXPERIMENTS, &c. 
 
 The question I have just treated, naturally 
 gives rise to another : Would it have been 
 more to our advantage, if accurate vision, in- 
 stead of being confined to one point of the 
 retina, had been possessed by every part of that 
 membrane? I answer, I think not, for the fol- 
 lowing reasons. 
 
 First ; The diffusion of such a property over 
 the whole retina would be of little use, unless 
 our power of attention was also increased. For 
 we should otherwise be still unable to perceive 
 more than one visible object at once, with di- 
 stinctness, since, by our present constitution, 
 we are capable of attending accurately to only 
 one thing at a time. The only benefit, indeed, 
 I can see to arise from such a condition of the 
 retina, is this; That our attention might be 
 shifted more quickly from picture to picture on 
 that membrane, than our eyes can be turned 
 from one external object to another. This ad- 
 vantage, however, would be far out-weighed by 
 an inconvenience accompanying it. For it is 
 a well-known fact, with respect to perception, 
 that we are capable of attending, more or less 
 accurately, to any particular impression upon 
 the senses, in proportion to the inferior force of 
 other impressions, which are at the same time 
 received. But in the supposed state of the 
 retina, there would be, almost always, several 
 
IN OPTICS. 91 
 
 impressions of the same strength as the one to 
 which we might desire particularly to attend ; 
 whereas, in its present state, the vivacity of the 
 impression from the object, to which we turn 
 the optic axis, most commonly surpasses, con- 
 siderably, that of every other upon the same 
 membrane ; by which means our attention is 
 rendered less liable to interruption. 
 
 Secondly ; The extension of accurate vision, 
 to every part of the retina, would deprive us, in 
 great measure, of the help, which we obtain, at 
 present, from the eye, in learning the thoughts 
 of other men. As far as I have been able to 
 observe, the changes produced by our internal 
 feelings, upon the state of the eye itself* are 
 very few, and relate only to the quantity of 
 moisture, which is diffused over its surface, and 
 the degree of fulness in the blood-vessels, which 
 are spread upon its white and glistening part. 
 Both of these circumstances, however, are 
 similarly altered by opposite passions, and, con- 
 sequently, neither of them can be regarded as 
 the appropriate expression of any. The whole 
 variety, then, of the expressions of feeling which 
 are justly attributed to the eye, must, I think, 
 depend upon its motions. Some of these are 
 the immediate effects of certain passions ; the 
 eye, for instance, being moved differently in 
 anger and in grief 5 and such may be esteemed 
 
EXPERIMENTS, &c. 
 
 as directly expressive of the passions by which 
 they are produced. But the far greater number 
 of them do little more, than merely point out 
 the external cause, or object of the sentiment, 
 which the changes of other parts of the counte- 
 nance declare to exist within us ; or distin- 
 guish certain external appearances depend- 
 ing upon a mental cause, from similar appear- 
 ances arising from a different source. Thus, 
 blushing is often distinguished from an ac- 
 cidental flush of the cheek, by the eye being 
 turned away from the person who occasions it. 
 That many of the expressions, which we at- 
 tribute to the eye, do in fact depend on changes 
 in other ^parts of the countenance, is evident 
 from the alterations we think induced upon it, 
 by the eyelashes falling off from disease, by a 
 slight inflammation of the edges of the eye-lids, 
 without its being communicated to the eye 
 itself, by artificially colouring the eye-brows, 
 and by many other similar circumstances. And 
 how essential to the right understanding of the 
 expressions of the other features, are the mo- 
 tions of the eyes, when conducted with design, 
 and properly directed, must be known to every 
 one, who has attended in discourse to the 
 countenances of very short-sighted people, and 
 more especially to those of persons afflicted with 
 blindness from a gutta serena, in which the eye, 
 
IN OPTICS. &3 
 
 : --'^\ ' .-.- * 
 
 with respect to its external condition, seems 
 
 without fault. But whatever is the assistance 
 the motions of the eye afford, in expressing our 
 internal feelings, the whole of it must ulti- 
 mately be referred to the circumstance of ac- 
 curate vision being confined to one point of the 
 retina ; since the intent of those motions is, to 
 bring the pictures of external objects upon the 
 most sensible part of that membrane. Their 
 necessity, therefore, would no longer exist, if 
 the same property were extended, and the ad- 
 vantages we at present enjoy from them, would, 
 consequently, cease. 
 
94 EXPERIMENTS, &c. 
 
 ARTICLE III. 
 
 On the Connexion between the different refractive States 
 of the Eyes, and, the different Inclinations of the Optic 
 Axes to each other. 
 
 I HAVE mentioned, in my Essay upon Single 
 Vision with Two Eyes *, that I had been con- 
 vinced, by experiments almost without number, 
 that every different degree of the mutual in- 
 clination of the optic axes, is attended by a dif- 
 ferent state of the refracting power of each eye. 
 The experiments I there alluded to were chiefly 
 of this sort. I placed a luminous point, most 
 commonly the reflected image of the flame of a 
 candle from the bulb of a small thermometer, 
 at such a distance, that when both my eyes 
 were accurately directed to it, its visible ap- 
 pearance to one of them was likewise that of a 
 point. Keeping then the axis of this eye fixed, 
 and making the other to cross it, sometimes 
 before and sometimes behind the luminous 
 point, I found that in both cases it appeared as 
 a surface to the eye, in the axis of which it was 
 
 * Page 66. 
 
IN OPTICS. 95 
 
 situated j and that the more remote from it was 
 the concurrence of the axes, the larger was the 
 luminous surface. Now when the axes met be- 
 fore the point, the apparent surface must have 
 been occasioned by the rays coming to a focus, 
 previously to their incidence upon the retina ; 
 because, when I passed my finger across the 
 eye by which it was seen, its parts disappeared, 
 in an order corresponding to the direction in 
 which the finger moved. The disappearance 
 of the parts was in an order, contrary to the 
 motion of the finger, when my optic axes inter- 
 sected each other beyond the point; which is 
 an equal proof, that the rays, in that case, 
 tended to a focus behind the retina. 
 
 One application of this fact has already been 
 shown*, and I shall now proceed to mention 
 several other phenomena in vision, which it 
 may serve either in whole, or in part, to ex- 
 plain. 
 
 1. It accounts for the following beautiful 
 observation made by Aguiloniusf, that if we 
 close one eye, and look with the other at an 
 object placed in its own axis, we shall not be 
 able to see this object distinctly, unless we also 
 direct to it the axis of the closed eye. For in 
 persons, who are neither presbytic nor myopic, 
 
 * Essay upon Single Vision, p. 66. 
 f Aguilonii Optica, page 84. 
 
96 
 
 EXPERIMENTS, &c. 
 
 the refractive states of the eyes are so adapted 
 to the mutual inclinations of the optic axes, 
 that pencils of rays flowing from bodies at mo- 
 derate distances are more accurately collected 
 upon the retina, when they are situated at the 
 intersection of those lines, than if their position 
 was, in any considerable degree, either nearer 
 or more remote. The reason given by Aguilonius 
 himself, is, that the mind perceives only those 
 objects distinctly, which are placed at the con- 
 course of the optic axes. But the following 
 experiment proves that the solution is true no 
 farther, than as it coincides with the one I have 
 advanced. Hold, in the axis of either eye, a 
 concave lens, at such a distance, that the letters 
 of a book, placed a little farther off, may appear 
 through it very indistinct to that eye, when 
 both axes are directed to any particular word. 
 View afterward the lens itself with both eyes, 
 and the letters will immediately become more 
 distinct. In this experiment then, an object is 
 more accurately perceived when distant from 
 the concourse of the optic axes, than when 
 situated exactly in it. 
 
 It may be said, perhaps, that the distinctness 
 of the letters is here to be attributed to the con- 
 traction of the pupil, which is occasioned by 
 the eyes being directed to a nearer object than 
 they were formerly. But that this is not the case* 
 
IN OPTICS. 97 
 
 maybe made evident by another experiment: 
 Place a convex lens in such a manner before 
 one eye, that the flame of a candle, at the 
 distance of two or three feet from the face, 
 may appear indistinctly terminated to that eye, 
 when both axes are pointed to it. The same 
 eye being kept fixed, let the two axes after- 
 ward meet beyond the flame, and it will now 
 be seen much better defined, though the pupil 
 is at the same time become larger. The insuf- 
 ficiency of the explanation of Aguilonius, is also 
 proved, by a circumstance frequently noticed 
 in persons who are very short-sighted ; for such 
 are observed, when they desire to view an ob- 
 ject with much attention, to hold it close to 
 one eye, and to turn the other aside ; in this 
 way occasioning the two axes to meet very re- 
 motely from the object. 
 
 2. The reason commonly given, why short- 
 sighted people view an object with one eye 
 only in the manner above-mentioned, is, that 
 by this means they avoid the uneasy straining 
 of the muscles, which must be employed to 
 direct both axes to the same point. But it is 
 evident they must derive from the practice this 
 farther advantage, that, as their optic axes are 
 now parallel to each other, or nearly so, they, 
 consequently, see the object in the least refrac- 
 tive state of their eyes. Pencils, therefore, will 
 
 H 
 
98 EXPERIMENTS, &c. 
 
 now have their focuses in the retina, the rays 
 of which would have crossed each other, before 
 they fell upon it, had both the axes been 
 directed to the object. 
 
 3. Spectacles were long employed, before the 
 manner in which they assisted sight was known. 
 About the year 1601, this was proposed as the 
 subject of a question to Kepler,* by his prin- 
 cipal patron at that time, Ludovie L. B. a Die- 
 trickstein, a learned nobleman of Austria. The 
 first answer he gave was, that convex glasses 
 were of use, by occasioning objects to appear 
 larger. But his patron observed, that if ob- 
 jects were rendered by them more distinct, be- 
 cause larger, no person would be benefited by 
 concave glasses, since these diminish objects. It 
 was not till three years after, that, in conse- 
 quence of finding out in what manner vision is 
 performed, he was able to give a just solution 
 of this problem, though his attention had been 
 directed to it during the whole of that interval. 
 According to the discovery he then made, con- 
 vex glasses were said by him to assist the sight 
 of presbytic persons, by so altering the directions 
 of rays diverging from a near object, that they 
 shall afterward fall upon the eye, as if they 
 had proceeded from a more remote one 5 and 
 
 * Paralipomena in Vitellionem, p. 200. 
 
IN OPTICS. 99 
 
 concave glasses to benefit the myopic, by 
 producing a contrary effect upon rays which 
 diverge from a distant object. Now it is ma- 
 nifest, that by this theory, to which I believe 
 no addition has been made by any succeeding 
 writer, precisely the same effects are attributed 
 to lenses, whether they be employed singly, or 
 in the form of spectacles. I am inclined, how- 
 ever, to think, that a difference, sometimes at 
 least, exists here, which has hitherto escaped 
 notice. For in regard to such spectacles as I 
 have tried upon myself, I have always found, 
 that, when I looked with them at objects placed 
 at moderate distances directly before me, my 
 optic axes passed through the glasses, more in- 
 wardly than their centres. With respect, there- 
 fore, to spectacles for long-sighted people, as 
 the inner halves of their glasses may be re- 
 garded as two prisms, whose refracting angles 
 face each other, to have allowed both my eyes 
 to receive through them pencils of rays from 
 the same point of an object, the intervals of my 
 pupils must have been less than was necessary 
 for that purpose in naked vision. The conse- 
 quence of which would be, an increase of the 
 refractive power of my eyes. Again ; as the 
 like parts of glasses in spectacles for short- 
 sighted persons, may be esteemed to be two 
 prisms, the refracting angles of which are 
 
100 EXPERIMENTS, &c. 
 
 turned from each other, the interval of the 
 pupils must have been increased, and the re- 
 fracting power of my eyes by this means di- 
 minished, when I looked at an object through 
 them, which was directly before rne. And 
 effects similar to what I have mentioned, must 
 have followed my viewing objects placed ob- 
 liquely, through glasses of both kinds. Here 
 then is one advantage, which persons, who see 
 with both eyes, either do or may enjoy from 
 spectacles, but which they cannot derive from 
 using single glasses. For if they are presbytic, 
 they can see an object by the means of them 
 with a higher refractive state of the eyes, than 
 if the optic axes met there, as in naked vision ; 
 and if myopic, with a less. It is also worthy 
 of remark, that this advantage does not ulti- 
 mately tend to increase the evil, which first 
 gives occasion for spectacles. On the con- 
 trary, if what every writer upon vision asserts 
 be true, that we are apt to become short or 
 long-sighted, according as we are much accus- 
 tomed to view near or distant objects, it must 
 serve to diminish that evil. In support of this 
 opinion, I shall mention a fact, with which I 
 have been made acquainted by Mr. George 
 Adams*, of this place, who is not only well 
 
 * Mathematical Instrument Maker to the King. 
 
IN OPTICS. 101 
 
 skilled in the theory of vision, but, from his 
 situation, as an artist, has better opportunities, 
 than most persons, of learning such matters. 
 The fact is this, that lie does not know a short- 
 sighted person, who has had occasion to in- 
 crease the depth of his glasses, if he began to 
 use them in the form of spectacles ; whereas 
 he can recollect several instances, where those 
 have been obliged to change their concave 
 glasses repeatedly, for others of higher powers, 
 who had been accustomed to apply them to one 
 eye only. This indeed may have happened by 
 accident ; but at any rate, the fact is worthy of 
 farther attention and inquiry. 
 
 It would seem, however, that the long-sighted 
 derive more benefit from the alteration in the 
 mutual inclinations of the optic axes, which, is 
 produced by spectacles, than the short-sighted. 
 For, as .the inner halves of the convex glasses 
 are to ,be regarded as prisms, with their re- 
 fracting angles continually increasing as we 
 approach their edges, if two objects, situated 
 at different distances, be viewed successively 
 through them, the inclination of the optic axes 
 to each other, when the nearer object is seen, 
 must bear a higher proportion to their inclina- 
 tion, when we look at the one more remote, 
 than the different inclinations of the optic axes 
 do to each other, when they are successively 
 
102 EXPERIMENTS, &c. , 
 
 directed to the same objects, without the inter- 
 vention of such glasses. Hence the nearer the 
 object is, the greater will be the effect of the 
 variation in the inclination of the axes produced 
 by spectacles with convex glasses ; which is 
 the order of things, the best adapted to the 
 wants of those who use them. But with respect 
 to short-sighted persons, since the refracting 
 angles of their glasses, considered as prisms, 
 decrease, in proportion as the objects seen 
 through them become more remote ; they must, 
 consequently, derive the least benefit from an 
 alteration in the mutual inclinations of the 
 optic axes occasioned by their spectacles, at 
 the time they most require it. 
 
 If it were asked, then, what is the real founda- 
 tion of the common reproach against spectacles 
 for long-sighted people ? I should answer, a 
 very different one from that, which is, for the 
 most part, assigned. For the change, in the 
 conformation of the eyes, which renders them 
 useful, seems to be one of those which nature 
 has destined to take place at a particular age, 
 and to which there is no gradual approach 
 through the preceding course of life. A per- 
 son, for instance, at forty, sees an object di- 
 stinctly, at the same distance that he did at 
 twenty. When he draws near to fifty, the 
 change I have spoken of commonly comes on, 
 
IN OPTICS. 
 
 and obliges him in a short time to wear specta- 
 cles. As it proceeds, he is under the necessity 
 of using others with a higher power. But, in- 
 stead of supposing that his sight is thus gra- 
 dually becoming worse, from a natural process, 
 he attributes the increase of the defect in it to 
 his too early and frequent use of glasses. Upon 
 the whole, I should draw this inference from 
 what has been said, that no person, whose sight 
 begins to grow long, ought to be, in the least, 
 prevented from enjoying the immediate advan^ 
 tage which, spectacles will afford him, by the 
 fear that they will ultimately injure his eyes j 
 not that I think the convexity of each glass, 
 considered by itself, can do 110 harm, but that 
 I believe the benefit, arising from the combina- 
 tion of the two, to be at least sufficient to com- 
 pensate it. Whether those, who have a ten- 
 dency to short-sight, should be also early in 
 their employment of spectacles, I shall not pre- 
 tend to say ; as there is not the same ground, 
 from theory, for supposing, that the benefit 
 arising from the combination of the two glasses 
 is able to over-balance the injury, produced by 
 the concavity of each considered separately. 
 
 All that I have said, however, upon the sub- 
 ject of spectacles, proceeds upon the supposi- 
 tion, that, when objects, placed directly before 
 us, at moderate distances, are viewed through 
 
104 EXPERIMENTS, &c. 
 
 them, the optic axes penetrate the glasses more 
 inwardly than their centres. But I can be by 
 no means sure, that the interval of the pupils 
 of other persons, bears the same proportion to 
 the interval of the centres of the lenses in spec- 
 tacles, as that of mine does. It concerns those, 
 therefore, who are choosing them, to have at- 
 tention to this circumstance. To me it appears 
 proper, that the glasses in spectacles, both for 
 long and short-sighted people, should be so far 
 asunder, that, when we look at a very remote 
 object directly before us, our optic axes may 
 pass exactly through their centres. For if the 
 centres of convex glasses be nearer to each 
 other, very remote objects will appear double ; 
 and if they are more distant, though the object 
 viewed be infinitely far from us, the optic axes 
 will, however, be inclined to one another, and 
 the refractive power of the eyes increased, 
 when this may be of disservice ; since there 
 are few eyes which are not able, even without 
 the aid of the convexity of a glass, to bring 
 parallel rays to a focus upon the retina. If the 
 centres of lenses in spectacles, for the short- 
 sighted, be less distant than what I have men- 
 tioned, the optic axes must be bent toward 
 each other, when very remote objects are seen, 
 and the refractive state of the eye, therefore, 
 heightened, which is the very reverse of whaj: 
 
IN OPTICS. 106 
 
 is here to be desired. Should the interval of 
 the centres of those lenses be greater, objects 
 at very considerable distances will be seen 
 double. 
 
 There are two other observations relative to 
 glasses for the sight, which I wish to add to 
 what I have already said upon this subject. 
 Thejfirst is, that the single convex glasses with 
 which some persons read, must be very inju- 
 rious, if they be sufficiently large, to admit the 
 same object to be seen with both eyes. For as 
 both axes will then pass through them, one on 
 each side of the centre, the interval of the pu- 
 pils will be widened, and the refracting power 
 of the eyes be diminished ; so that here a dis- 
 advantage is to be added to the prejudice of the 
 convexity of the glass, not a benefit to be placed 
 against it, as in the case of common spectacles 
 for the long-sighted. If, indeed, the defect in 
 sight does not arise from the conformation of 
 the eye, but from a want of transparency in its 
 cornea or humours, then such glasses, by mag- 
 nifying objects, will be useful, for the same 
 reason, that, in a very faint light, we can read 
 a book of a large print, with more ease than 
 one of a smaller. The second observation is, 
 that if flat-sided prisms were fixed in spectacle- 
 frames, with their refracting angles toward each 
 other, they would assist the long-sighted some- 
 
106 EXPERIMENTS, &c. 
 
 what, without producing the evil which is said 
 to arise from the convexity of lenses ; and spec- 
 tacles of this kind might, with more propriety, 
 I think, than any others, be called presemers. 
 A like combination of such prisms, but with 
 their angles turned the other way, might, when 
 the object was moderately distant, be of service 
 to the short-sighted. But objects, very remote, 
 would be made by them to appear double. 
 
 
IN OPTICS. 107 
 
 ARTICLE IV. 
 
 On the Limits of perfect or distinct Vision. 
 
 DR. Jurin*, I believe, was the first who di- 
 stinguished between perfect and distinct vision ; 
 confining the former term to those cases, where 
 the rays of a single pencil are collected to a sin- 
 gle point of the retina ; and marking, by the 
 latter, the perception we have of visible objects, 
 when the rays of the pencils, diverging from 
 them, though not collected to single points of 
 the retina, yet occupy so small portions of it, as 
 to allow the objects to be distinctly seen. But 
 as few authors have adopted this division,' I 
 shall, in the present article, use both terms in 
 the sense, which he has appropriated to the 
 first. Neither of them is indeed free from ob- 
 jection, since bodies to be distinctly or perfectly 
 seen, not only require, that their pictures should 
 be accurately formed upon the retina, but that 
 they should fall upon a particular part of it. 
 
 Although it has long been a subject of in- 
 quiry, within what limits of distance objects 
 are distinctly perceived by sight, yet the only 
 
 * Essay on distinct and indistinct Vision. 
 
108 EXPERIMENTS, &c. 
 
 experiments I have met with in books, which 
 have been made, with any tolerable show of ac- 
 curacy, to determine this matter, are those of 
 Dr. Porterfield. I shall not here say what 
 they are, as his Treatise is in every body's 
 hands, but shall only mention the principal 
 conclusions which he drew from them ; .first, 
 that objects could be distinctly seen by him, 
 that is, the pencils of rays which came from 
 them could be accurately collected to points 
 upon the retina, when their distances from his 
 eye did not exceed twenty-seven inches, and 
 were not less than seven ; and secondly, that, 
 as often as the axes of both eyes were directed 
 to any one point, situated within those distances, 
 the rays proceeding from it had their focus in 
 each retina. 
 
 As the results of some experiments, which I 
 have made upon the same subject, differ from 
 these conclusions of Dr. Porterfield, I have read 
 over what he has written upon the matter with 
 more than ordinary attention, and I think I can 
 thence show reason, why they should not be 
 received without caution. For, in the first 
 place, his experiments are related so circum- 
 stantially, and with such an appearance of ac- 
 curacy in the making of them, that you would 
 scarcely suppose he left the least possible room 
 for error. And yet, after finishing his account 
 
IN OPTICS. 109 
 
 of them, he tells us, that he would have repeated 
 them with more care and exactness*, had he not 
 been interrupted. Secondly, his experiments 
 were made upon one eye only, though his con- 
 clusions apply to both eyes ; an inaccuracy 
 which gives occasion to suspect others. Lastly, 
 he says, that he could not see an object distinctly 
 at the distance of seven inches, unless both axes 
 were pointed to another object, at only half that 
 distance. Had he then directed both axes to 
 an object seven inches distant, which he does 
 not mention he ever did, it must consequently 
 have been seen indistinctly ; and yet one of his 
 conclusions states, that objects, distant from 
 about seven, to about twenty-seven inches, were 
 always distinctly seen, when the axes of both 
 eyes were directed to them. Such are the rea- 
 sons which lead me to think, that the whole of 
 the difference, between the results of the expe- 
 riments of Dr. Porterfield and myself, is not to 
 be attributed to a difference in the structure of 
 our eyes. 
 
 The experiments, which I made upon this 
 subject, were with luminous points. They 
 proved to me,Jlrst, that, when both optic axes 
 are directed to any object, placed at a less 
 distance from my eyes than about seventeen 
 
 Treatise on the Eye, Vol. I. p. 423.' 
 
110 EXPERIMENTS, &c. 
 
 inches, my vision of it by the left eye is in- 
 distinct, from the rays of light tending to fo- 
 cuses behind the retina ; secondly, that my vision 
 by the same eye is perfect, if the object seen, 
 and to which both axes are turned, be from 
 about seventeen to about nineteen inches di- 
 stant ; thirdly, that the vision of my left eye 
 becomes again imperfect, if the object be moved 
 to a greater distance than that of nineteen 
 inches, the rays being now collected to focuses, 
 previously to their falling upon the retina ; and 
 fourthly, that I have, by my right eye, imperfect 
 vision of all objects, to which I direct both axes, 
 unless their distances be so great, ' that the rays 
 of each pencil, proceeding from them, may be 
 regarded as parallel. , 
 
 A conclusion is furnished by these experi- 
 ments, similar to one, which was drawn by Mr. 
 Delahire *, from some made by himself; namely, 
 that each eye sees objects distinctly only at one 
 distance ; as I take for granted, that, in every 
 case of ordinary vision, both axes are directed 
 to the object which is viewed. But Mr. Dela- 
 hire drew a second conclusion from his experi- 
 ments, which he seems to have regarded only as 
 another expression of the first, but which, in 
 truth, includes a very different fact. It was, 
 
 * Memoires de Mathematiquc et de Physique, 4to. p. 2Q8. 
 
IN OPTICS. Ill 
 
 that the refractive state of the eye is always 
 the same, whether we look at a very near or a 
 very distant object. The following observations, 
 however, will prove the contrary, at the same 
 time that they show, in what I farther differ 
 from Dr. Porterfield. 
 
 1. Though an object, to which both axes are 
 pointed, does not appear distinct to my left eye, 
 unless it be from about seventeen to about 
 nineteen inches distant ; nor to my right eye, 
 unless it be at a very considerable distance ; 
 yet 1 find, that when the axes are made to meet 
 at a point, about two inches distant from a line 
 connecting the two pupils, which however can- 
 not be effected without much straining, my left 
 eye will now see an object distinctly, which is 
 only about seven inches from it, and my right 
 eye will at the same time see an object distinctly, 
 the distance of which is about ten inches. I 
 find also, that my left eye is made to see an 
 object distinctly, though placed more than nine- 
 teen inches from it, if I direct both axes to a 
 point still more remote. 
 
 2. I formerly mentioned, that every degree 
 of the mutual inclination of the optic axes is 
 attended, by a particular state of the refracting 
 power of each eye. But I must now remark, 
 that these states are sometimes subject to slight 
 variations, while the inclinations of the optic 
 
112 EXPERIMENTS, &c. 
 
 axes to each other remain the same. For 
 I find, that, when a luminous point, to which 
 both axes are turned, is distinctly seen by my 
 left eye, I can, by certain efforts not easily to 
 be described, but without changing the position 
 of either axis, make it afterward appear as a 
 surface, and this too, at one time, from the rays 
 coming to a focus too soon, and at another, too 
 late, for perfect vision*. One instance of these 
 variations deserves to be minutely described, as 
 it proves, that the refractive power of the eyes 
 is subject to greater changes, than what are 
 shown by any experiments I have met with in 
 authors. When I look attentively at a bright 
 star, with the optic axes parallel to each other, 
 it appears to my left eye a surface of some ex- 
 tent, and to my right eye, though not a point, 
 yet a surface of very small extent, as small as- 
 
 * The variations, however, seem produced in such a man- 
 ner, that the middle of the set belonging to one degree of 
 the mutual inclination of the optic axes, is always differen t 
 from the middle of the set belonging to another degree of 
 their inclination ; and that, when no other effort is made, 
 than to direct both axes to the same object, the eyes always 
 assume the middle state of the refractive power, which ac- 
 companies that particular inclination of the axes. No argu- 
 ment, therefore, can hence be derived against the applications 
 I formerly made of the general fact, respecting the connexion 
 of the refractive states of the eyes with the mutual inclina- 
 tions of the optic axes. 
 
IN OPTICS. 113 
 
 the sphericity of the cornea and crystalline, the 
 various refrangibility of the different kinds of 
 light, and the width of the pupil at night, can 
 be supposed to allow; for I find, that, if I now 
 pass a needle across the axis of the right eye, 
 its shadow will not be seen. But should I, after 
 this, withdraw my accurate attention from the 
 star, and view it in the state of sight we have, 
 when we are said to be in a reverie, in which, 
 though our eyes are open, we are yet scarcely 
 conscious of seeing surrounding objects, the 
 appearance to the right eye expands itself, and 
 if a needle be again passed before this eye, its 
 shadow will be observed to move over the star, 
 in a direction contrary to that of the needle 
 itself; a sure indication that the rays of light 
 now tend to a focus behind the retina. In the 
 same state of things, the appearance -of the star 
 to the left eye contracts, and if a needle be 
 held before the eye, no shadow is seen ; a sign 
 that the rays are collected to a focus on the 
 retina ; whereas they had formerly crossed one 
 another before they reached that membrane. 
 
 Upon the whole then it is manifest, from the 
 experiments I have related, that my left eye 
 can collect to focuses in the retina, rays which 
 proceed from objects at every distance what- 
 soever, not less than seven inches; that my 
 right eye can collect to focuses in the retina, 
 
 i 
 
114 EXPERIMENTS, &c. 
 
 rays which proceed from objects at every di- 
 stance whatsoever, not less than ten inches, and 
 even such as are somewhat convergent, since it 
 can make those, which are parallel, to meet be- 
 fore they fall upon the retina ; and lastly, that, 
 while both the optic axes are directed to a 
 point within the limits of distinct vision, the 
 rays proceeding from it are never accurately 
 collected to focuses in both retinas, and scarcely 
 ever to a focus in either retina. These are like- 
 wise the > principal circumstances, in which my 
 experiments differ in their results from those of 
 Dr. Porterfield. 
 
 In making such experiments with luminous 
 points, one or other of two appearances very 
 constantly occurs, neither of which, as far as I 
 know, has been spoken of by any preceding 
 author. The most proper way of mentioning 
 what they are, is, perhaps, to show what ought 
 to happen in those situations, in which they 
 are observed. 
 
 When a beam of white light passes, obliquely, 
 from one medium into another of different re- 
 fractive power, its variously coloured rays must 
 begin to diverge from each other, at the point 
 of the beam's incidence upon the latter me- 
 dium. In achromatic telescopes, the mutual 
 separation of these rays is checked, and its 
 farther increase prevented, before it becomes 
 
IN OPTICS. 115 
 
 perceptible to sense, by the contrary refractions 
 which they undergo, from passing, successively, 
 through the different parts of the object-glass. 
 Hence, some have imagined, that, since objects, 
 in ordinary vision, are seen without colour, as 
 far as this depends on the refractions of the 
 eye, nature has furnished us with an instru- 
 ment, constituted upon principles similar to 
 those of the object-glass of an achromatic tele- 
 scope. But every one, the least acquainted 
 with the structure of the eye, must know, that 
 this cannot be the case, as the refractions in it 
 are all made one way*. And there are experi- 
 mental proofs, that compounded light is always 
 separated into its parts, by passing through the 
 eye. For if we interpose any opake substance 
 between us and a luminous body, so that only 
 a very small portion of this may remain visible, 
 it will appear to consist of three differently 
 coloured parts, red, yellow, and blue* The 
 reason, therefore, of objects being, for the most 
 part, seen colourless, must be elsewhere sought f. 
 Now let us suppose, that a luminous point is 
 the only object which is seen at any one time ; 
 should the focus of its mean refrangible rays be 
 
 * There are indeed some exceptions to this, but not of 
 sufficient consequence to affect the present argument. 
 
 f Dr. Maskelyne has very learnedly treated this subject ii* 
 the Philosophical Transactions, vol. Ixxix. part 2. 
 
116 EXPERIMENTS, &c. 
 
 anterior to the retina, the middle of its picture 
 upon that membrane must be chiefly composed 
 of the less refrangible rays ; and this must be 
 the reason, that, when I look attentively at a 
 bright star with my left eye, the centre of it 
 always appears of a light orange colour. As 
 the beams, however, from the luminous point, 
 which enter the eye near to its axis, suffer but 
 little refraction, the brightness of their white 
 light, will, in great measure, overpower the 
 colour given to the middle of the picture upon 
 the retina, by the less refrangible rays of those, 
 which enter the eye at a distance from its axis. 
 Were you then to intercept the former beams, 
 the effect I have mentioned of the latter, must 
 be more observable ; and hence it is, that when 
 I place a pin or needle between my eye and 
 a luminous point, the rays of which come to 
 a focus before they fall upon the retina, the 
 shadow, instead of appearing black, is always 
 of a red or deep orange colour ; which is one 
 of the phenomena respecting luminous points, 
 to which I have alluded. 
 
 On the other hand, should the focus of the 
 mean refrangible rays of a luminous point lie 
 behind the retina, the middle of the picture 
 there will be principally formed of the more re- 
 frangible rays ; and if the beams, which enter 
 the eye near to its axis, be also in the present 
 
IN OPTICS. 117 
 
 case intercepted, the effect of the latter rays, in 
 giving colour to the middle of the picture, will 
 consequently be rendered more evident. Hence 
 it is, that, when a luminous point is not suf- 
 ficiently remote for distinct vision, the seeming 
 shadow upon it, occasioned by any small opake 
 object held before my eye, is always blue; and 
 this is the second of the appearances, which I 
 said are frequently to be observed, in experi- 
 ments upon luminous points. 
 
AN 
 
 ESSAY 
 
 ON 
 
 DEW, 
 
 AND 
 
 SEVERAL APPEARANCES 
 
 CONNECTED WITH IT. 
 
TO 
 
 JAMES DUNSMURE, ESQUIRE, 
 
 MERCHANT IN LONDON. 
 
 MY DEAR SIR, 
 
 Without your aid, I should, in all 
 probability, never have acquired the knowledge, upon 
 which the following Essay is principally grounded ; since 
 I could not, I believe, have found any other place, con- 
 sidering that I was obliged to be daily in London, so well 
 fitted for my experiments, as that which you permitted 
 me to use during a very long time, though manifestly to 
 the great inconvenience of yourself and your family. I 
 beg leave to assure you, that I feel this kindness most 
 strongly, and that my gratitude for it will never cease to 
 exist. 
 
 I am, 
 
 My dear Sir, 
 
 Your most obedient Servant, 
 
 and faithful Friend, 
 WILLIAM CHARLES WELLS. 
 
 Augu$tt5, 1814. 
 
The following notice was prefixed by the Author to the 
 second edition, published in 1815. 
 
 The infirm state of the Author's health having prevented 
 him, since the publication of the former edition of his 
 Essay on Dew, from making experiments in the open air 
 during the night, and his reading having in the meanwhile 
 been directed to other objects, the present edition of that 
 Essay will be found to contain almost nothing more than 
 
 . the other. The chief difference between the two arises 
 from a change in the form of several of his expressions. 
 He has, for instance, altered the expression of ' saturation 
 with moisture' to that of ' repletion with moisture', in 
 order to avoid the appearance of maintaining, that com- 
 mon air is capable of dissolving water; a tenet uncon- 
 nected with his theory. Sometimes he has subjoined to 
 the phrase, which he now employs, on that subject, the 
 words ' in a pellucid state ;' when this addition has not 
 been made, he wishes it to be understood. 
 
ESSAY ON DEW, &c. 
 
 INTRODUCTION. 
 
 I WAS led, in the autumn of 1784, by the event 
 of a rude experiment, to think it probable, that 
 the formation of dew is attended with the pro- 
 duction of cold. In 1788, a paper on hoarfrost, 
 by Mr. Patrick Wilson of Glasgow, was pub- 
 lished in the first volume of the Transactions 
 of the Royal Society of Edinburgh, by which 
 it appeared, that this opinion had been enter- 
 tained by that gentleman, before it had oc- 
 curred to myself. In the course of the same 
 year, Mr. Six of Canterbury mentioned in a 
 paper communicated to the Royal Society, that, 
 on clear and dewy nights, he always found the 
 mercury lower in a thermometer laid upon the 
 ground, in a meadow in his neighbourhood, 
 than it was in a similar thermometer suspended 
 in the air, six feet above the former ; and that, 
 upon one night, the difference amounted to 5 
 of Fahrenheit's scale. Mr. Six, however, did 
 
124 ESSAY 
 
 not suppose, agreeably to the opinion of Mr. 
 Wilson and myself, that the cold was occasioned 
 by the formation of dew ; but imagined, that it 
 proceeded, partly from the low temperature of 
 the air, through which the dew, already formed 
 in the atmosphere, had descended, and partly 
 from the evaporation of moisture from the 
 ground, on which his thermometer had been 
 placed. The conjecture of Mr. Wilson, and 
 the observations of Mr. Six, together with many 
 facts, which I afterwards learned in the course 
 of reading, strengthened my opinion ; but I 
 made no attempt, before the autumn of 1811, 
 to ascertain by experiment if it were just, though 
 it had, in the mean time, almost daily occurred 
 to my thoughts. Happening, in that season, 
 to be in the country on a clear and calm night, 
 I laid a thermometer upon grass wet with dew, 
 and suspended a second, in the air, two feet 
 above the other. An hour afterwards, the 
 thermometer on the grass was found to be 8 
 lower, by Fahrenheit's division, than the one 
 in the air. Similar results having been obtained 
 from several similar experiments, made during 
 the same autumn, I determined, in the next 
 spring, to prosecute the subject with some de- 
 gree of steadiness, and with this view went 
 frequently to the house of one of my friends, 
 who lives in Surrey. At the end of two months, 
 
ON DEW, &c. 125 
 
 I fancied that I had collected information 
 worthy of being published; but fortunately, 
 while preparing an account of it, I met, by 
 accident, with a small posthumous work of Mr. 
 Six, printed at Canterbury in 1794, in which are 
 related differences observed on dewy nights, 
 between thermometers placed upon grass and 
 others in the air, that are much greater than 
 those mentioned in the paper presented by him 
 to the Royal Society in 1788. In this work, 
 too, the cold of the grass is attributed, in agree- 
 ment with the opinion of Mr. Wilson, altogether 
 to the dew deposited upon it. The value of 
 my own observations appearing to me now 
 much diminished, though they embraced many 
 points left untouched by Mr. Six, I gave up 
 my intention of making them known. Shortly 
 after, however, upon considering the subject 
 more closely, I began to suspect, that Mr. Wil- 
 son, Mr. Six, and myself, had all committed an 
 error, in regarding the cold, which accompanies 
 dew, as an effect of the formation of that fluid. 
 I, therefore, resumed my experiments, and 
 having, by means of them, I think, not only 
 established the justness of my suspicion, but 
 ascertained the real cause both of dew, and of 
 several other natural appearances, which have 
 hitherto received no sufficient explanation, I 
 venture now to submit, to the consideration of 
 
126 ESSAY ON DEW, &c. 
 
 the learned, an account of some of my labours, 
 without regard to the order of time, in which 
 they were performed, and of various conclusions 
 which may be drawn from them, mixed with 
 facts and opinions already published by others, 
 
PART I. 
 
 OF THE PHENOMENA OF DEW. 
 
 SECTION I. 
 
 Of Circumstances which influence the Production of Dew. 
 
 ARISTOTLE* and many other writers have 
 remarked, that dew appears only on calm and 
 serene nights. The justness of this observa- 
 tion, however, has not been universally ad^ 
 mitted. For M usschenbroek f says, that dew 
 forms in Holland, while the surface of the 
 country is covered with a low mist ; but, asf 
 he mentions at the same time, that it is de- 
 posited upon all bodies indiscriminately, the 
 moisture, of which he speaks, cannot properly 
 be called dew, as will be more distinctly seen 
 hereafter. Other writers of considerable reputa- 
 tion have also regarded clearness of the atmo- 
 sphere, as not being requisite for the production 
 of dew, misled, I believe, partly by theory, and 
 
 * Meteor. Lib. I. c. x. et De Mundo. c. iii. 
 t Nat. Phil. T. ii. De Rore. 
 
128 ESSAY 
 
 partly by observing on misty mornings copious 
 dews, which had been produced during preced- 
 ing clear nights. Respecting this point I can 
 aver, after much experience, that I never knew 
 dew to be abundant, except in serene weather. 
 In regard to the necessity of the air being still, 
 I know of no person who rejects it, except Mr. 
 Prieur*, a late French author of little con- 
 sideration, and he affirms, in opposition to the 
 most common observation, that a fresh wind is 
 requisite for the production of dew. 
 
 The remark of Aristotle, however, is not to 
 be received in its strictest sense, as I have fre- 
 quently found a small quantity of dew on grass, 
 both on windy nights, if the sky was clear, or 
 nearly so, and on cloudy nights, if there was no 
 wind. If, indeed, the clouds were high, and 
 the weather calm, I have sometimes seen on 
 grass, though the sky was entirely hidden, no 
 very inconsiderable quantity of dew. Again ; 
 according to my observation, entire stillness of 
 the atmosphere is so far from being necessary 
 for the formation of this fluid, that its quantity 
 has seemed to me to be increased, by a very 
 gentle motion in the air. Dew, however, has 
 never been seen by me, on nights both cloudy 
 and windy. 
 
 * Journal de 1'Ecole Poly technique, Tom. ii. 409. 
 
ON DEW, &c. 129 
 
 If, in the course of the night, the weather, 
 from being calm and serene, should become 
 windy and cloudy, not only will dew cease to 
 form, but that, which has formed, will either 
 disappear, or diminish considerably. 
 
 In calm weather, if the sky be partially co- 
 vered with clouds, more dew will appear, than 
 if it were entirely covered, but less than if it 
 were entirely clear. 
 
 Dew probably begins, in this country, to ap- 
 pear upon grass, in places shaded from the sun, 
 during clear and calm weather, soon after the 
 heat of the atmosphere has declined. My op- 
 portunities, however, for making such observa- 
 tions have not been numerous, since, while 
 pursuing this subject, I seldom went into the 
 country, till late in the afternoon ; but I have 
 frequently felt grass moist, in dry weather, 
 several hours before sunset. On the other 
 hand, I have scarcely ever known dew to be 
 present in such quantity upon grass, as to ex- 
 hibit visible drops, before the sun was very near 
 the horizon, or to be very copious, till some time 
 after sunset. It also continues to form, in 
 shaded places, after sunrise; but the interval 
 between sunrise, and its ceasing to form, is, 
 according to my observation, which, upon this 
 point, has not been extensive, considerably 
 shorter, than that between its first appearance 
 
130 ESSAY 
 
 in the afternoon, and sunset. Contrary, how- 
 ever, to what happens at sunset, if the weather 
 be favourable, more dew forms a little before, 
 and, in shaded places, a little after sunrise, than 
 at any other time. Musschenbroek, therefore, 
 errs greatly when he says, that dew does not 
 form after the sun has risen. The preceding 
 observations, on the early appearance of dew in 
 the afternoon, are to be restricted to what hap- 
 pens to grass, or other substances highly attrac- 
 tive of dew placed on the ground ; for it occurs 
 much later on similar substances, which are 
 elevated a few feet above the ground, though 
 upon these it continues to form, as long after 
 the rising of the sun, as upon the others, if they 
 be equally sheltered from the rays of that body. 
 The formation of dew, after it has once com- 
 menced, continues during the whole night, if 
 the weather remain still and serene. Mr. 
 Prieur, indeed, of whom I have already spoken, 
 asserts, that dew forms only in the evening and 
 morning, and that any which occurs in the 
 former season always disappears in the course 
 of the night. I can affirm, however, from long 
 experience, that grass, after having been dewed 
 in the evening, is never found dry until after 
 sunrise, unless the weather has, in the mean 
 time, changed. Upon one serene and still 
 night, I placed fresh parcels of wool upon grass 
 
ON DEW, &c. Ml 
 
 every hour, and by weighing each of them, after 
 exposure for an hour, found, that they had all 
 attracted dew. 
 
 When dew forms upon a smooth dense body 
 as glass, and it is only by means of such a body, 
 that the process can be accurately observed, the 
 appearances are altogether similar to those, 
 which occur on a like body, when exposed to 
 the steam of water, a little warmer than itself. 
 The exposed surface has first its lustre dimi- 
 nished, by a slight damp uniformly spread over 
 it. As the moisture increases, it gathers into 
 irregularly shaped flat drops, which are, at first, 
 very small, but afterwards enlarge and run into 
 one another, forming streamlets, by means of 
 which a great part escapes from the body which 
 had received it. 
 
 During nights, that are equally clear and 
 calm y dew often appears in very unequal quan- 
 tities, even after allowance has been made, for 
 any difference in their lengths. One great 
 source of these differences is very obvious. 
 For, it being manifest, whatever theory be 
 adopted concerning the immediate cause of 
 dew, that the more replete the atmosphere is 
 with moisture, previously to the operation of 
 that cause, the more copious will the precipita- 
 tion of water be, after this operation has com- 
 menced, all the circumstances, which tend to 
 
 K 2 
 
132 ESSAY 
 
 increase the quantity of moisture in the atmo- 
 sphere, must likewise tend to increase the pro- 
 duction of dew. Thus dew, in equally calm 
 and clear nights, is more abundant shortly after 
 rain, than during a long tract of dry weather. 
 It is more abundant, also, throughout Europe, 
 with perhaps a few exceptions, and in some 
 parts of Asia and Africa, during southerly and 
 westerly winds, than during those, which blow 
 from the north and the east. Aristotle* says, 
 that Pontus is the only country, in which dew 
 is more copious during a northerly, than during 
 a southerly wind. But a similar fact occurs in 
 Egypt ; for dew is scarcely ever observed there, 
 except while the Etesian winds prevail. Both 
 cases, however, though contrary to the letter, 
 are consonant with the spirit of the rule ; since 
 the north wind, in one country, proceeds from 
 the Euxine sea, and, in the other, from the Me- 
 diterranean. Another circumstance, of the same 
 kind with the blowing of wind from the south 
 and the west, as shewing that the air contains 
 much moisture, is the lessening of the weight of 
 the atmosphere. My experience on this point 
 has not, indeed, been great, as the falling of the 
 mercury in the barometer is very commonly 
 attended with wind or clouds, both unfavourable 
 
 * Meteor. Lib. 1 . c. x. 
 
ON DEW, &c. 133 
 
 to the production of dew ; but still the greatest 
 dew, I have ever witnessed, occurred while the 
 barometer was sinking. A corresponding ob- 
 servation is made by Mr. de Luc, who says, 
 that rain may be foretold, when dew is uncom- 
 monly abundant, in relation to the climate and 
 season*. 
 
 To the greater or less quantity of moisture in 
 the atmosphere, at the time of the action of the 
 immediate cause of dew, are likewise to be re- 
 ferred several other facts respecting its copious- 
 ness, the explanation of which is, perhaps, not 
 so apparent, as in the preceding examples. 
 
 In the first place ; dew is commonly more 
 plentiful in spring and autumn, than in summer; 
 the reason is, that a greater difference is ge- 
 nerally found between the temperatures of the 
 day and the night, in the former seasons of the 
 year, than in the latter. In spring, this cir- 
 cumstance is prevented often from having a 
 considerable effect, by the opposite influence of 
 northerly and easterly winds ; but, during still 
 and serene nights in autumn, dew is almost 
 always highly abundant. 
 
 In the second place ; dew is always very co- 
 pious, on those clear and calm nights, which 
 are followed by misty or foggy mornings j the 
 
 * Rech. sur les Mod. de 1' Atm. $ 725. 
 
134 ESSAY 
 
 turbidness of the air in the morning shewing, 
 that it must have contained, during the preced- 
 ing night, a considerable quantity of moisture. 
 
 Thirdly ; I have observed dew to be unusually 
 plentiful on a clear morning, which had suc- 
 ceeded a cloudy night. For the air, having in 
 the course of the night lost little or no moisture, 
 was in the morning more charged with watery 
 vapour, than it would have been, if the night 
 had also been clear. 
 
 Fourthly ; heat of the atmosphere, if other 
 circumstances are favourable, which, according 
 to my experience, they seldom are in this 
 country, occasions a great formation of dew. 
 For, as the power of the air, to retain watery 
 vapour in a pellucid state, increases considerably 
 faster, while its temperature is rising, than in 
 proportion to the heat acquired, a decrease of 
 its heat, in any small given quantity, during the 
 night, must bring it, if the temperature be high, 
 much nearer to the point of repletion, before it 
 be acted upon by the immediate cause of dew, 
 than if the temperature were low. We read, 
 accordingly, in the writings of those, who have 
 travelled into hot climates, of a copiousness of 
 dew frequently observed by them there, which 
 very much exceeds what occurs, at any time, in 
 this country. But even here, dew, though for 
 the most part scanty in our hottest season, is 
 
ON DEW, &c. 135 
 
 sometimes very abundant during it, an example 
 of which occurred to me on the night, common 
 to the 29th and 30th of July 1813 ; for on that 
 night, notwithstanding its shortness, more dew 
 appeared, than has ever been observed by me 
 on any other. 
 
 In the last place ; I always found, when the 
 clearness and stillness of the atmosphere were 
 the same, that more dew was formed between 
 midnight and sunrise, than between sunset and 
 midnight, though the positive quantity of mois- 
 ture in the air, must have been less in the 
 former, than in the latter time, in consequence 
 of a previous precipitation of part of it. The 
 reason, no doubt, is the cold of the atmosphere 
 being greater in the latter, than in the prior 
 part of the night. 
 
 But there are many circumstances, influ- 
 encing the quantity of dew, which, though 
 much more open to accurate observation, than 
 those tiitherto mentioned, are yet much less 
 easy to be understood. 
 
 In my first attempts to compare the quantities 
 of dew formed during different times, or in dif- 
 ferent situations, I attended only to the appear- 
 ance, which it made on bodies having smooth 
 surfaces. But quickly seeing this method to 
 be very imperfect, I next employed wool to 
 collect dew from the atmosphere, and found it 
 
136 ESSAY 
 
 well adapted for my purpose, as it readily admits 
 amongst its fibres the moisture, which forms on 
 its outer parts, and retains what it receives so 
 firmly, that I never but once had occasion to 
 suspect, that it suffered any portion of what it 
 had thus acquired to pass entirely through it. 
 The wool, which I used, was white, moderately 
 fine, and already imbued with a little moisture, 
 from having been long exposed to the air of a 
 room, in which no fire was kept. I divided it 
 into parcels of 10 grains each, and, immediately 
 before exposure, pulled the fibres of every 
 parcel somewhat asunder, so as to give it the 
 form of a flattened sphere, the greatest dia- 
 meter of which was about 2 inches. As in 
 doing this, I went by the judgment of my sight 
 alone, some little inequality, in point of size, 
 must have existed among different parcels, but 
 none, I think, sufficient to affect the accuraqy 
 of my conclusions from the experiments, in 
 which they were employed, more especially as 
 my conclusions scarcely ever rested upon single 
 trials. 
 
 Previously to mentioning the results of any 
 of my experiments with these parcels of wool, 
 I think it right to describe the place, where by 
 far the greater part of my observations on dew 
 were made. This was a garden in Surrey, 
 distant, by the public road, about three miles 
 
ON DEW, &c. 137 
 
 from the bridge over the Thames at Blackfriars, 
 but not more than a mile and a quarter, from a 
 densely built part of the suburbs on the south 
 side of that river. The form of the garden was 
 oblong, its extent nearly half an acre, and its 
 surface level. At one end was a dwelling- 
 house of moderate size, at the other a range of 
 low buildings ; on one side a row of high trees, 
 on the other a low fence, dividing it from 
 another garden. If this fence had been absent, 
 the garden would have been on the latter side 
 entirely open. Within it were some fruit trees, 
 but, as it had not been long made, their size 
 was small. Towards one end, there was a grass- 
 plat, in length 62 feet, and nearly 1.6 broad, the 
 herbage of which was kept short by frequent 
 mowing. The rest of the garden was employed 
 for the production of culinary vegetables. All 
 of these circumstances, however trifling they 
 may appear, had influence on my experiments, 
 and most of them, as will hereafter be seen, 
 must have rendered the results less remarkable, 
 than they would have been, if they had occurred 
 on a wide open plain, considerably distant from 
 a large city. 
 
 I now proceed to relate the influence, which 
 several differences in the situation, mechanical 
 state, and real nature of bodies, have upon the 
 production of dew. 
 
138 ESSAY 
 
 I. One general fact relative to situation is, 
 that whatever diminishes the view of the sky, 
 as seen from the exposed body, occasions the 
 quantity of dew, which is formed upon it, to be 
 less than would have occurred, if the exposure 
 to the sky had been complete. 
 
 I placed, on several clear and still nights, 10 
 grains of wool upon the middle of a painted 
 board, 4^ feet long, 2 feet wide, and 1 inch 
 thick, elevated 4 feet above the grassplat, by 
 means of 4 slender wooden props of equal 
 height; and, at the same time, attached, loosely, 
 10 grains of wool to the middle of its under- 
 side. The two parcels were consequently only 
 an inch asunder, and were equally exposed to 
 the action of the air. Upon one night, how- 
 ever, I found, that the upper parcel had gained 
 14 grains in weight, but the lower only 4. On 
 a second night, the quantities of moisture, ac- 
 quired by like parcels of wool, in the same 
 situations as in the first experiment, were 19 
 and 6 grains ; on a third, 1 1 and 2 ; on a 
 fourth, 20 and 4 ; the smaller quantity being 
 always that, which was gained by the wool at- 
 tached to the lower side of the board. 
 
 I bent a sheet of pasteboard into the shape of 
 a house-roof, making the angle of flexure 90 
 degrees, and leaving both ends open, This was 
 placed one evening, with its ridge uppermost, 
 
ON DEW, c. 139 
 
 upon the same grassplat, in the direction of the 
 wind, as well as this could be ascertained. I 
 then laid 10 grains of wool on the middle of 
 that part of the grass, which was sheltered by 
 the roof, and the same quantity on another part 
 of the grassplat fully exposed to the sky. In 
 the morning, the sheltered wool was found to 
 have increased in weight only 2 grains, but 
 that, which had been exposed to the sky, 16 
 grains. 
 
 In these experiments, the view of the sky 
 was almost entirely cut off from the situations, 
 in which little dew was formed. In others, 
 where it was less so, the quantity gained, was 
 greater. Thus, 10 grains of wool, placed upon 
 the spot of the grassplat, which was directly 
 under the middle of the raised board, and which 
 enjoyed, therefore, a considerable oblique view 
 of the sky, acquired during one night 7, during 
 a second 9, and during a third 12 grains of 
 moisture, while the quantities gained, during 
 the same times, by equal parcels of wool, laid 
 upon another part of the grassplat, which was 
 entirely exposed to the heavens, were 10, 16, 
 and 20 grains. 
 
 As no moisture, falling like rain from the at- 
 mosphere, could, on a calm night, have reached 
 the wool in any of the situations, where little 
 dew was formed, it may be thought, that the 
 
140 ESSAY 
 
 substances, under which the wool was placed, 
 prevented, mechanically, the access of that fluid. 
 But on this supposition it cannot be explained, 
 why some dew was always found in the most 
 sheltered places, and why a considerable quan* 
 tity occurred upon the grass under the middle 
 of the raised board. A still stronger proof of the 
 want of justness in this supposition is afforded 
 by the following experiment. I placed, up- 
 right, on the grassplat a hollow cylinder of 
 baked clay, the height of which was 2^ feet, 
 and diameter 1 foot. On the grass, surrounded 
 by the cylinder, were laid 10 grains of wool, 
 which, in this situation, as there was not the 
 least wind, would have received as much rain, 
 as a like quantity of wool fully exposed to the 
 sky. But the quantity of moisture, obtained 
 by the wool surrounded by the cylinder, was 
 only a little more than 2 grains, while that ac- 
 quired by 10 grains of fully exposed wool was 
 16. This occurred on the night, during which 
 the wool under the bent pasteboard gained only 
 2 grains of moisture. 
 
 Dew, however, will, in consequence of other 
 varieties of situation, form in very different 
 quantities, upon substances of the same kind, 
 although these should be similarly exposed to 
 the sky. 
 
 In the first place ; it is requisite, for the most 
 
ON DEW, &c. 141 
 
 abundant formation of dew, that the substance 
 attracting it should rest on a stable horizontal 
 body of some extent. Thus, upon one night, 
 while 10 grains of wool, laid upon the raised 
 board, increased 20 grains in weight, an equal 
 quantity, suspended in the open air, 5j feet 
 above the ground, increased only 1 1 grains, 
 notwithstanding that it presented a greater sur- 
 face to the air than the other parcel. On an- 
 other night, 10 grains of wool gained on the 
 raised board 19 grains, but the same quantity 
 suspended in the air, on a level with the board, 
 only 13; and on a third, 10 grains of wool ac- 
 quired, on the same board, 2 J grains of weight, 
 during the time in which other 10 grains, hung 
 in the air, at the same height, acquired only \ 
 a grain. 
 
 In the second place ; the quantities of dew 
 attracted by equal masses of wool, similarly ex- 
 posed to the sky, and resting on equally stable 
 and extended bodies, oftentimes vary consider- 
 ably, in consequence of some difference in the 
 other circumstances of these bodies. 10 grains 
 of wool, for instance, having been placed upon 
 the grassplat, on a dewy evening, 10 grains 
 upon a gravel walk which bounded the grass- 
 plat, and 10 grains upon a bed of bare garden 
 mould, immediately adjoining the gravel walk; 
 in the morning, the wool on the grass was 
 
142 ESSAY 
 
 found to have increased 16 grains in weight, 
 but that on the gravel walk only 9, and that on 
 the garden mould only 8 On another night y 
 during the time that 10 grains of wool, laid 
 upon grass, acquired 2^ grains of moisture, the 
 same quantity gained only 3- a grain upon the 
 bed of garden mould, and a like quantity, 
 placed upon the gravel walk, received no acces- 
 sion of weight whatever. 
 
 Two objections will probably be made against 
 the accuracy of these, as well as my other ex- 
 periments with wool. One is, that wool placed 
 on grass may, by a kind of capillary attraction 
 receive dew previously formed on the grass, in 
 addition to its own. To this I answer, that 
 wool in a china saucer, placed on the grass, ac- 
 quired very nearly as much weight, as an equal 
 parcel immediately touching the grass. The 
 second objection is, that a part of the increased 
 weight in the wool might arise from its im- 
 bibing moisture, as a hygroscopic substance. I 
 do not deny, that some weight was given to the 
 wool in this way; but it may be safely affirmed, 
 that this quantity must have been very small. 
 For, on very cloudy nights, apparently the best 
 fitted to increase the weight of hygroscopic 
 substances, wool upon the raised board would, 
 in the course of many hours, acquire little or 
 no weight j and in London, 1 have never found 
 
ON DEW, &c. 143 
 
 10 grains of wool, exposed to the air on the 
 outside of one of my chamber windows, to in- 
 crease, during a whole night, more than ^ a 
 grain in weight. When this weight was gained, 
 the weather was clear and still ; if the weather 
 was cloudy and windy, the wool received either 
 less or no weight. This window is so situated, 
 as to be, in great measure, deprived of the 
 aspect of the sky. 
 
 It being shewn, that wool, though highly at- 
 tractive of dew, was prevented, by the mere 
 vicinity of a gravel walk, or a bed of garden 
 mould, for only a small part of it actually 
 touched those bodies, from acquiring nearly as 
 much dew, as an equal parcel laid upon grass, 
 it may be readily inferred, that little was formed 
 upon themselves. In confirmation of this con- 
 clusion, I shall mention, that I never saw dew 
 upon either of them. Another fact of the same 
 kind is, that, while returning to London from 
 the scene of my experiments about sunrise, I 
 never observed, if the atmosphere was clear, 
 the public road, or any stone pavement on the 
 side of it, to be moistened with dew, though 
 grass within a few feet of it, and painted doors 
 and windows of houses not far from it, were 
 frequently very wet. If, indeed, there was a 
 foggy morning, after a clear and calm night, 
 even the streets of London would sometimes be 
 
144 ' ESSAY 
 
 moist, though they had been dry the day before, 
 and no rain had in the meanwhile fallen. This 
 entire, or almost entire, freedom of certain si- 
 tuations from dew depends, however, much 
 more upon extraneous circumstances, than upon 
 the nature of the substances found there ; for 
 river sand, though of the same nature with 
 gravel, when placed upon the raised board, or 
 upon grass, attracted dew copiously. 
 
 A third difference, from situation, in the 
 quantity of dew collected by similar bodies, 
 similarly exposed to the sky, depends upon 
 their position with respect to the ground. Thus, 
 a substance placed several feet above the 
 ground, though in this situation later dewed, 
 than if it touched the earth, would, notwith- 
 standing, if it lay upon a stable body of some 
 extent, such as the raised board lately men- 
 tioned, acquire more dew during a very still 
 night, than a similar substance lying on grass. 
 
 A fourth difference of this kind occurred 
 among bodies placed on different parts of the 
 raised board. For one, that was placed at the 
 leeward end of it, generally acquired more dew 
 than a similar body at the windward extremity. 
 
 II. Difference in the mechanical state of 
 bodies, though all other circumstances be similar, 
 has likewise an effect on the quantity of dew, 
 which they attract. Thus, more dew is formed 
 
ON DEW, &c. 145 
 
 upon fine shavings of wood, than upon a thick 
 piece of the same substance. It is chiefly for a 
 similar reason, I believe, that fine raw silk, fine 
 unwrought cotton, and flax, were found by me 
 to attract somewhat more dew, than the wool 
 I employed, the fibres of which were thicker, 
 than those of the other substances just men- 
 tioned. 
 
 III. Bright metals, in consequence of some 
 circumstance in their constitution, attract dew 
 much less powerfully than other bodies ; all of 
 which, after allowance has been made for any 
 difference, which may exist in their mechanical 
 states, seem to attract dew in quantities not 
 very unequal, if they be similarly situated. 
 
 Musschenbroek was the first, who distinctly 
 remarked this peculiarity of metals ; but Dufay *, 
 I believe, published it before him, referring, 
 at the same time, the discovery to its proper 
 author. Both Musschenbroek and Dufay, how- 
 ever, made too large an inference from their 
 experiments ; for they asserted, that dew never 
 appears on the upper surface of bright metals, 
 whereas the contrary has since been observed 
 by many persons, and I have myself known dew 
 to form on gold, silver, copper, tin, platina, 
 iron, steel, zinc, and lead. Dew, however, * 
 
 * Mem. de 1'Acad. Fran. 1736. 
 
 L 
 
146 ESSAY 
 
 when it does form upon metals, commonly 
 sullies only the lustre of their surface ; and 
 even when it is sufficiently abundant to gather 
 into drops, these are almost always small and 
 distinct. Two other facts of the same kind 
 are ; fast, that the dew, which has formed 
 upon a metal, will often disappear, while other 
 substances in their neighbourhood remain wet ; 
 and secondly, that a metal, which has been pur- 
 posely moistened, will often become dry, though 
 similarly exposed with bodies which are attract- 
 ing dew. This inaptitude to attract dew, in 
 metals, is communicated to bodies of a very 
 different nature, which touch or are near to 
 them. For I have found, that wool laid upon 
 a metal will acquire much less dew, than an 
 equal quantity laid upon grass in the immediate 
 vicinity. 
 
 A large metallic plate, lying on grass, resists 
 the formation of dew more powerfully than a 
 very small one similarly situated. I conclude 
 from various collateral facts, that a considerable 
 difference in the thickness of two pieces of 
 metal, exposing equal surfaces to the sky, will 
 be attended with a similar consequence, where- 
 ever they be placed, though I have no observa- 
 tion, which proves this directly. If, however, 
 a large and a very small plate be suspended 
 horizontally, at the same height, in the air, the 
 
ON DEW, &c. 147 
 
 small plate will resist the formation of dew 
 more powerfully than the large. 
 
 If a metal be closely attached to a substance 
 of some thickness, which attracts dew power- 
 fully, the attraction of the metal itself for dew, 
 instead of being increased from this circum- 
 stance, becomes diminished, provided the metal 
 cover the whole of the upper surface of the 
 other body. If only a part of this body be co- 
 vered, the production of dew on the metal is 
 forwarded by the conjunction, and this some- 
 what in proportion, to the quantity of surface 
 in the lower body left uncovered. The justness 
 of the first of these observations is proved by 
 the following experiment. I joined, in the 
 form of a cross, two pieces of very light wood, 
 each 4 inches long, a third of an inch in 
 breadth, and 1 line in thickness. To one side 
 of this cross I fastened, by means of mucilage, 
 a square piece of gilt paper, and then exposed 
 the instrument to the sky, with its metallic side 
 uppermost, on a dewy night, by suspending it, 
 in a horizontal position, about 6 inches above 
 the ground. A few hours after, the unattached 
 parts of the metalled paper were found covered 
 with minute drops of dew, while those, which 
 adhered to the cross, were dry. 
 
 A large metallic plate, laid upon grass, was 
 dewed with more difficulty on its upper surface, 
 
 *L 2 
 
148 ESSAY 
 
 than a similar plate elevated a few inches above 
 the grass, by means of slender props, which 
 allowed the air to pass freely under the metal. 
 But the case with respect to small pieces was 
 the reverse ; for I have often seen, covered 
 with dew, the metallic sheath of a small ther- 
 mometer lying upon grass, while the similar 
 sheath of another thermometer, suspended in 
 the air, remained dry. 
 
 Removing a metal several times, in the course 
 of the night, from one part of the grassplat to 
 another, facilitated its being dewed. The same 
 effect was produced on gilt and silvered paper, 
 by first exposing them to the sky, for some 
 time, with the bare side uppermost, and then 
 turning them. 
 
 If a piece of glass, covered on one side with 
 a metal, be placed upon the ground, with this 
 side downwards, the upper surface will attract 
 dew, precisely as if no metal were attached to 
 the lower surface. 
 
 The upper surfaces of metals are most readily, 
 and most copiously dewed, on those nights, and 
 in those parts of the night, during which other 
 substances are the most readily, and the most 
 copiously dewed. 
 
 If a metallic plate had been laid upon grass, 
 before dew began to form anywhere, its lower 
 side, notwithstanding, always became moist in 
 
ON DEW, &c. 149 
 
 the course of the night ; and the same effect 
 was almost always observed, if the plate had 
 been placed horizontally in the air, a few inches 
 above the grass. While the undersides were 
 thus moist, the upper surfaces were very often 
 dry. If, however, the plate was elevated several 
 feet in the air, the condition of both sides was 
 always the same, whether this was dry or moist. 
 
 The remarks hitherto made, on the relation 
 of metals to dew, apply to the class generally; 
 but it is now to be mentioned, that they do not 
 all resist the formation of that fluid, with the 
 same force. 
 
 I saw, for example, platina one night dis- 
 tinctly dewed, while gold, silver, copper and 
 tin, though similarly situated, were entirely 
 dry ; and I have also several times seen these 
 four metals free from dew, while iron, steel, 
 zinc, and lead were covered with it. 
 
 I once supposed, in consequence of the dif- 
 ficulty with which metals are dewed, that they 
 might in all circumstances resist, in a greater 
 degree than other bodies, the condensation of 
 watery vapour upon their surface ; and I after- 
 wards found, that Le Roi* asserts this to be 
 the case. But having exposed at the same time, 
 to the steam of warm water, pieces of glass and 
 
 * Mem. de 1'Acad. Fran. 1751. 
 
150 ESSAY 
 
 of metal, I did not see, that moisture formed 
 in the least more readily, upon the former than 
 upon the latter. I have since learned, that 
 Saussure* once entertained a similar suspicion, 
 which was also proved by an experiment to be 
 groundless. 
 
 All my experiments, hitherto spoken of, were 
 made in the country. But Le Roi having said, 
 that dew is never deposited by the air of cities, 
 I determined to ascertain, if his assertion was 
 just. With this view, I frequently exposed, at 
 night, 10 grains of wool upon a slight wooden 
 frame, placed in such a manner, between two 
 ridges of the top of my house, which is situated 
 in one of the most crowded districts of London, 
 as to be 3 feet distant from the nearest part of 
 the roof. The event was, that, upon clear and 
 calm nights, dew was always acquired by the 
 wool, though never in any considerable quan- 
 tity j probably, however, more from the wooden 
 frame being^ nearly surrounded by buildings, 
 much more elevated than itself, than from any 
 particular condition of the air in cities. The 
 formation of dew, in this situation, proceeded 
 much less regularly than in the country. For, 
 
 * Hygronometrie, page 829. 
 
ON DEW, &c. 151 
 
 upon one evening, 10 grains of wool gained 
 in it 3 grains of moisture, in 1 hour and 18 
 minutes, though I scarcely ever knew a greater 
 quantity to be collected by a similar parcel of 
 wool, in the same place, during a whole night. 
 These experiments will no doubt seem to many 
 superfluous, since dew may be observed every 
 fine evening, upon grass in London. But as 
 dew upon grass is said by Le Roi to proceed 
 from the ground, and not from the atmosphere, 
 the argument derived from its appearance there, 
 in cities, against his assertion is thus eluded by 
 him. 
 
 The last subject, which I shall here touch 
 upon, is that of hoarfrost. 
 
 This substance has, I believe, from the time 
 of Aristotle*, been uniformly, and, according 
 to my observations, justly, considered as frozen 
 dew. I shall, therefore, frequently refer here- 
 after to the experiments of the late Mr. Patrick 
 Wilson of Glasgow respecting it, as if they had 
 been actually made upon that fluid. Indeed, 
 several of my experiments upon dew were only 
 imitations of some, which had been previously 
 made upon hoarfrost, by that ingenious and 
 most worthy man. 
 
 * Meteor. Lib. I. c, x. 
 
152 ESSAY 
 
 SECTION II. 
 
 Of the Cold connected with the Formation of Dew* 
 
 DEW is often spoken of as being cold, by 
 popular writers. Thus Cicero and Virgil apply 
 to it the epithet of 'gelidus,' Milton that of 
 ' chill/ and Collins that of cold/ Of the same 
 import is a passage in Herodotus, in which it is 
 said, that, in Egypt, the crocodile passes a great 
 part of the day on dry land, but the whole of 
 the night in the Nile, this being warmer than 
 the atmosphere, and the dew. Among philoso- 
 phers, however, Mr. Wilson was the first, I be- 
 lieve, who ever suspected the existence of such 
 a conjunction. 
 
 In my experiments on the temperature of 
 bodies moistened with dew, small thermometers 
 were employed, (the largest being only 8 inches 
 long) having globular bulbs, which, in most of 
 them, were not more than from 2 to 2|- lines in 
 diameter. Their scales, which were marked in 
 the manner of Fahrenheit, were of ivory or 
 wood, and were furnished, almost all of them, 
 with hinges. They were always employed 
 naked, except I wished to know the effect of 
 covering them with any particular substance. 
 
 By means of these instruments I have very 
 
ON DEW, &c. 153 
 
 many times, during serene and still nights, exa- 
 mined the temperature of dewed grass, and 
 have constantly observed it to be less than that 
 of the air, anywhere between 1 inch and 9 feet 
 above the ground, the latter being the greatest 
 height, at which I ever marked the heat of the 
 atmosphere, in these experiments. I generally, 
 however, compared the temperature of dewed 
 grass with that of the air 4 feet above the 
 ground ; and on nights, that were calm and 
 clear, very frequently found the grass, at the 
 ordinary place of my observations, 7, 8, or 9 
 degrees colder than the air at that height. Se- 
 veral times it was 10 and 11 colder than the 
 air, and once 12. These differences are not 
 so great, as those related in Mr. Six's post- 
 humous work. But, in his experiments, the 
 temperature of grass was compared with that 
 of the air 7 feet above the ground, which, in 
 clear and calm nights, may be regarded as J a 
 degree warmer than the air at the height of 4 
 feet. Besides ; the most considerable differ- 
 ences, mentioned by Mr. Six, occurred in winter, 
 when he says a greater degree of cold is oc- 
 casioned by dew, than at any other time ; 
 whereas very few of my experiments, on the 
 temperature of grass, were instituted in that 
 season. In the last place ; my experiments 
 
154 ESSAY 
 
 were almost always made on very short grass, 
 while Mr. Six's thermometers were laid upon 
 long grass bent, by strong pressure, towards 
 the earth ; in which state they marked a tem- 
 perature 1, 2, and 3 degrees lower, than that 
 shewn by similar thermometers placed upon 
 grass, less than an inch in height. Had it not 
 been for these circumstances, and the unfitness, 
 in various respects, besides the shortness of the 
 grass, for the production of a great cold, of the 
 common scene of my operations, I believe that, 
 in consequence of my thermometers being much 
 better adapted to mark a superficial, or transi- 
 tory cold, than those of Mr. Six, I should at 
 some time have seen a difference several degrees 
 greater, than the greatest ever seen by that 
 gentleman, which was one of 13j. In con- 
 firmation of this opinion, I shall mention, that 
 having, during a short visit to a more distant 
 part of the country, exposed, in the evening, a 
 thermometer upon the surface of an open grass 
 field, I found it soon after, although the grass 
 was short, and the weather warm, 14 lower 
 than a similar thermometer, suspended in the 
 air, 4 feet above the grass. If to this quantity 
 be added ^ a degree, on account of the dif- 
 ference in elevation between our suspended 
 thermometers, the cold, connected with dew, 
 
 
ON DEW, &c. 155 
 
 observed by me this night on grass, will exceed 
 the greatest ever observed by Mr. Six by 1 
 degree. 
 
 According to a few observations made by me, 
 the greater coldness of grass, than that of the 
 air, begins to appear, in clear and calm weather, 
 in places, sheltered in the afternoon from the 
 sun, but still open to a considerable portion of 
 the sky, soon after the heat of the atmosphere 
 has declined. A similar coldness continues 
 upon grass in still and serene mornings, for 
 some time after the rising of the sun, in places 
 shaded from its direct light, but otherwise open 
 to the sky. My experiments on this point have 
 also not been many, and none of them were 
 made in winter f which, I presume, are the rea- 
 sons, that I never observed a cold, from this 
 cause, later in the morning, than an hour after 
 sunrise. The surface of snow, however, was 
 once, in the depth of winter, observed by Mr. 
 Wilson of Glasgow to be considerably colder 
 than the air, till a little after midday*. 
 
 In cloudy nights, particularly if there was 
 wind, the grass was never much colder than 
 the air. On such nights, the temperatures of 
 both were sometimes the same ; at other times 
 that of the grass was the higher of the two, 
 
 * Paper in Phil. Trans. 1781. 
 
156 ESSAY 
 
 even when the grass was wet from preceding 
 rain, and when, consequently, it must have 
 been, in some measure, cooled by evaporation. 
 On one such night, the grass was found to be 
 4 colder than the earth an inch beneath the 
 surface of the plat, which afforded a sufficient 
 reason for the grass itself being warmer than 
 the air. In windy weather, however, if the sky 
 was clear, some degree of cold, in addition to 
 that of the air, was always observed upon the 
 grass ; and in calm weather, very high clouds, 
 though sufficiently extensive and dense, to con- 
 ceal the sky completely, would yet frequently 
 allow of the grass being several degrees colder 
 than the air. I once observed, upon a night of 
 this kind, a difference of 5 between the tem- 
 peratures of those bodies. 
 
 If the night became cloudy, after having 
 been very clear, though there might be no 
 change with respect to calmness, a considerable 
 alteration in the temperature of the grass always 
 ensued ; and this sometimes very suddenly. 
 Upon one such night, the grass, after having 
 been 12 colder than the air, became only 2 
 colder than it, the temperature of the air being 
 the same at both observations. On a second 
 night, grass became 9 warmer in the space of 
 an hour and a half. On a third night, in less 
 than 45 minutes, for the whole change occurred 
 
ON DEW, &c. 157 
 
 while I was absent 45 minutes, the temperature 
 of the grass rose 15, while that of the neigh- 
 bouring air increased 3j. During a fourth 
 night, the temperature of the grass at half-past 
 9 o'clock was 32. In 20 minutes afterwards, 
 it was found to be 39, the sky having in the 
 mean time become cloudy. At the end of 20 
 minutes more, the sky being clear, the tempera- 
 ture of the grass was again 32. These were 
 the most remarkable of my observations on this 
 subject ; but I may add to them, that I have 
 frequently seen, during nights that were gene- 
 rally clear, a thermometer lying on the grass- 
 plat rise several degrees, upon the zenith being 
 occupied only a few minutes by a cloud. On 
 the other hand, upon two nights I observed a 
 very great degree of cold to occur on the 
 ground, in addition to that of the atmosphere, 
 during short intervals of clearness of sky, be- 
 tween very cloudy states of it. 
 
 I did not speak in the preceding section of 
 another obscure state of the atmosphere, that 
 occasioned by fog, or mist, as the moisture de- 
 posited in it attaches to all bodies, indiscri- 
 minately ; on which account, I was unable to 
 determine, whether or not dew forms during its 
 continuance. But, with respect to the con- 
 nexion of this condition of the atmosphere with 
 cold, I have to remark, that I have several 
 
158 ESSAY 
 
 times, on its appearance betwixt daybreak and 
 sunrise, found the difference between thermo- 
 meters on grass and in the air, which had been 
 considerable during the night, to diminish 
 greatly. I never, indeed, observed it to vanish, 
 but this I used to impute to the air being not 
 very much obscured. I have now, however, 
 reason to doubt the justness of this conclusion; 
 for on the evening of the 1st of January in the 
 present year, 1814, I found, during a dense fog, 
 while the weather was very calm, a thermo- 
 meter lying on grass, thickly covered with 
 hoarfrost, 9 lower than another suspended in 
 the air, 4 feet above the former. On the fol- 
 lowing evening, when the air was equally calm, 
 but the fog sufficiently attenuated to allow me 
 to see that the sky was almost entirely covered 
 with clouds, the difference between two ther- 
 mometers, similarly placed with the former, 
 was only 1. On comparing the observations 
 of these two evenings, I conclude, that on the 
 first few or no clouds existed above the fog, 
 and consequently that fog, if there be no clouds 
 above it, may, in a very calm air, admit of the 
 appearance of a considerable degree of cold, at 
 night, upon the surface of the earth, in addition 
 to that of the atmosphere. Mr. Six, indeed, 
 says, while speaking of the cold connected with 
 dew, in his paper in the Philosophical Transac- 
 
ON DEW, &c. 
 
 159 
 
 tions for 1788, " fogs did not, as far as I could 
 perceive, at all impede, but rather increase, the 
 refrigeration." But this was a mistake ; which 
 in all probability arose from his ascribing the 
 effect of a clear night to an ensuing foggy 
 morning, as he examined his thermometers only 
 in the daytime. He afterwards discovered his 
 error $ for, in his posthumous work, thick fogs 
 are ranked among the circumstances, which 
 always impede, and sometimes prevent alto- 
 gether, the appearance of a cold upon the sur- 
 face of the earth, greater than that of the atmo- 
 sphere. During a very dense fog, Mr. Wilson 
 found no difference, at night, between a ther* 
 mometer laid upon snow, and another suspended 
 in the air*. 
 
 When, during a clear and still night, different 
 thermometers were examined, at the same time, 
 which had been placed in different situations, 
 those which were situated, where most dew was 
 formed, were always found to be the lowest. 
 Thus, upon one such night, I found a thermo- 
 meter placed upon a little wool, lying upon the 
 middle of the upper side of the raised board, to 
 be 9 lower than another thermometer, in con- 
 tact with an equal quantity of wool, attached to 
 the middle of the underside of the board. On 
 
 * Edin. Phil. Trans. I. 170. 
 
160 ESSAY 
 
 two other nights, the difference between two 
 thermometers in the same situations was 8* I 
 found also, on two other serene and calm nights, 
 a spot of grass covered by the pasteboard roof, 
 and another spot surrounded by the earthen 
 cylinder, to be both 10 warmer than neigh- 
 bouring grass fully exposed to the sky. Think- 
 ing it possible, that the cylinder, which had 
 been exposed to the sun the preceding day, 
 might still possess some of the heat, which it 
 had then imbibed, I placed near to it, on an- 
 other night, a cylinder made of very thin paste- 
 board ; but this was equally efficacious with the 
 earthen one, in preventing cold from occurring 
 on grass. When the exposure was greater than 
 in the preceding examples, and more dew was 
 in consequence formed, the cold was also 
 greater, but still less than where the exposure 
 was complete. For instance, upon the night 
 during which 10 grains of wool, placed upon 
 the middle of the grass, which was sheltered 
 by the raised board, had gained 7 grains, and 
 the same quantity on grass fully exposed to the 
 sky had gained 10 grains, the difference be- 
 tween the temperatures of the two portions of 
 grass was only 2j. 
 
 The same correspondence was observed, when 
 the differences in the quantity of dew did not 
 depend, as in the preceding instances, upon 
 
ON DEW, &c. 161 
 
 any diversity of exposure to the sky. Thus, the 
 mercury in a thermometer placed upon wool, 
 lying on the raised board, was found to be at 
 the 44th degree, while that in another, pendent 
 in the air, at the same height from the ground, 
 and wrapped in wool, was at the 48th. Wool 
 also, on the raised board *, was commonly a little 
 colder than the same substance on grass, when 
 the night was very still ; and the leeward end 
 of that board was generally colder than the 
 windward extremity. 
 
 But, the most remarkable examples of this 
 kind were exhibited by the gravel walk, and the 
 bare garden mould. In still and serene nights, 
 the surfaces of these bodies were always warmer 
 than the neighbouring grass, and frequently 
 warmer than the air. On one night of this 
 description, I observed, 2|- hours after sunset, 
 the surface of the gravel walk to be 16J, and 
 that of the garden mould to be 12j, warmer 
 
 * The greater cold of the raised board, in my experiments, 
 most probably depended on the grass being very short; since 
 Mr. Wilson Found, that snow on the ground was colder than 
 the same body on a raised board. If 1 , 2, or 3 degrees were 
 added to the cold of the grass at my place of observation, 
 agreeably to the difference found by Mr. Six, between the 
 temperatures of long and short grass in dewy nights, the cold 
 on my raised board would, upon such nights, have been 
 always less than that of the grassplat. 
 
 M 
 
162 ESSAY 
 
 than grass very near to them, and similarly ex- 
 posed to the heavens. As the night proceeded, 
 clouds formed and accumulated ; in consequence 
 of which the difference at sunrise, between the 
 temperatures of the grass and the gravel walk, 
 was only 6, and between those of the grass and 
 the mould only 4, the temperature of the grass 
 having in the mean time increased considerably, 
 while that of the other bodies had decreased a 
 little. At another time, shortly before sunrise, 
 a very clear morning having succeeded a cloudy 
 night, I found the gravel walk to be 10 and the 
 garden bed to be 9 warmer than neighbouring 
 grass, which was 8 colder than the air. Both 
 of these examples occurred in summer, and I 
 believe, that such considerable differences will 
 occur in that season only. It was on the first of 
 these two nights, that 10 grains of wool gained 
 only \ a grain of moisture on the mould, and 
 that the same quantity gained no weight on the 
 gravel walk. That the unfitness of the gravel 
 walk, however, to become cold, like its unfitness 
 to attract dew, arose from its situation, and not 
 from the nature of the substance of which it was 
 made, is proved by this circumstance, that river 
 ^sand, placed on the raised board, was on 4 dif- 
 ferent nights, none of them highly favourable for 
 the production of cold, 7, 7, 8, and 8^ degrees 
 colder than the air at the same height. 
 
ON DEW, &c. 163 
 
 It may be added here, that I have always 
 found, on dewy nights, the temperature of the 
 earth, \ an inch or an inch beneath its surface, 
 much warmer than the grass upon it. On five 
 such nights the differences were from 12 to 16 
 degrees. The earth, at the above-mentioned 
 depth, was also almost constantly warmer on 
 dewy nights than the air; sometimes it was con- 
 siderably so, for I once observed it to be 10 
 warmer, at another time 9, and at a third 7-J. 
 An exception will no doubt occur, if very mild 
 weather should follow a lorig frost ; but of this 
 I have had no experience. 
 
 In the experiments upon my housetop in 
 London, I always found, during clear and calm 
 nights, wool lying on the wooden frame to be 
 colder than the air, at the same height; but the 
 difference was seldom more than 3. On the 
 evening, however, during which dew formed 
 there more copiously than usual, the difference 
 was 5. That the smallness of these differences 
 was not wholly occasioned by any thing special 
 in the air of cities was afterwards proved, by my 
 finding others much greater, in a garden nearly 
 in the middle of London, from which almost the 
 whole of the sky was visible. 
 
 Metals, likewise, furnish proofs of the con- 
 nexion of dew with a cold in the substance, on 
 
 M 2 
 
ESSAY 
 
 \ 
 
 which it forms, superior to that of the neigh- 
 bouring atmosphere. My observations, how- 
 ever, on the temperature of metals, when ex- 
 posed to the sky on dewy nights, were less 
 numerous, than those on several other subjects 
 treated in this Essay, by reason of the less fre- 
 quent opportunity I enjoyed of making them ; 
 and many of those, which I did make, were 
 afterwards found by me to have been impro- 
 perly conducted. I thought, for instance, for 
 some time, that the temperature of a metal, on 
 a dewy night, might easily be learned in the 
 way, in which I had been accustomed to ascer- 
 tain the temperature of dewed grass. But, ob- 
 serving dew one night on the glass tube of a 
 thermometer, which was lying on a metal placed 
 upon grass, while the metal itself was free from 
 moisture, I conceived it probable, that the cold 
 then indicated by the thermometer was not the 
 real temperature of the body, to which it was 
 applied. To determine the point, I placed on 
 the same metal a second thermometer, covered 
 with gilt paper, upon which this was found at 
 three observations to be 6J, 7, and 7 higher 
 than the other. In this experiment, the bulb 
 of the naked thermometer, from being very 
 small, did not project as far as the outer surface 
 of the scale, and, consequently, did not come 
 
ON DEW, &c. 165 
 
 in contact with the metal. But even when the 
 ball of a thermometer was applied directly to a 
 metal, on a clear and calm night, a temperature 
 was marked by it, commonly 2 and 3, and some- 
 times more degrees less than that marked by 
 a similar , thermometer, inclosed in gilt paper, 
 and similarly placed. I found it likewise ne- 
 cessary, in this inquiry, to correct the tempera- 
 ture of the air, as given by a naked thermo- 
 meter. For, on still and serene nights, a ther- 
 mometer inclosed in a case of gilt or silvered 
 paper, and suspended in the air 4 feet above 
 the grassplat, was usually observed to be 1 J or 
 2 higher than a bare thermometer, of the same 
 construction, suspended near to it. The dif- 
 ference of two such thermometers, thus placed, 
 was once observed by me to be 2|-, and once 
 3^. It may be thought, perhaps, that these 
 differences were caused by the metalled case 
 obstructing the transmission of the temperature 
 of the air to the inclosed instrument. But that 
 this was not the reason is shewn by my observ- 
 ing, that on cloudy nights there existed no dif- 
 ference between the two thermometers ; that, 
 even on clear nights, a thermometer contained 
 in a case of white paper, somewhat thicker than 
 the metalled, was always nearly of the same 
 temperature with a naked one which was sus- 
 pended close to it 5 and that, when a difference 
 
166 ESSAY 
 
 did exist between the two latter, the thermo- 
 meter in the white paper case was commonly 
 lower than the other. 
 
 The estimation of the heat, both of air and 
 of metals, on a dewy night, is liable to errors 
 from other causes. As these, however, are 
 trifling, I shall not mention them, but proceed 
 to state the results of my observations, upon 
 the temperature of metals exposed to the sky at 
 night, though unable to vouch for their entire 
 accuracy. 
 
 Thin bright metallic plates, the least having 
 a surface of 25 square inches, and some of them 
 a surface of more that 100 such inches, were 
 several times observed, while lying on grass 
 which was attracting dew, to be 1 and 2, and 
 once 3, degrees warmer than the air 4 feet 
 above them. At other times, their temperature 
 was the same with that of the air. In both of 
 these cases their upper surfaces were always 
 free from dew. Metals thus situated were, 
 consequently, often much warmer than the 
 grass, which surrounded them. I made no ex- 
 periments on this point, during the nights, on 
 which occurred the greatest instances of cold 
 on grass, relatively to the temperature of the 
 air ; but I found, notwithstanding, during one 
 night, a metal on grass to be 10 warmer than 
 the exposed grass near to it. On two other 
 
ON DEW, &c. 167 
 
 nights, the differences were 9 and 8. The 
 superiority of the heat of metals on grass over 
 that of the air, when it did exist, was evidently 
 connected with the temperature of the grass, 
 which they covered, and this again with that of 
 the earth under the same portion of grass ; for 
 this portion was always a little warmer than the 
 metal, but not so warm as the earth. 
 
 On the other hand, metals, on which dew 
 was forming while they lay upon grass, were 
 always colder than the air. In like manner, if 
 one metal upon the grassplat were dewed, while 
 another similarly situated remained dry, the 
 former was always colder than the latter. 
 
 When a metal lying on the grassplat became 
 dewed, the grass under it was always colder 
 than that under another metal, which was un- 
 dewed. 
 
 A metal, while receiving dew, in consequence 
 of being elevated in the air, was always colder 
 than a similar metal, which remained undewed 
 on the grass. 
 
 The greatest instances of cold, observed by 
 me on metals, occurred at times, when other 
 bodies near to them had become considerably 
 colder than the atmosphere. 
 
 The cold, however, contracted by metals, 
 from exposure to the sky in a clear and still 
 night, was always less than that of other bodies 
 
168 ESSAY 
 
 similarly situated, the greatest excess of cold 
 ever observed by me, in the larger metallic 
 plates, from this cause, over that of the air, 
 being not more than 3 or 4 degrees. If much 
 smaller pieces were placed upon grass, the re- 
 sult was different. For I have found a small 
 thermometer placed in this situation, while in- 
 closed in a sheath of gilt paper, to be only 3 
 less cold than the surrounding grass, during a 
 night favourable to the production of cold on 
 the surface of the earth. 
 
 I collected only a few facts respecting the 
 comparative temperatures of different metals, 
 when they were exposed together to the sky, 
 on dewy nights ; but such as I did collect tend 
 to prove, that the most readily dewed metals 
 become colder than the air, sooner than those, 
 which receive dew with greater difficulty. 
 
 Many of the experiments, which have been 
 mentioned in this section, shew, that when bo- 
 dies, which had been equally exposed to the 
 night air, were examined at the same time, 
 those which were most dewed were also the 
 coldest. No such correspondence, however, 
 was found in the experiments of different 
 nights, or even of different parts of the same 
 night. Thus, during two nights, on which 
 
ON DEW, &c. 
 
 grass was 12 and 14 colder than the air, there 
 was little dew; while on the night, which 
 afforded the most copious dew ever observed 
 by me, the cold possessed by the grass, beyond 
 that of the air, was for the most part only 3 
 and 4 ; and I have always seen less dew about 
 sunset, than about sunrise, when the weather 
 has been calm and clear at both times, though 
 there is commonly, in this country at least, a 
 greater difference between the temperature of 
 grass and of air in the evening, than in the 
 morning. I had early observed, also, bodies 
 exposed to the sky, on a cloudy but calm night, 
 to be sometimes 2 or 3 colder than the air, 
 without having any appearance of dew; and 
 when two metals possessing different relations 
 to dew were exposed together, I have seen the 
 one, which was the fitter to attract that fluid, 
 colder than the other, though both were dry. 
 
 I shall conclude this part of my Essay, with 
 relating the results of some experiments, which 
 were made for the purpose of ascertaining the 
 tendencies of various bodies to become cold, 
 upon exposure to the sky at night. Unfor- 
 tunately, the weather was not always favourable 
 to my views ; but what occurred appears, to me, 
 notwithstanding, worthy of being related. 
 
170 ESSAY 
 
 In the observations hitherto given by me on 
 the cold connected with dew, the temperature 
 of grass has been chiefly considered, partly be- 
 cause my first experiments had been made upon 
 it, and partly from a wish, which arose after- 
 wards, to compare my own experiments with 
 those of Mr. Six, which had been confined to 
 that substance. I found it, however, very unfit 
 to furnish the means of comparing the degrees 
 of cold produced at night on the surface of the 
 earth, at different times and places ; as its state 
 on different nights, on the same parts of the 
 plat I commonly made use of, and in different 
 parts of the plat on the same nights, was often 
 very unequal, in point of height, thickness and 
 fineness, all of which circumstances influenced 
 the degree of cold produced by it. I observed, 
 in consequence, a much greater uniformity in 
 the results of experiments made with various 
 other bodies, whose condition, when first ex- 
 posed to the air, was always the same. Of these, 
 the most productive of cold were the filamentous 
 and downy, as wool of moderate fineness, very 
 fine raw silk, very fine unspun cotton, fine flax, 
 and swandown, all of which were not only more 
 steadily cold, upon clear and calm nights, than 
 grass, but also gave rise to a greater degree of 
 cold, than was almost at any time observed upon 
 it, even in its best state. Among the bodies of 
 
ON DEW, &c. 171 
 
 this class, wool produced the least cold, and I 
 formerly mentioned that it attracted less dew, 
 than silk, cotton, and flax. The last mentioned 
 substances, and swandown, were found equal, 
 or nearly so, in their tendency to become cold. 
 Swandown, however, exhibited the greatest cold 
 rather more frequently than any of the rest ; on 
 which account, and from its being more easily 
 managed, as it was used while adhering to the 
 skin of the bird, I at length scarcely ever em- 
 ployed any other body of the same class. On 
 the night, during which grass was observed to 
 be 14 colder than the air, swandown, lying 
 upon a neighbouring piece of grass, was still 
 one degree lower. This difference of 15, be- 
 tween the temperature, at night, of a body on 
 the surface of the earth, and that of the air, a 
 few feet above the earth, is the greatest which 
 I have hitherto seen. 
 
 Fresh, unbroken straw, and shreds of white 
 paper, though not properly to be ranked among 
 filamentous substances, were also found to be a 
 little more productive of cold, than the wool 
 which I used. 
 
 The next class consisted of bodies in the state 
 of a powder, more or less fine. These were 
 clean river sand, glass, chalk, charcoal, lamp- 
 black, and a brown calx of iron. Chalk pro- 
 duced the least, and the three last substances, 
 
172 ESSAY 
 
 the greatest cold. They were all, however, in- 
 ferior in this respect to bodies of the first class. 
 
 Solid bodies, having a surface exposed to the 
 sky, of at least 25 inches square, formed a third 
 class, on which such experiments were made. 
 The particular substances of this description, 
 subjected to trial, were glass, brick, cork, oak- 
 wood, and wax ; all of which were, likewise, 
 found inferior to the filamentous substances. 
 From these last experiments it follows, that 
 when a glass bulb of a thermometer is applied 
 at night to a body exposed to a clear sky, the 
 temperature exhibited by the instrument will 
 not be accurately that of the body in question, 
 except the disposition of the latter to become 
 cold, in such a situation, be the same as that of 
 glass. An example of this- fact has been given 
 in this Essay*. 
 
 My principal experiments, however, of this 
 kind were made with snow. 
 
 On the 25th of January 1813, the ground 
 being then covered with snow about an inch 
 deep, I went to my usual place of experiment 
 in the country ; but, during 8 hours that I at- 
 tended to my thermometers, the whole sky was 
 constantly overcast with clouds. The atmo- 
 sphere was, for the greater part of that time, 
 very still, and a thermometer on the snow was 
 * Page 164. 
 
ON DEW, &c. 
 
 173 
 
 generally about lower, than another in the 
 air. That this difference was not owing to 
 evaporation was proved by the thermometer on 
 the snow always rising, from a half to a whole 
 degree, whenever the air was a little moved, 
 and falling the same quantity, as soon as a great 
 stillness again took place. 
 
 I had no opportunity of renewing my observa- 
 tions upon snow, before the beginning of the 
 present year, 1814. The state of my health 
 rendering it improper, that I should incur 
 much fatigue, or be long exposed to night air, 
 I restricted myself to the making a few experi- 
 ments, in the large garden in Lincoln's-Inn 
 Fields. I went thither, for the first time, on 
 the evening of the 4th of January, immediately 
 after a considerable snowfall had ceased, wish- 
 ing to begin my observations, before any cold 
 should arise on the snow's surface, from ex- 
 posure to the sky. This was desirable on an- 
 other account ; for Mr. Kirwan, in direct op- 
 position to indisputable facts, most clearly stated 
 by Mr. Wilson, had said, that the great cold, 
 observed by that gentleman on snow, was occa- 
 sioned by this substance having retained the 
 temperature of the high region, from which it 
 had fallen*. The result of my inquiry was, 
 that the surface of the snow, and the air 4 feet 
 * On Temperatures, p. 30. 
 
174 ESSAY 
 
 above it, had precisely the same heat. The 
 depth of the snow was 4 inches. 
 
 My next experiment took place on the even- 
 ing of the 6th, the intervening day having been 
 snowy. The sky was clear, but the air had a 
 considerable motion. The heat of the atmo- 
 sphere, at the height of 4 feet, was at 9% h. 26; 
 while that of the surface of the snow, and of 
 swandowri lying upon it, was 22. The depth 
 of the snow was now about 5 inches. 
 
 On the 7th, a little after sunset, the heat of 
 the air in the garden was 23, that of the surface 
 of snow 19, but that of swaridown lying upon 
 the snow only 15. There was then a gentle 
 breeze ; some parts of the sky were covered with 
 clouds, and the lower atmosphere was a little 
 obscure. While the exposed surface of the 
 snow was 19> a part of its surface, which had 
 been covered, about 20 minutes, with a piece 
 of pasteboard, was 22. Grass, at the bottom 
 of the snow, was 31, and the earth an inch be- 
 neath the grass 32. 
 
 After this, there was no fit time for observa- 
 tion until the 13th. The thermometers were 
 exposed at 8 h. on the evening of that day, the 
 sky being then without clouds ; but the stars 
 were not bright, and there was a perceptible 
 motion in the air. At 83- h. the temperature of 
 the air was 22^, that of the surface of the snow 
 
ON DEW, &c. 175 
 
 13, and that of swandown, lying on the snow, 
 8. At 9h. the air was 23J, . snow 17, and 
 swandown 15. The sky being now, in great 
 measure, covered with high thin clouds, my 
 experiments ceased. At lO^h. the sky was 
 very bright, and the atmosphere very calm ; but 
 it was not then convenient to me to renew my 
 observations. Had I repeated them at that 
 time, I should probably have found a difference, 
 between the temperature of the swandown and 
 air, several degrees more considerable than 
 the one of 14^, which had already occurred on 
 this evening, and consequently greater than the 
 greatest observed by Mr. Wilson, between the 
 temperatures of snow and of the atmosphere, 
 which was one of 16. 
 
 The next favourable evening was that of the 
 21st. Much snow having in the meanwhile 
 fallen, its depth was now more than a foot. 
 The thermometers were observed 5 times be- 
 tween 4 h. 15m. and 4 h. 55 m. At 4 of those 
 times, the swandown was 13, and at one of 
 them 13^, colder than the air, the heat of 
 which at the 4 first observations was 26, and 
 at the last 25^-. The temperature of the sur- 
 face of the snow, during the whole period of 
 observation, was 17, and consequently 4 times 
 it was 4, and once 5, less cold, than that of 
 the swandown. The atmosphere was altogether 
 
176 ESSAY ON DEW, &c. 
 
 free from clouds, and nearly quite calm, but # 
 good deal hazy. 
 
 Before another proper evening arrived, my 
 health became so infirm, that I was obliged to 
 relinquish this pursuit. I conclude therefore 
 my account of it, with two remarks. 1. If Mr. 
 Wilson had been accustomed to examine the 
 temperature of swandown, or any similar sub- 
 stance, placed upon snow, he would, probably, 
 have observed a cold, on the surface of the 
 earth, exceeding that of the atmosphere by 20* 
 or more, on the night of his actually observing 
 an excess of 16. 2. Since upon one evening, 
 when the atmosphere was neither very clear nor 
 very still, a difference of 14^ was found by me, 
 between the temperatures of air and of swan* 
 down, which is only \ a degree less than the 
 greatest difference I have ever observed, be- 
 tween the same substances on the stillest and 
 clearest nights in summer, a corroboration is 
 hence derived of a conclusion, made by Mr. 
 Six from his experiments, that the greatest dif- 
 ferences at night, in point of temperature, be- 
 tween bodies on the surface of the earth, and 
 the atmosphere near to it, are those which take 
 place in very cold weather. 
 
PART II. 
 
 OF THE THEORY OF DEW. 
 
 
 DEW, according to Aristotle*, is a species of 
 rain, formed in the lower atmosphere, in conse- 
 quence of its moisture being condensed by the 
 cold of the night into minute drops. Opinions 
 of this kind, respecting the cause of dew, are 
 still entertained by many persons, among whom 
 is the very ingenious Mr. Leslie of Edinburgh f. 
 A fact, however, first taken notice of by Gers- 
 ten, who published his treatise on dew in 1733, 
 proves them to be erroneous ; for he found, that 
 bodies a little elevated in the air often become 
 moist with dew, while similar bodies, lying on 
 the ground, remain dry, though necessarily, 
 from their position, as liable to be wetted, by 
 whatever falls from the heavens, as the former. 
 Shortly after the appearance -of Gers ten's 
 treatise, Musschenbroek made the remark, 
 already mentioned in this Essay, that metals 
 will be free from dew, while other bodies attract 
 it copiously. This philosopher contented himself 
 
 * Meteor. Lib. 1. c. x. et De Mundo. c. iii. 
 
 f Relations of Heat and Moisture, p. 37, and 132. 
 
 N 
 
178 ESSAY 
 
 with publishing his discovery, but his friend 
 Dufay concluded from it, that dew is an electric 
 phenomenon, since it leaves untouched the 
 bodies, which conduct electricity, while it ap- 
 pears upon those, which cannot transmit that 
 influence. If dew, however, were to form on 
 the latter only, its quantity would never be suf- 
 ficiently great, to admit its being distinctly seen -, 
 for the non-conductors, as soon as they became 
 in the least moist, would be changed into con- 
 ductors. Charcoal, too, it is now known, though 
 the best solid conductor of electricity after the 
 metals, attracts dew very powerfully ; and, in 
 the last place, contrary to the assertion of Du- 
 fay, dew frequently forms upon metals them- 
 selves. 
 
 Other authors have ascribed the production 
 of dew to electricity, for reasons different from 
 that of Dufay. But there are several considera- 
 tions, which seem to me to prove, that no such 
 opinion can be just. 1. When dew is produced 
 in a clear atmosphere, the portion of air, by 
 which it is deposited, must necessarily be un- 
 able, at that moment, to retain, in a state of pel- 
 lucid vapour, all the moisture, which it had 
 immediately before held in that form. But I 
 know of no experiment, which shows, that air, 
 by becoming positively electrical, which is said 
 to be its condition on the evenings, during which 
 
ON DEW, &c. 179 
 
 dew is most abundant, is rendered less able, than 
 it had previously been, to contain watery vapour 
 in a state of transparency. 2. Bodies in similar 
 circumstances, as far as electricity is concerned, 
 acquire very different quantities of dew. Wool 
 placed on the raised board, for example, attracted 
 very much more dew, than wool attached to the 
 lower side of the same board, and even considera- 
 bly more than the same substance freely sus- 
 pended in the air, and entirely exposed to the 
 sky. 3. Dew forms in different parts of the 
 night, in quantities no way proportioned to the 
 degrees of electricity found in the atmosphere 
 at the same times. Thus, it is commonly more 
 copious in the morning than in the evening, 
 notwithstanding that the air is observed to be, 
 in the latter season, more highly electrical than 
 in the former. 4. I have several nights held 
 a glass bottle, upon which dew was forming, 
 close to the top of a Bennett's electrometer, 
 which had been previously kept in a dry place ; 
 but I never saw the slips of gold leaf to move 
 in consequence. It is very probable, however, 
 that more refined experiments will show, that 
 electrical appearances attend the production of 
 dew. These, perhaps, accompany every change 
 in the chemical form of bodies. But the facts, 
 which have been stated, seem sufficient to esta- 
 blish, that any such appearances, which may be 
 
 N 2 
 
ISO ESSAY 
 
 hereafter remarked, during the formation of 
 dew, must be considered as effects, and not 
 as the cause, of the conversion of the watery 
 vapour of a clear atmosphere into a fluid. 
 
 A remaining argument applies equally to all 
 the theories, which have hitherto been made 
 public on the cause of dew. This is, that none 
 of them include the important fact, that its pro- 
 duction is attended with cold ; since no explana- 
 tion of a natural appearance can be well founded, 
 which has been built without the knowledge of 
 one of its principal circumstances. It may seem 
 strange to many, that neither Mr. Wilson, nor 
 Mr. Six, applied this fact to the improvement 
 of the theory of dew. But according to their 
 view of the subject, no such use could have been 
 made of it by them, as they held the formation 
 of that fluid to be the cause of the cold observed 
 with it. I had many years, as was formerly 
 mentioned, held the same opinion ; but I began 
 to see reason, not long after my regular course 
 of experiments commenced, to doubt its truth, 
 as I found that bodies would sometimes become 
 colder than the air, without being dewed ; and 
 that, when dew was formed, if different times 
 were compared, its quantity, and the degree 
 of cold which appeared with it, were very far 
 from being always in the same proportion to 
 each other. The frequent recurrence of such 
 
ON DEW, &c. 181 
 
 observations at length converted the doubt of 
 the justness of my ancient opinion, into a con- 
 viction of its error, and at the same time occa- 
 sioned me to conclude, that dew is the produc- 
 tion of a preceding cold in the substances, upon 
 which it appears. Wishing, however, to obtain 
 proofs, more striking in degree, of the validity 
 of these inferences, than such as had been 
 afforded to me by casual observation, while 
 attending to other parts of my subject, I insti- 
 tuted the experiments which will be next re- 
 lated. 
 
 I had frequently remarked, early in the even- 
 ing, a considerable degree of cold on substances 
 exposed in calm weather to a clear sky, and I 
 had also sometimes seen, early in the evening, 
 the raised board altogether dry, while the grass 
 was much moistened. I therefore determined 
 to make the experiments in view on the raised 
 board, and to commence them as soon as the 
 sun should cease to shine upon it. The first 
 day I went to the country for this purpose, the 
 19th of August 1813, almost every circumstance 
 was favourable to its completion. There had 
 been no rain for three weeks; the wind was 
 northerly ; and the barometer was rising ; all 
 which indicated, that the atmosphere contained 
 little moisture. The air too was extremely still. 
 The only appearance in the least unfavourable 
 
182 ESSAY 
 
 was, that the sky was not entirely free from 
 clouds; but these were few, of small extent, 
 thin, and high. 
 
 At 6h. 25m. immediately after the sun had 
 ceased to shine upon the spot, where my experi- 
 ments were to be carried on, though the time 
 of its setting was still 47 minutes distant, I 
 placed upon the raised board 1 grains of wool, 
 and a small bag, made of the skin of a swan's 
 breast with the down adhering, and stuffed with 
 wool, the whole weighing nearly 5 drachms. 
 On each of these substances the naked bulb of 
 a small and delicate thermometer was laid. A 
 similar thermometer, with its bulb also naked, 
 was suspended in the air, over the grassplat, at 
 the same height with the board. Two thermo- 
 meters were placed in other situations, as will 
 be seen in the annexed Table. After an ex- 
 posure of 20 minutes, the wool was 7 colder 
 than the air, but the swandown bag only 6, no 
 doubt in consequence of its comparatively great 
 quantity of matter. Neither, however, had 
 gained the least weight, according to the scales 
 employed by me, which were sensibly moved 
 by the 16th of a grain. These observations 
 were repeated several times during the follow- 
 ing hour, as will be seen by the Table, at none 
 of which, except the last, was either the wool 
 or swandown found in the least heavier, than 
 
ON DEW, &c. 
 
 183 
 
 when first placed on the board. At this last 
 observation, the wool, though 9i colder than 
 the air, was still without any increase in weight ; 
 but the swandown, which was 1 colder than 
 the wool, had gained \ a grain. My experi- 
 ments now properly ceased ; but having suffered 
 the thermometers, which had been placed on the 
 wool and swandown, and in the air, to remain 
 in those situations, I examined them again at 
 8h. 45m., that is, 2h. 0m. after they had 
 been first exposed. The wool, which was still 
 9j colder than the air, had gained somewhat 
 less than a grain ; and the swandown, which 
 was now llj colder than the air, had gained 
 2 grains, including the J grain already men- 
 tioned. When these last observations were 
 made, the sky was entirely cloudless, and the 
 
 atmosphere very calm. 
 
 < 
 
 TABULAE VIEW OF OBSERVATIONS 
 
 on the Evening of dugust 19, 1813. 
 
 
 6h.45<n. 
 
 7h. 
 
 7h. 20m. 
 
 7b. 40m. 
 
 Bh. 46m, 
 
 Heat of air 4 feet above the grass 
 
 60f 
 
 60$ 
 
 59 
 
 58 
 
 54* 
 
 " - wool on the raised board 
 
 53$ 
 
 54| 
 
 51* 
 
 48$ 
 
 44* 
 
 swandowri on the same 
 
 54* 
 
 53 
 
 51 
 
 47| 
 
 49} 
 
 surface of the raised board 
 
 58 
 
 57 
 
 55f 
 
 
 
 grassplat* 
 
 53 
 
 51 
 
 4<$ 
 
 49 
 
 42 
 
 * In these experiments, contrary to what usually happens, 
 the grass was almost constantly colder than the filamentous 
 substances, although they were placed upon the raised board 
 
184 ESSAY 
 
 Similar experiments made at the same place, 
 on the evenings of the 25th of August and 17th 
 of September, in the same year, had results, 
 which were also similar but less in degree ; the 
 greatest difference between the temperature of 
 wool or swandown, while they were without any 
 increase of weight, and the temperature ^e 
 air, having been, on the first of those even .,s, 
 only 4, and on the second only 5. The rea- 
 sons were, in great measure, if not wholly, that 
 a considerable part of the sky was covered with 
 clouds, and that the air was commonly in that 
 state of motion, which is denominated a gentle 
 breeze. 
 
 On the evening of my first experiments, I had 
 omitted to measure the heat of the raised board, 
 before the thermometers were placed upon it. 
 This was attended to on the two latter evenings, 
 on the first of which its upper surface was found, 
 at the commencement of the experiments, 4 
 warmer than the air ; on the second, both it and 
 the air were of the same temperature. Again ; 
 on the first of the latter evenings, 10 grains of 
 wool, to which 3 grains of water had been added, 
 having been laid on the raised board, near the 
 thermometers ; at the end of 45 minutes the 
 parcel was found to have lost 2^ grains of mois- 
 ture by evaporation, during the time, that dry 
 wool had become several degrees colder than 
 the air. 
 
ON DEW, &c. 185 
 
 A fourth experiment of , this kind was made 
 by me on the 7th of January, 1814, in the 
 garden of Lincoln's-Inn-Fields, by placing 10 
 grains of wool on a sheet of pasteboard, which 
 lay upon the snow. At the end of 35 minutes 
 the wool was 5 colder than the air, without 
 possessing any additional weight. 
 
 Having thus shown the justness of my former 
 conclusion, that the cold, observed with dew, 
 is the previous occurrence, and, consequently, 
 that the formation of this fluid has precisely 
 the same immediate cause, as the presence of 
 moisture upon the outside of a glass or metallic 
 vessel, when a liquid considerably colder than 
 the air has been poured into it shortly before ; 
 I shall next apply this fact to the explanation 
 of several atmospherical appearances. 
 
 I. The variety in the quantities of dew, which 
 were found by me upon bodies of the same 
 kind, exposed to the air during the same time 
 of the night, but in different situations, is now 
 seen to have been occasioned by the diversity 
 of temperature, which existed among them. 
 
 II. Agreeably to the opinion of Mr. Wilson 
 and Mr. Six, the cold connected with dew 
 ought always to be proportional to the quan- 
 tity of that fluid ; but this is contradicted by 
 experience. On the other hand, if it be 
 granted, that dew is water precipitated from the 
 
186 ESSAY 
 
 atmosphere, by the cold of the body on which it 
 appears, the same degree of cold, in the pre- 
 cipitating body, may be attended with much, 
 with little, or with no dew, according to the 
 existing state of the air in regard to moisture ; 
 all of which circumstances are found actually 
 to take place. 
 
 III. The formation of dew, indeed, not only 
 does not produce cold, but like every other pre- 
 cipitation of water from the atmosphere, pro- 
 duces heat. I infer this, partly because very 
 little dew appeared upon the two nights of the 
 greatest cold I have ever observed on the sur- 
 face of the earth, relatively to the temperature 
 of the air, both of them having occurred after a 
 long tract of dry weather ; and partly from the 
 most dewy night, which I have ever seen, having 
 been attended, during the greater part of it, 
 with no considerable degree of cold. On this 
 night, the difference between the temperatures 
 of grass and of air was at first 7^, the dew 
 being then not very abundant. But, after the 
 dew had become very abundant, the difference 
 of those temperatures never exceeded 4, and 
 was frequently only 3. 
 
 With the view of obtaining, though indirectly, 
 some knowledge of the quantity of cold, which 
 had been prevented, by the formation of dew, 
 from appearing on the surface of the earth, in 
 
ON DEW, &c. 187 
 
 the night just spoken of, I made the following 
 experiment. To 10 grains of wool having the 
 same form and extension, as the parcels em- 
 ployed for the collection of that fluid, were 
 added 21 grains of water, this being the quan- 
 tity of moisture, which had been attracted by 
 10 grains of wool, lying on the grassplat, in the 
 space of 8 hours on that night. The wet wool 
 having been then placed in a china saucer, laid 
 on a feather-bed in a room, the door and win- 
 dows of which were shut, its heat during the 
 following 8 hours was, at frequent examina- 
 tions, uniformly found to be about 4 less, 
 than that of a dry china saucer on the same 
 bed ; the temperature of the air in the room 
 not having altered more than \ a degree, in the 
 course of the experiment. At the end of the 
 8 hours, the wool still retained 2j grains of 
 moisture. If this quantity had also evaporated, 
 the cold uniformly produced during the 8 hours 
 would, in all probability, have been about 4j. 
 From this experiment, therefore, I think it may 
 be inferred, that the mean quantity of cold, 
 which was prevented, by the formation of dew, 
 from appearing on the ground, during the night 
 which has been mentioned, was also aoout 4^, 
 But, as the production of dew, during some 
 parts of the night, was at a greater rate, than 
 that of 211 grains for 8 hours, 1 or 2 degrees 
 
188 ESSAY 
 
 may be added for those times, which will raise 
 the effect of the dew in diminishing the ap- 
 pearance of cold during them to about 6, on 
 the supposition, which cannot be far from the 
 truth, that dew had been attracted as copiously 
 by the grass, as by wool which lay upon it. 
 
 The less difference commonly observed be- 
 tween the temperatures of grass and of air, in 
 the morning, than what occurs in the evening, 
 is likewise to be, in part, attributed to a greater 
 quantity of dew appearing in the former, than 
 in the latter season. 
 
 A more remarkable fact, deriving an explana- 
 tion from the same source, is the greater dif- 
 ference which takes place in very cold weather, 
 if it be calm and clear, between the tempera- 
 tures of the air and of bodies on the earth, at 
 night, than in equally clear and calm weather 
 in summer; since, in very cold weather, any 
 diminution of the temperature of a portion of 
 air, in contact with a cold body, will be at- 
 tended, in consequence of the well known re- 
 lations of the atmosphere to moisture, with a 
 much less formation of water, than an equal di- 
 minution would be in summer, supposing the 
 air, before it touches the cold body, to be at 
 both times equally near to its point of repletion 
 with moisture. 
 
 IV. In very calm nights, a portion of air, 
 
ON DEW, &c. 
 
 which comes in contact with cold grass, will 
 not, when the surface is level, immediately quit 
 it, more especially, as this air has become spe- 
 cifically heavier than the higher, from a diminu- 
 tion of its heat, but will proceed horizontally, 
 and be applied successively to different parts of 
 the same surface. The air, therefore, which 
 makes this progress, must at length have no 
 moisture to be precipitated, unless the cold of 
 the 'grass which it touches should increase. 
 Hence in great measure is to be explained, why 
 on such nights, as have been just mentioned, 
 more dew was acquired by substances placed 
 on the raised board, than by others of the same 
 kind on the grass, though it began to form 
 much sooner in the latter than in the former 
 situation, those on the raised board having re- 
 ceived air, which had previously deposited less 
 of its moisture. 
 
 A reason is now also afforded, why a slight 
 agitation of the atmosphere, when very preg- 
 nant with moisture, should increase the quan- 
 tity of dew ; since fresh parcels' of air will 
 hence be more frequently brought into contact 
 with the cold surface of the earth, than if the 
 atmosphere were entirely calm. 
 
 V. Dew, in agreement with the immediate 
 cause which has been assigned by me for its 
 production, can never be formed, in temperate 
 
190 ESSAY 
 
 climates, upon the naked parts of a living and 
 healthy human body, during the night ; since 
 their heat is never less in this season, in such 
 climates, than that of the atmosphere. I have, 
 in fact, never perceived dew on any naked part 
 of my own body at night, though my attention 
 was much occupied, for three years, with every 
 thing relative to this fluid, and though I had 
 been, during that period, much exposed to the 
 night air. On the other hand, in very* hot 
 countries, the uncovered parts of a human body 
 may sometimes, from being considerably colder 
 than the air, condense the watery vapour of 
 the atmosphere, and hence be covered with a 
 real dew, even in the day-time. 
 
 VI. Hygrometers formed of animal or vegeta- 
 ble substances, when exposed to a clear sky at 
 night, will become colder than the atmosphere ; 
 and hence, by attracting dew, or, according to 
 an observation of Saussure*, by merely cooling 
 the air contiguous to them, mark a degree of 
 moisture, beyond what the atmosphere actually 
 contains. This serves to explain an observation 
 made by Mr. De Lucf, that in serene and 
 calm weather, the humidity of the air, as de- 
 termined by an hygrometer, increases about, 
 
 * Hygronometrie, p. 25. 
 
 f Introduction a la Physique Terrestre, II, 491. 
 
ON DEW, &c. 
 
 and after sunset, with a greater rapidity, than 
 can be attributed to a diminution of the general 
 heat of the atmosphere. 
 
 These examples are sufficient to show the 
 value of the fact, that bodies become colder than 
 the neighbouring air, before they are dewed, 
 in explaining many atmospherical appearances. 
 To this point, the investigation of the cause of 
 dew might have been carried at any time, since 
 the invention of thermometers ; but its com- 
 plete theory could not possibly, in my opinion, 
 have been attained, before the discoveries on 
 heat were made, which are contained in the 
 works of Mr. Leslie and Count Rumford. 
 
 The experience of most persons, respecting 
 the communication of heat among bodies in the 
 open air, is confined to what happens during 
 the day; at which time, those that are situated 
 near to one another are always found to possess 
 the same temperature, unless some very evident 
 reason for the contrary should exist. To 
 many, therefore, it may appear incredible, that 
 a perfectly dry body, placed in contact, on all 
 sides, with other bodies of the same tempera- 
 ture with itself, shall afterwards, without un- 
 dergoing any chemical change, become much 
 colder than they are, and shall remain so for 
 
192 
 
 ESSAY 
 
 many hours \ yet these circumstances are found 
 to occur in substances attractive of dew, when 
 laid on the surface of the earth, in a still and 
 serene night, and are in perfect agreement 
 with the doctrine of heat, now universally ad- 
 mitted to be just. 
 
 To render this more easy of apprehension, 
 let a small body which radiates heat freely, and 
 possesses a temperature, in common with the 
 atmosphere, higher than 32, be placed, while 
 the air is clear and still, on a slow conductor of 
 heat lying on the surface of a large open plain, 
 and let a firmament of ice be supposed to exist 
 at any height in the atmosphere ; the conse- 
 quence must be, that the small body will, from 
 its situation, quickly become colder than the 
 neighbouring air. For, while it radiates its 
 own heat upwards, it cannot receive a sufficient 
 quantity from the ice to compensate this loss ; 
 little also can be conveyed to it from the earth, 
 as a bad conductor is interposed between them^ 
 and there is no solid, or fluid except the air, to 
 communicate it laterally either by radiation or 
 conduction. This small body, therefore, unless 
 it shall receive from the air, nearly as much 
 heat as it has emitted, which, considering the 
 little that can be communicated from one part 
 of the atmosphere to another, in its present 
 calm state, must be regarded as impossible, will 
 
ON DEW, &c. 193 
 
 become colder than the air, and condense the 
 watery vapour of the contiguous parts "of it, if 
 they should contain a sufficient quantity to 
 admit of this effect. But events similar to these 
 occur, when dew appears in an open and level 
 grass field, during a still and serene night. The 
 upper parts of the grass radiate their heat into 
 regions of empty space, which consequently 
 send back no heat in return ; its lower parts, 
 from the smallness of their conducting power, 
 transmit little of the earth's heat to the upper 
 parts, which at the same time receiving only a 
 small quantity from the atmosphere, and none 
 from any other lateral body, must remain colder 
 than the air, and condense into dew its watery 
 vapour, if this be sufficiently abundant, in re- 
 spect to the decreased temperature of the grass*. 
 
 This subject may be further illustrated by a 
 reference to what happens in the experiment, 
 which has been used to prove the reflection of 
 cold. 
 
 In the simplest form of this experiment, a 
 
 * I have adopted in this explanation the hypothesis of Mr. 
 Prevost of Geneva, on the constant radiation of heat by bo- 
 dies in contact with the atmosphere, even at the time that 
 they are exposed to the influence of bodies warmer than them- 
 selves; as it appears to agree perfectly with all the pheno- 
 mena of the communication of heat, which do not depend 
 upon conduction. I shall hereafter make frequent use of this 
 hypothesis. 
 
 O 
 
194 ESSAY 
 
 small body, the bulb of a thermometer, possess- 
 ing the temperature of the atmosphere, is placed 
 before a larger cold body, rendered equal in 
 effect to one still larger, by means of a concave 
 metallic mirror. In this situation, the small 
 body radiates heat to the larger, without re- 
 ceiving an equivalent from it, and, in conse- 
 quence, becomes colder than the air through 
 which its heat is sent, notwithstanding that it 
 is continually gaining some heat, both from the 
 air which surrounds it, and from the walls and 
 contents of the apartment, in which the experi- 
 ment is made. Dew, therefore, would as readily 
 form upon the thermometer in this experiment, 
 as it would upon one suspended in the open air 
 at night, under a clear sky, provided that the 
 two instruments were equally colder than the 
 atmosphere, and that this was in both cases 
 equally near to being replete with moisture *. 
 
 * The invention of this experiment having been ascribed 
 a few years ago to Mr. Pictet of Geneva, various English 
 writers have shown, that it occurs in several much older 
 foreign authors. But I have not seen any mention made of 
 its having been also long since known in this country. That 
 it was so appears from the following extract of a letter, 
 written by Mr. Oldenburgh to Mr. Boyle in 1665. " I met 
 the other day in the Astrological Discourse of Sir Christopher 
 Heydon, with an experiment, which he affirms to have tried 
 himself, importing, that cold accompanies reflected light, by 
 employing burning spherical concaves, or parabolical sections, 
 
ON DEW, &c. 195 
 
 Regarding now as established, that bodies 
 situated on or near to the surface of the earth 
 become, under certain circumstances, colder 
 than the neighbouring air, by radiating more 
 heat to the heavens, than they receive in every 
 way*, I shall in the first place offer a few re- 
 marks on the extent and use of this occurrence, 
 and shall afterwards apply the knowledge, of it 
 to the explanation of several more of the ap- 
 pearances described in the former part of this 
 Essay, and of some others, which have not 
 hitherto been mentioned by me. 
 
 Which, he saith, will as sensibly reflect the actual cold of snow 
 or ice, as they will the heat of the sun." Boyle's Works, 
 folio, vol. V. p. 345. 
 
 * Count Rumford offered the following conjecture, in a 
 paper printed in the Philosophical Transactions for 1804. 
 fe The excessive cold which is known to reign, in all seasons, 
 on the tops of very high mountains, and in the higher regions 
 of the atmosphere, and the frosts at night, which so fre- 
 quently take place on the surface of the plains below, in very 
 clear and still weather, in spring and autumn, seem to indi- 
 cate, that frigorific rays arrive continually at the surface of 
 the earth, from every part of the heavens." But he gave no 
 experiments to prove, that such a communication actually 
 exists between the heavens and the earth at night. Neither 
 does it appear from any of his writings which I have seen, 
 that he ever supposed, that the surface of the earth is more 
 cooled by these frigorific rays, than the air through which 
 they pass, or that some solid bodies are more cooled by them 
 than others. 
 
 O 2 
 
196 ESSAY 
 
 Radiation of heat by the earth to the heavens 
 must exist at all times ; but, if the sun be at 
 some height above the horizon, the degree of 
 which is hitherto undetermined, and probably 
 varies according to season, and several other 
 circumstances, the heat emitted by it to the 
 earth will overbalance, even in places shaded 
 from its direct beams, that which the earth 
 radiates upwards. I suspended at midday, on 
 the 24th of July, 1813, in the open air over a 
 grassplat, while the sky was wholly covered 
 with very dense clouds, and the weather calm, 
 two delicate thermometers, one of which was 
 naked, but the other cased in gold paper. At 
 two observations, having an interval of 10 mi- 
 nutes between them, the thermometer in the 
 gilt case was 2 lower than that which was naked. 
 A white paper case was then drawn over the 
 gilt one, upon which, after 5 minutes, the 
 covered instrument was observed to be at the 
 same height with the naked. The outer white 
 case having, in the next place, been taken from 
 the covered thermometer, but that which was 
 gilt suffered to remain, the two instruments 
 were in a few minutes found again to differ 2. 
 A thermometer on the grassplat was, during 
 these experiments, higher than the naked in- 
 strument in the air by 2, and than that in the 
 gilt case by 4. It is evident, therefore, that 
 
ON DEW, &c. 
 
 heat radiated by the sun must, on this day, 
 have been transmitted in considerable quantity 
 through the thickest clouds ; since not only 
 was the earth's surface warmer than the air, 
 but a small body, covered with a substance not 
 readily admitting the entrance of radiant heat, 
 was colder than a similar body which was unco- 
 vered. In like manner, I observed at noon, on 
 the 2nd of January, 1814, during the prevalence 
 of a dense fog, a thermometer placed upon 
 swandown, which was lying upon grass thickly 
 incrusted with hoarfrost, to be 2 warmer than 
 the air, and 1 warmer than the grass*. 
 
 In a calm and serene night, however, when 
 consequently little impediment exists to the 
 escape, by radiation, of the earth's heat to the 
 heavens, and when no heat can be radiated by 
 the sun to the place of observation, an immense 
 degree of cold would occur on the ground, if 
 the following circumstances did not combine 
 to lessen it. 1 . The incapacity of all bodies to 
 prevent, entirely, the passing of heat, by con- 
 duction, from the earth to substances placed 
 upon them. Q. The heat radiated to these 
 
 * Another fact of the same kind, which occurred at the 
 same time, is that, although the temperature of the air was 
 3O, the hoarfrost on trees rapidly decreased, the solid matter 
 of the trees intercepting radiant heat, which had penetrated 
 through 4he fog from the sun, and converting it into heat of 
 temperature. 
 
198 ESSAY 
 
 substances by lateral objects. 3. The heat com- 
 municated to the same substances by the air. 
 4. The heat which is evolved, during the con- 
 densation of the watery vapour of the atmo- 
 sphere into dew. 
 
 The extent of the effect of all these checks 
 upon the production of cold, by the nightly 
 radiation of heat from bodies on the surface of 
 the earth, cannot, in the present state of our 
 knowledge, be properly estimated ; but facts 
 show that, notwithstanding their operation, the 
 cold originating in this source must be often 
 very considerable. 
 
 1. Mr. Wilson once observed a difference of 
 16, from this cause, between the temperatures 
 of snow and of air. In taking the latter tem- 
 perature, however, he employed a naked ther- 
 mometer, on which account, in consequence of 
 what has already been mentioned by me, about 
 2 are to be added to the 16 noted by him, in 
 order to obtain the real difference between the 
 heat of the snow and the air at that time*. 
 
 2. If Mr. Wilson, as was formerly said, had 
 
 * As bright metals, when suspended in the air, and exposed 
 to a clear sky on a calm night, become colder than the sur- 
 rounding atmosphere, a thermometer covered with metalled 
 paper, and placed in the circumstances which have been just 
 mentioned, will mark a temperature less than that of the air 
 near to it. But, as the difference must be small, and as I 
 know of no way to estimate it accurately, I have hitherto 
 always neglected to consider it. 
 
ON DEW, &c. 199 
 
 laid a thermometer on any downy substance in 
 contact with the snow, he would, in all proba- 
 bility, have found a cold indicated by it at least 
 20 greater than that of the air, as marked by a 
 naked instrument, and consequently at least 22 
 greater than the real cold of the surrounding 
 atmosphere. 
 
 3. Mr. Wilson's place of observation was not 
 very favourable to the occurrence of a great 
 cold, from radiation of heat at night, it being 
 near to a large smoky city, in the immediate 
 vicinity also, as appears to me from what he 
 says of it, of one or more considerable buildings, 
 and in a climate abounding in moisture. 
 
 4. None of Mr. Wilson's experiments, in 
 which a very great degree of cold occurred, 
 were made within an hour or two after sunset, 
 during which time, according to my observa- 
 tion, the most considerable differences between 
 the temperatures of the air, and of bodies on 
 the surface of the earth, commonly happen. 
 
 If, then, such experiments should be made 
 in an atmosphere still colder than that, in which 
 Mr. Wilson made his, on a large plain remote 
 from any city, and free from objects of every 
 kind, that are elevated above the ground, and 
 in a country remarkable for the dry ness of its 
 air, all which circumstances may be found in 
 Russia during the winter j a difference of at 
 
ESSAY 
 
 least 30 would probably appear, on some still 
 and serene night, between a small thermometer 
 placed with its bulb naked*, on the middle, or 
 leeward side of a stratum of a downy substance, 
 occupying a space upon a grass field, or bed of 
 snow, one or two square yards in extent, and a 
 similar thermometer inclosed in a case of gilt 
 paper, and suspended in the air a few feet above 
 the other. Two thermometers, thus placed, 
 would, I think, be sometimes found even in 
 this country to differ not much less than 30. 
 I have myself never made any such experiments 
 with a downy substance, which had a surface of 
 more than a few square inches, or in a very cold 
 night, when the atmosphere was clear and calm, 
 and the scene of observation remote from large 
 masses of building. 
 
 But even a cold of 30 appears not to be the 
 greatest, that can be thought to occur, from the 
 radiation of heat to the heavens, at night, by 
 substances on the surface of the earth. For 
 experiments by Mr. Pictetf, Mr. Sixf, and I 
 may add by myself, establish that, in exception to 
 the common rule, the heat of the atmosphere in 
 clear and calm nights increases with the distance 
 
 * The effect would, perhaps, be a little increased, by cover-* 
 ing the bulb with a very thin layer of lamp-black, 
 f Essai sur le Feu, c. x. 
 J Phil. Trans. 1784, and 
 
ON DEW, &c, 201 
 
 from the earth. Agreeably to Mr. Six's ex- 
 periments, the atmosphere at the height of 220 
 feet is often, upon such nights, 10 warmer than 
 what it is 7 feet above the ground. If, there- 
 fore, I am able to show, as I expect I shall be 
 in the course of a few pages, that the air at the 
 smaller height becomes colder than that of the 
 greater, from its vicinity to the surface of the 
 earth, previously rendered cold by radiating its 
 heat to the heavens, it will follow, that these 
 10 must be added to the quantity of cold 
 already mentioned ; and, consequently, that a 
 body on the ground may become, at night, at 
 least 40 colder than the air two or three hun- 
 dred feet above it, by the radiation of its heat 
 to a clear sky. 
 
 I shall add, with the greatest diffidence, a few 
 words upon a final cause of the radiation of heat 
 from the earth at night, and upon some of the 
 circumstances which modify its action, though 
 fully conscious of the danger of error, which is 
 always incurred in the attempt to appreciate 
 the works of our Creator. 
 
 The "heat which is radiated by the sun to the 
 earth, if suffered to accumulate, would quickly 
 destroy the present constitution of our globe *. 
 This evil is prevented by the radiation of heat 
 
 * Count Rumford says j ' ' May it not be by the action of 
 these [frigorific] rays, that our planet is cooled continually, 
 and enabled to preserve the same mean temperature for ages, 
 
202 ESSAY 
 
 by the earth to the heavens, during the night, 
 when it receives from them little or no heat 
 in return. But, through the wise economy of 
 means, which is witnessed in all the operations 
 of Nature, the prevention of this evil is made 
 the source of great positive good. For the sur- 
 face of the earth, having thus become colder 
 than the neighbouring air, condenses a part of 
 the watery vapour of the atmosphere into dew, 
 the utility of which is too manifest to require 
 my speaking of it. I may remark, however, 
 that this fluid appears chiefly where it is most 
 wanted, on herbage, and low plants, avoiding, 
 in great measure, rocks, bare earth and con- 
 siderable masses of water*. Its production too, 
 
 notwithstanding the immense quantities of heat that are ge- 
 nerated at its surface, by the continual action of the solar 
 rays?" Phil. Trans. 1804, p. 181. 
 
 * I have no direct observations for the foundation of this 
 assertion concerning considerable masses of water. But, I 
 hold it, notwithstanding, to be just ; because, as soon as the 
 surface of the water is in the least cooled by radiation, the 
 particles composing it must fall downwards, from their in- 
 creased gravity, and be replaced by others that are warmer. 
 The whole mass, therefore, can never, in the course of a 
 single night, be sufficiently cooled to condense into dew any 
 great quantity of the watery vapour of the atmosphere. Be- 
 sides ; I have found, that even a small mass of water, as will 
 be more particularly mentioned in the last part of this Essay, 
 sometimes acquires no weight from the reception of dew, in 
 the space of a whole night favourable to the formation of 
 that fluid. 
 
ON DEW, &c. 203 
 
 by another wise arrangement, tends to prevent 
 the injury, that might arise from its own cause ; 
 since the precipitation of water, upon the tender 
 parts of plants, must lessen the cold in them, 
 which occasions it. I shall observe in the last 
 place, that the appearance of dew is not confined 
 to any one part of the night, but occurs during 
 its whole course, from means the most simple 
 and efficacious. For after one part of the air 
 has deposited its moisture, on the colder surface 
 of the earth, it is removed, in consequence of 
 that agitation in the atmosphere which exists 
 during its stillest states, and gives place to an- 
 other having its quantity of water undiminished ; 
 and, again, as the night proceeds, a portion of 
 air, which had before deposited all the moisture, 
 which circumstances at that time permitted, is 
 rendered fit, by the general increase of the cold 
 of the atmosphere, to give out a fresh parcel, 
 when it comes anew into contact with the 
 ground. 
 
 I. The first fact, which I shall here attempt 
 to explain, is the prevention, either wholly or 
 in part, of cold, from radiation, in substances 
 on the ground, by the interposition of any solid 
 body between them and the sky. This evi- 
 dently appears to arise in the following manner. 
 The lower body radiates its heat upwards, as if 
 no other intervened between it and the sky j 
 
204 ESSAY 
 
 but the loss, which it hence suffers, is more or 
 less compensated by what is radiated to it, from 
 the body above, the under-surface of which 
 possesses always the same, or very nearly the 
 same temperature as the air. In this way there- 
 fore, is to be accounted for the warmth of the 
 substances, which were sheltered from the sky 
 by the raised board, the pasteboard roof, and 
 the hollow cylinders of earth and pasteboard. 
 In these examples, the interposed substances 
 cannot be supposed to have remitted more heat 
 than they received. But in situations where 
 large masses of bare solid matter exist, which 
 are warmer than the atmosphere, from the heat 
 of the preceding day or other causes, a greater 
 heat will be received by the exposed body, than 
 what is radiated by itself. For example, it 
 seems certain to me, that the houses, surround- 
 ing LincolnVInn Fields, had an influence upon 
 my thermometers, during my experiments there 
 at night, beyond what arose from their merely 
 returning a quantity of heat, equivalent to that, 
 which they received from the surface of the 
 garden. It is not, however, absolutely requi- 
 site, that a body should be itself exposed to the 
 sky on a clear and calm night, in order to be- 
 come colder than the atmosphere ; exposure to 
 the influence of another body, so situated, is 
 sufficient for the production of a slight degree 
 
ON DEW, &c. 
 
 of this effect. Thus, I have always found wool 
 attached to the underside of my raised board, 
 on such a night, to be a little colder than the 
 air ; and it has appeared to me a sufficient rea- 
 son for the fact, that the wool in this situation 
 was, in some degree, exposed to the influence 
 of grass, which had become considerably colder 
 than the atmosphere, by radiating its heat to 
 the sky. 
 
 II. No direct experiments can be made to 
 ascertain the manner, in which clouds prevent, 
 or occasion to be small, the appearance of a 
 cold at night, upon the surface of the earth, 
 greater than that of the atmosphere ; but it 
 may, I think, be firmly concluded, from what 
 has been said in the preceding article, that they 
 produce this effect, almost entirely, by radiating 
 heat to the earth, in return for that which they 
 intercept in its progress from the earth towards 
 the heavens. For although, upon the sky be- 
 coming suddenly cloudy during a calm night, 
 a naked thermometer, suspended in the air, 
 commonly rises 2 or 3 degrees, little of this rise 
 is to be attributed to the heat evolved by the 
 condensation of watery vapour in the atmo- 
 sphere, as was supposed by Mr. Wilson * ; since, 
 in consequence of the ceasing of that part of 
 the cold indicated by the thermometer, which 
 
 * Edin. Phil. Trans. I. 15;. 
 
206 
 
 was owing to its own radiation to a clear sky, 
 the temperature of the atmosphere may seem 
 to increase 2, or more, notwithstanding that it 
 has received no real addition. Besides; the 
 heat which is extricated by the condensation of 
 vapour, during the formation of a cloud, must 
 soon be dissipated ; whereas the effect of greatly 
 lessening, or preventing altogether, the appear- 
 ance of a superior cold on the earth to that 
 of the air, will be produced by a cloudy sky r 
 during the whole of a long night. 
 
 Dense clouds, near the earth, must possess 
 the same heat as the lower atmosphere, and will 
 therefore send to the earth, as much, or nearly 
 as much heat as they receive from it by radia- 
 tion. But similarly dense clouds, if very high, 
 though they equally intercept the communica- 
 tion of the earth with the sky, yet being, from 
 their elevated situation, colder than the earth, 
 will radiate to it less heat than they receive from 
 it, and may, consequently, admit of bodies on 
 its surface becoming several degrees colder than 
 the air. In the first part of this Essay, an ex- 
 ample was given of a body on the ground be- 
 coming at night 5 colder than the air, though 
 the whole sky was thickly covered with high 
 clouds*. 
 
 * Mr. Prevost of Geneva, in his work on Radiant Heat, 
 p. 382, has already in this way, conjecturally, accounted for 
 the effect of clouds, in diminishing, at night, the cold of the 
 
ON DEW, &c. 207 
 
 Islands, and parts of continents close to the 
 sea, being, by their situation, subject to a cloudy 
 sky, will, from the smaller quantity of heat lost 
 by them through radiation to the heavens at 
 night, in addition to the reasons commonly 
 assigned, be less cold in winter, than countries 
 considerably distant from any ocean. 
 
 III. Fogs, like clouds, will arrest heat, which 
 is radiated upwards by the earth, and, if they 
 be very dense, and of considerable perpendi- 
 cular extent, may remit to it as much as they 
 receive. Accordingly, Mr. Wilson found no 
 
 atmosphere, and of the surface of the earth ; but he seems 
 not to have known, that their effect on the temperature of 
 the latter is much greater than that which they produce upon 
 the air. My explanation of this influence of clouds, on the 
 temperature of the surface of the earth, during the night, is 
 a direct consequence from the facts, which I had observed 
 respecting the prevention of cold on the ground from radia- 
 tion, by the interposition of solid bodies between it and the 
 heavens, and occurred to me in 1812. Mr. Prevost's work, 
 indeed, was published in IS 9; but I did not see it before 
 the summer of 1813 ; when it was lent to me by his relation 
 Dr. Marcet of London, who at the same time said, that he 
 believed there was no other copy of it in Great Britain, ex- 
 cept one, which had been sent by himself to Edinburgh. 
 
 Note to second edition.'] I did not know, until after the 
 first edition of this Essay was printed, that Mr. Prevost had 
 published his opinion on the effect of clouds in preventing 
 the occurrence of cold at night in the atmosphere, and upon 
 the surface of the earth, as early as 1792, in a work entitled 
 6 Recherches sur la Chaleur.' 
 
208 ESSAY 
 
 difference at night, in very foggy weather, be- 
 tween the temperature of the surface of snow, 
 and that of the air. Several observations by 
 myself tend to confirm that of Mr. Wilson. An 
 instance, however, as was formerly said, oc- 
 curred to me of a difference at night of 9 
 between the temperatures of grass crusted over 
 with hoarfrost, and of air, during a very dense 
 fog. A fact, remarked by Mr. Leslie, respect- 
 ing fogs, serves to explain this apparent ano- 
 maly. For it was found by that philosopher*, 
 from experiments made with his photometer, 
 that in mists and low fogs the diminution of the 
 sun's heat is small, when compared with what 
 occurs, when the sky is obscured by a dense 
 body of clouds ; and it will, I presume, be 
 readily granted, that the same state of the at- 
 mosphere, which allows the heat of the sun to 
 pass copiously, will also give a ready transit to 
 heat radiated by the earth. Now there are 
 several reasons for believing, that the fog, dur- 
 ing which grass was 9 colder than the air, did 
 not ascend far above the ground. 1. The baro- 
 meter had been falling for some days before, 
 and it is a matter of common observation, that 
 great fogs seldom occur, except it be high. 
 2. On the day preceding the observation, the 
 
 * On Heat and Moisture, p. 57. 
 
ON DE\V, &c. 209 
 
 air, after having been extremely foggy for 
 nearly a week, had become clear enough to 
 allow the sun's being distinctly seen during the 
 whole of the afternoon, though there was still 
 a sufficient obscurity in the lowermost parts of 
 the atmosphere, to obstruct considerably the 
 view of objects on the ground and very near to 
 it. 3. On the day following the observation, 
 the fog was again much less ; on the next it 
 disappeared, and was succeeded by snow. It 
 is to be mentioned likewise, that on the even- 
 ing in question the state of the grass, which 
 was the subject of experiment, was unusually 
 favourable to the production of cold ; since, 
 contrary to general experience, it was as cold 
 as swandown. If, then, the latter substance, 
 from the much greater regularity of the ap- 
 pearances exhibited by it, be taken as the 
 standard, by which the occurrences of different 
 nights are to be compared together, it will 
 follow, that the fog of which I am speaking, 
 though it did not prevent, must have lessened, 
 the production of cold from radiation. For, 
 on the preceding evening, when there was little 
 fog, the atmosphere being equally still on both, 
 the difference between swandown and the air 
 was 12; and on another, a fortnight after, the 
 difference at the same place of observation, be- 
 tween thermometers in the same situations, was 
 
210 ESSAY 
 
 , the air being now free from fog. If the 
 atmosphere had been as still on this, as on the 
 former evenings, a greater difference would 
 doubtless have been seen. I conclude, there- 
 fore, that fogs do not in any instance furnish a 
 real exception to the general rule, that what- 
 ever exists in the atmosphere, capable of stop- 
 ping or impeding the passage of radiant heat, 
 will prevent or lessen the appearance at night 
 of a cold on the surface of the earth, greater 
 than that of the neighbouring air. 
 
 It follows also, from what has been said in 
 this article, that the water deposited upon the 
 earth, during a fog at night, may sometimes 
 be derived from two different sources, one of 
 which is a precipitation of moisture from a con- 
 siderable part of the atmosphere, in consequence 
 of its general cold \ the other, a real formation 
 of dew, from the condensation, by means of the 
 superficial cold of the ground, of the moisture 
 of that portion of the air, which comes in con- 
 tact with it. In such a state of things, all 
 bodies will become moist, but those especially, 
 which most readily attract dew in clear weather*. 
 I have had no opportunity, however, of trying 
 
 * The moisture observed at night by Musschenbroek in 
 Holland, and called by him dew, appears to me to have been 
 of this kind. See this Essay, p. 127. 
 
ON DEW, &c. 
 
 211 
 
 this conclusion by the test of observation, since 
 it occurred to me. 
 
 IV. When bodies become cold from radia- 
 tion, the degree of effect observed must de- 
 pend, not only on their radiating power, but in 
 part also on the greater or less ease, with which 
 they can derive heat, by conduction, from 
 warmer substances in contact with them. Thus 
 grass, on a clear and still night, was constantly 
 colder, sometimes very much colder, than the 
 gravel walk, though a small quantity of sand, 
 placed upon grass, was always nearly as cold as 
 this substance. In this case, the difference in 
 temperature, between the gravel walk and sand, 
 evidently depended on the different quantities 
 of heat, which they received from the parts 
 beneath. A like reason is to be given for dew 
 appearing in greater quantity on shavings of 
 wood, than on the same substance in a more 
 dense and compact form ; and for filamentous 
 and downy substances becoming colder than all 
 others, even than lampblack, which is placed 
 by Mr., Leslie, at the head of the best solid 
 radiators of heat. For the lampblack exposed 
 by me, being about 2 lines in depth, possessed, 
 in consequence, a fund of internal heat, which 
 would more readily pass to its cold surface, 
 than the heat of the lower parts of the downy 
 substances would to their upper surface. 
 
212" ESSAY 
 
 This subject is illustrated by the following 
 experiment. On a dewy evening, I depressed 
 into soft garden mould a drinking glass, having 
 a thick flat bottom, until its brim was upon a 
 level with the surrounding earth, and at the 
 same time placed a similar vessel, with its 
 cavity also towards the sky, on the surface of 
 the mould. In the morning, the inside of the 
 depressed glass was entirely dry, while that 
 of the other was dewed. I then applied the 
 bulb of a small thermometer to the inside of 
 the bottom of each vessel, on which I found the 
 heat of that part of the depressed one to be 56, 
 but of the same part of that which stood on 
 the mould only 49 J. At this time the tem- 
 perature of the air was 53. The cause, there- 
 fore, was evident, both of the wetness of the 
 first vessel, arid of the dryness of the second. 
 
 From this source also is to be derived the 
 reason, why the prominent parts of various 
 bodies were observed by Mr. Wilson to be 
 crusted with hoarfrost, while their more re- 
 tired and massy parts were free from it *. 
 
 V. Bodies, exposed in a clear night to the 
 sky, must radiate as much heat to it during the 
 prevalence of wind, as they would do if the air 
 were altogether still. But in the former case, 
 
 * Paper in Phil. Trans. 178O. 
 
ON DEW, &c. 213 
 
 little or no cold will be observed upon them 
 above that of the atmosphere, as the frequent 
 application of warm air must quickly return a 
 heat equal, or nearly so, to that which they had 
 lost by radiation. A slight agitation of the air 
 is sufficient to produce some effect of this 
 kind ; though, as has already been said, such 
 an agitation, when the air is very pregnant with 
 moisture, will render greater the quantity of 
 dew, one requisite for a considerable produc- 
 tion of this fluid being more increased by it, 
 than another is diminished. 
 
 VI. A small body, as a thermometer, sus- 
 pended in the air, will even in the calmest night 
 exhibit but little cold from radiation, since it is 
 continually exposed to the application of fresh 
 parcels of warmer air, both from the progres- 
 sive motion of this fluid, and from the down- 
 ward motion produced in it by the superior 
 gravity of such portions, as have been cooled 
 by contact with the suspended body. On. the 
 other hand, a thermometer upon a board, raised 
 above the earth and possessing a surface of 
 several square yards, will have its cold from 
 radiation much less diminished than the former, 
 as it is exposed to no loss from a downward 
 motion of the air, and as the air, which ap- 
 proaches it horizontally, must, almost always, 
 have had its temperature previously lowered, 
 
214 ESSAY 
 
 by passing over another part of the board. The 
 reason then of the lee side of the raised board 
 being often colder than the windward is ob- 
 vious. 
 
 VII. There is a remark by Theophrastus *, 
 which has been confirmed by other writers, that 
 the hurtful effects of cold occur chiefly in hollow 
 places. If this be restricted to what happens 
 on serene and calm nights, and it does not, I 
 believe, hold true in any other circumstances, 
 two reasons from different sources are to be 
 assigned for it. The first is, that the air being 
 stiller in such a situation, than in any other, the 
 cold, from radiation, in the bodies which it 
 contains, will be less diminished by renewed 
 applications of warmer air ; the second, that 
 from the longer continuance of the same air in 
 contact with the ground, in depressed places 
 than in others, less dew will be deposited, and 
 therefore less heat extricated during its forma- 
 tion. It will be seen in the last part of this 
 Essay, that, in the East Indies-, depressions in 
 the earth are artificially made, for the purpose 
 of increasing the cold, which appears in serene 
 nights. On this subject, however, it is to be 
 observed, that if the depressed or hollow places 
 be deep, in proportion to their horizontal ex- 
 tent, a contrary effect must follow ; as a case 
 
 * Lib. v. c. xvi. 
 
ON DEW, &c. 215 
 
 will occur more or less similar to that which 
 existed in some experiments formerly related 
 by me, in which a small portion of grass was 
 surrounded by a hollow cylinder. 
 
 VIII. An observation closely connected with 
 the preceding, namely that, in clear and still 
 nights, frosts are less severe upon hills, than in 
 neighbouring plains*, has excited more atten- 
 tion, chiefly from its contradicting what is com^ 
 monly regarded an established fact, that the 
 cold of the atmosphere always increases with 
 the distance from the earth. This inferior cold 
 of hills is evidently a circumstance of the same 
 kind, with that ascertained 'by Mr. Pictet and 
 Mr. Six, respecting the increasing warmth, in 
 clear and calm nights at all seasons of the year, 
 of the different strata of the atmosphere, in pro- 
 portion as these are more elevated above the 
 earth. As the greater cold of the lower air is 
 the less complicated fact, I shall attempt to ex- 
 plain it in the first place. Mr. Pictet, indeed, 
 furnishes an explanation himself, by ascribing it 
 to the evaporation of moisture from the ground. 
 But to show that this is not just, it need only 
 be mentioned, that the appearance never occurs 
 in any considerable degree, except upon such 
 
 * Theophrastus also remarks, that it freezes less on hills 
 than on plains, but without mentioning, that this happens 
 only on calm and serene nights. Lib. v. c< xx. 
 
216 ESSAY 
 
 nights as are attended with some dew, and that 
 its great degrees are commonly attended with a 
 copious formation of that fluid ; since it cannot 
 be thought, that the same stratum of air will 
 deposit moisture on the ground, from an in- 
 sufficiency of heat, at the very time it is re- 
 ceiving moisture from the ground, in the state 
 of pellucid vapour, as this presupposes, that it 
 is not yet replete with water. 
 
 Our atmosphere has been very generally re- 
 garded, as incapable of being heated directly 
 by the rays of the sun, principally because these 
 give no heat to any particular portion of it, in 
 which they are brought to a focus. I do not 
 know, whether this experiment was ever made 
 with all the accuracy of which it is susceptible ; 
 but, granting that it has been thus made, my 
 opinion is, notwithstanding, that no reliance 
 can be placed in it. For as air, if heated at all 
 by concentrated sunbeams, must be heated by 
 them in a very slight degree, during the time 
 that their focus may be looked upon as sta- 
 tionary, otherwise the present question would 
 not have arisen, it is necessary for conducting 
 the experiment properly, that, during the whole 
 of it, the same individual small portion of air 
 shall constantly receive that focus ; but this, 
 for various manifest reasons, cannot possibly 
 happen. Viewing, therefore, the argument 
 
ON t)EW, &c. 217 
 
 founded upon this experiment as without force, 
 I shall now offer several considerations, which 
 seem to prove, that air is actually heated by the 
 sunbeams, which enter it. 
 
 1 . Air both reflects and refracts light, and all 
 other bodies, as far as I know, acquire heat, 
 while they act thus on the light of the sun. 
 
 2. Air suffocates or absorbs the sun's light, 
 which it cannot be supposed to do, without in- 
 creasing in temperature. 
 
 3. If air, considered as an uniform fluid, 
 were even incapable of gaming heat directly 
 from the sun's rays, heat would be communi- 
 cated by them to it, through the intervention 
 of the innumerable particles of solid matter, 
 which the trivial experiment of receiving a sun- 
 beam into a darkened room shows to be present 
 in the atmosphere. Should it be said, that this 
 appearance may occur only in the neighbour- 
 hood of the earth, from the accidental admix- 
 ture of solid matter raised from its surface by 
 winds, or in any. other way, the answer is, that, 
 as my inquiry is concerning the existence of a 
 certain condition of the atmosphere, it matters 
 not how this originates. Nothing more can be 
 demanded, than that it should always be found, 
 which I believe to be the case ; since, if I can 
 trust my memory with respect to what took 
 place many years ago, I should say, that such 
 
218 ESSAY 
 
 particles are to be seen, by means of the sun's 
 light, in the air over the middle of the Atlantic 
 ocean. These particles then must receive heat 
 from the sunbeams, which impinge upon them, 
 and this they will communicate to the con- 
 tiguous pellucid air. 
 
 4. Unless it be admitted, that the atmosphere 
 is capable of intercepting part of the heat, 
 which is radiated into it by the sun, and of 
 converting this into heat of temperature, I deem 
 it impossible to find a sufficient reason, for the 
 great warmth which exists, after a long calm, 
 in air incumbent upon the Atlantic and Pacific 
 oceans, at the distance of a thousand miles or 
 more from any considerable body of land. It 
 cannot be derived from the neighbouring water, 
 since this is colder than the lower atmosphere ; 
 and no one will suppose it to be the same heat, 
 which the air had acquired from the last con- 
 tinent it had passed over, many days before. 
 But, if even this were supposed, another difc 
 ficulty would remain to be removed, which is, 
 that, during the whole of the calm, the air is 
 cooled every night, and again becomes warm 
 in the day *. 
 
 * One reason is hence apparent for the great coldness of 
 the high regions of the atmosphere ; since the air in them 
 must be less fit, than that of the lower strata, to arrest heat 
 which is radiated into it. 
 
ON DEW, &c. 219 
 
 Should what has been said be thought suf- 
 ficient to establish, that the air arrests part of 
 the sun's heat, which is radiated into it bound 
 up with light, two consequences must also be 
 allowed. The first is, that air will exert a 
 greater power of the same kind upon heat ra- 
 diated into it without light, since the sun's heat 
 passes instantaneously through many bodies, 
 which refuse a similar way to heat radiated by 
 terrestrial substances ; the other, that air must 
 be as capable of becoming cold by radiating its 
 own heat*, as of becoming warm from heat 
 radiated into it, as these two properties are uni- 
 formly observed to exist together, and to be 
 proportional to each other. The truth of the 
 latter conclusion may also be inferred from this 
 fact, that in still and calm weather the heat of 
 the air, a few feet above the earth, will some- 
 times decrease, even in this country, 18 or 20 
 degrees between sunset and sunrise, though no 
 change of wind has in the meantime occurred > 
 for the inconsiderable conducting power, which 
 air is now known to possess, will permit only 
 a small part of this diminution to arise from 
 heat passing, by means of that power, from the 
 
 * Mr. Prevost says : " On peut supposer queles molecules 
 3e 1'air rayonnent." Du Calorique Rayonnant, p. 24. 
 
220 ESSAY 
 
 atmosphere to the colder earth. Mr. Leslie *, in- 
 deed, ascribes this effect to the descent of cold 
 air from the higher regions of the atmosphere ; 
 but if this were just, a less cold ought to be 
 found, on a clear and still night, in the lower 
 than in the higher strata, which is contrary to 
 the uniform results of numerous experiments 
 by Mr. Pictet and Mr. Six. Winds too, which 
 produce such a mixture, always lessen the noc- 
 turnal decrease of temperature in the lowermost 
 part of the atmosphere. 
 
 Having thus shown, that air is capable, both 
 of absorbing heat, which is radiated into it, and 
 of radiating heat, which had before formed a 
 part of its temperature, I proceed to apply the 
 knowledge of these facts, to the explanation of 
 the phenomenon observed by Mr. Pictet and 
 Mr. Six. 
 
 This phenomenon occurs on those nights 
 only, which permit bodies, on the surface of 
 the earth, to become cold by radiating their 
 heat to the heavens. On other nights, when 
 .bodies, thus situated, were not colder than the 
 air, I have observed the atmosphere, within the 
 limits of 9 feet from the ground, the boundary 
 of my own experiments, to decrease a little in 
 
 * On Heat and Moisture, p. 11, and 132. 
 
ON DEW, &c. 221 
 
 temperature, as the distance from the earth 
 increased. Mr. Six likewise found, that, on 
 cloudy nights, the air was sometimes colder 
 220 feet above the ground, than at the distance 
 of 9 feet from it. When, therefore, the earth 
 has become colder, from radiation, than the 
 neighbouring air, in consequence of the latter 
 having, by reason of its small radiating power, 
 emitted a less proportion of its heat to the 
 heavens, the warmer air must radiate a part of 
 its heat to the earth, without receiving a full 
 compensation, and will therefore become colder, 
 than it otherwise would have been. In propor- 
 tion too as the air is nearer to the earth, must 
 the cold of the former from this cause be the 
 greater. My own conception of this matter is 
 facilitated*, by contemplating the occurrence 
 of an opposite effect, when the earth is warmer 
 than the air. Let it be supposed then, that 
 while the earth, in this state, radiates upwards 
 a quantity of heat, a foot in depth of the in- 
 cumbent air is capable of stopping a 1000th of 
 what it hence receives, and of converting it 
 into heat of temperature. The consequence 
 must be that the next foot, from receiving only 
 999 parts of what had been emitted by the 
 earth, will not be so much heated as the first 
 
 * The same facility is afforded by considering cold as a 
 body. 
 
222 ESSAY 
 
 foot, though it should absorb the same pro- 
 . portional quantity of what enters it. In this 
 way, every successive foot will acquire a less 
 quantity of heat than the preceding, and a 
 state of the atmosphere be produced, like to 
 that which is actually observed in a calm and 
 sunny day. In the day, however, the pheno- 
 mena, from the heating of air by rays from the 
 earth, are somewhat confused by the warmed 
 portions rising upwards, and mixing with what 
 is colder ; whereas, at night, the air, which has 
 been cooled by radiating heat to the earth, is 
 rendered, by an increase of gravity, the more 
 fit to retain its low position. I have here, for 
 the sake of simplifying the argument, taken no 
 notice of the cooling of any considerable mass 
 of the air, in consequence of the actual contact 
 of its lowermost stratum with the earth, or by 
 the conduction of the temperature of one por- 
 tion of it to another. But, in a calm state of 
 the atmosphere, these effects must be incon- 
 siderable, though it appears to me impossible, 
 in the present state of our knowledge, to de- 
 termine them with any precision. 
 
 According to the view, which has been given 
 by me of this subject, the heat of the air, in a 
 clear and calm night, ought to increase, within 
 the limits of the phenomenon, in some de- 
 creasing geometrical ratio, as the atmosphere 
 
ON DEW, &c. 223 
 
 ascends ; and this conclusion is so far con- 
 firmed, by the observations of Mr. Pictet and 
 Mr. Six taken together, that the increase of 
 temperature is found to be greater in a given 
 space very near to the earth, than in an equal 
 space more remote from it. 
 
 To return to the immediate object of this 
 article, the fact is certain, whatever may be 
 thought of my explanation of it, that, in every 
 clear and still night, the air near to the earth is 
 colder than that which is more distant from it, 
 to the height at least of 220 feet, this being the 
 greatest to which Mr. Six's experiments relate. 
 If then a hill be supposed to rise from a plain, 
 to the height of 220 feet, having upon its 
 summit a small flat surface covered with grass ; 
 and if the atmosphere, during a calm and serene 
 night, be admitted to be 10 warmer there, than 
 it is near the surface of the low ground, which 
 is a less difference, according to the observa- 
 tions of Mr. Six, than what sometimes occurs 
 in such circumstances, it is manifest, that, 
 should both the grass upon the hill, and that 
 upon the plain, acquire a cold of 10 by radia- 
 tion, the former will, notwithstanding, be 10 
 warmer than the latter. 
 
 But the equality here supposed to be in the 
 cold acquired by grass, in two such situations, 
 
224 ESSAY 
 
 can seldom exist. For, according to an ob- 
 servation made by Aristotle*, and since fre- 
 quently repeated, the air of high places is much 
 more agitated, than that upon low ground. 
 The frequent renewal, therefore, from this 
 cause, of the air in contact with the grass on 
 the hill, will prevent it from ever becoming 
 much colder than the general mass of the atmo- 
 sphere, at the same height. Consequently, any 
 diminution in this way of the 10 of cold, for- 
 merly supposed to occur there from radiation, 
 must be added to the difference of temperature 
 in the grass in the two situations. 
 
 What has hitherto been said refers only to 
 the occurrences on the very summit of the hill. 
 With respect to its sides, these can be only a 
 little colder than the atmosphere upon a level 
 with them, even in its calmest state. For, in 
 fche first place, they do not enjoy the full aspect 
 of the sky; and, in the second, the air, whicli 
 is cooled by contact with them, will, from its 
 increased gravity, slide down their declivity, 
 and thus make room for the application of new 
 and warm parcels to the same surface. The 
 motion too, thus excited in the air, near to the 
 sides of the hill, must occasion a motion in that 
 
 * Meteor, lib. 1. c. x. 
 
ON DEW, &c. 225 
 
 Upon the summit, which may, in some measure, 
 account for the last-mentioned observation of 
 Aristotle, as far as relates to what happens in a 
 clear night. 
 
 The height of the hill, in this example, has 
 been supposed to be small, to make it accord 
 with that of the stations, whose temperatures 
 were compared by Mr. Six with the heat of the 
 air near the ground. But observations of the 
 same kind will apply to hills of much greater 
 elevation. For granting, first, that the air at 
 the height of 220 feet is never more than 10 
 colder, than that near to the earth, which is not 
 probable, and is indeed contradicted by some 
 of Mr. Six's observations ; and again, that the 
 increase of the air's heat, in a calm and serene 
 night, ceases precisely at the greatest height, 
 to which Mr. Six carried his observations, which 
 is also improbable ; still a reduction, to the ex- 
 tent of 10, in the temperature of the air near 
 to the earth, will render the cold of this low 
 portion of the atmosphere greater than that of 
 any other portion, which is not more than 2500 
 or 3000 feet above the former, if the estimate 
 be just, which makes a declension in the heat 
 of the atmosphere of 1 for every 250, or 300 
 feet of its height, when no counteracting cause 
 exists. 
 
 The remarks, however, which have been 
 
 Q 
 
226 ESSAY 
 
 offered on the greater warmth of hills at night, 
 in a certain state of weather, are strictly ap- 
 plicable to those only, which are insulated, and 
 of inconsiderable lateral extent ; and it is upon 
 such chiefly, if not solely, that this phenomenon 
 has been observed. The superiority of the cold 
 of a low plain, from radiation, over that of a 
 wide expanse of hilly ground, will, for obvious 
 reasons, be less; and no superiority of this kind 
 will probably exist in the former situation, when 
 the high ground is not only extensive, but flat 
 on the top, forming what is called a table-land ; 
 unless indeed, which seems to be actually the 
 case, the air of such an elevated country should 
 be commonly more agitated, than that of lower 
 places equally level. 
 
 An explanation may be now easily given of 
 an observation by Mr. Jefferson of Virginia *, 
 which, however, had also been made by Ari- 
 stotle t, and Plutarch J, that dew is much less 
 copious on hills, than it is upon plains. For 
 allowing, at first, the surface of the ground to 
 be in both situations equally colder than the air 
 which is near to it ; still, as the production of 
 dew must be in proportion to the whole de- 
 pression of the temperature of the air which 
 
 * Notes on Virginia, p. 132. f Meteor. Lib. 1 . c. x. 
 J De Primo Frigido. 
 
ON DEW, &c. 227 
 
 furnishes it, below what its heat had been in 
 the preceding day, and as one part of this de- 
 pression, the general cooling of the atmosphere, 
 is much more considerable on the plain than on 
 the hill, moisture must necessarily be deposited 
 more copiously in the former than in the latter 
 place. If the greater agitation of the atmo- 
 sphere, and the less quantity of moisture, during 
 clear weather, in its higher region than in the 
 lower, be added, it may readily be inferred, 
 that dew shall sometimes be altogether wanting 
 on a hill, though abundant on a plain at its 
 foot, agreeably to what has been actually ob- 
 served by Mr. Jefferson. 
 
 IX. The leaves of trees often remain dry 
 throughout the night, while those of grass are 
 covered with dew. As this is a similar fact to 
 the smallness of dew on hills, I shall in account- 
 ing for it do little more, than enumerate the 
 circumstances on which it depends. 
 
 1. The atmosphere is several degrees warmer 
 near the upper parts of trees on dewy nights, 
 than close to the ground. 2. The air in the 
 higher situation is more agitated, than that in 
 the lower. 3. The air at a little distance from 
 the ground, from being nearer to one of its 
 sources of moisture, will on a calm evening 
 contain more of it, than that which surrounds 
 the leaves of elevated trees. 4. Only the leaves 
 
228 ESSAY 
 
 of the very tops of trees are fully exposed to the 
 sky. 5. The declension of the leaves fron* an 
 horizontal position will occasion the air, which 
 has been cooled by them, to slide quickly away, 
 and be succeeded by warmer parcels. 6. The 
 length of the branches of the trees, the tender- 
 ness of their twigs, and the pliancy of the foot- 
 stalks of their leaves, will cause in the leaves an 
 almost perpetual motion, even in states of air 
 that may be denominated calm. I have hence 
 frequently heard, during the stillness of night, 
 a rustling noise in the trees, which formed one 
 of the boundaries of the ordinary place of my 
 observations, while the air below seemed with- 
 out motion. 
 
 Nearly in the same manner is to be ex- 
 plained, why shrubs and bushes also receive 
 dew more readily than lofty trees. 
 
 X. Bright metals, exposed to a clear sky in a 
 calm night, will be less dewed on their upper 
 surface than other solid bodies ; since of all 
 bodies they will, in such a situation, lose the 
 smallest quantity of heat by radiation to the 
 heavens, at the same time that they are capable 
 of receiving, by conduction, at least as much 
 heat as any others from the atmosphere, and 
 more than any others from the warmer solid 
 substances, which they happen to touch. 
 
 If the exposed pieces of metal be not very 
 
ON DEW, &e. 229 
 
 small, another reason will contribute somewhat 
 to their being later and less dewed than other 
 solid substances. For, in consequence of their 
 great conducting power, dew cannot form upon 
 them, unless their whole mass be sufficiently 
 cold to condense the watery vapour of the 
 atmosphere ; while the same fluid will appear 
 on a bad conductor of heat, though the parts a 
 very little beneath the surface are wanner than 
 the air *. 
 
 From the same ready passage of heat from 
 one part of a metal to another, a metallic plate 
 suspended, horizontally, in the air several feet 
 above the ground, will be found dewed on its 
 lower side, if the upper has become so ; while 
 the lower surface of other bodies, more attrac- 
 tive of dew, but worse conductors of heat, are 
 without dew in a similar situation. 
 
 A metal placed at night in the air, near to 
 the ground, is, for the most part, sufficiently 
 cold to condense, on its underside, the vapour 
 which arises from the warmer earth ; though 
 
 * I hence think it probable, that dew will sometimes form 
 on the bulb of a thermometer, before the mercury in it is 
 cooled below the temperature of the air. It seems certain 
 to me, also, that dew may appear upon substances, which, 
 from the thinness of the layer of matter their cold is con- 
 fined to, will produce little or no sensible effect upon a ther- 
 mometer that is applied to them. 
 
230 ESSAY 
 
 its upper surface may be dry, from possessing 
 the same, or almost the same temperature, as 
 the atmosphere near to it. 
 
 As the temperature of metals is never much 
 below that of the neighbouring air, a slight 
 diminution of their cold from radiation will 
 often occasion them to evaporate the dew, 
 which they had previously acquired, though 
 other substances, which had been more cooled 
 by radiation, are still attracting dew. For a 
 like reason, a metal, which has been purposely 
 wetted, will often become dry at night, while 
 other substances are becoming moist. 
 
 A substance highly attractive of dew, such 
 as wool, if laid upon a metal, will derive heat 
 from it, and will therefore acquire less dew, 
 than an equal portion of the same substance 
 laid upon grass. 
 
 A large metallic plate will be less readily 
 dewed while lying on grass, than if it were 
 placed in the air, though only a few inches 
 above the grass ; because, in the former situa- 
 tion, it receives freely, by means of its great con- 
 ducting power, heat from the earth ; whereas, 
 when placed in the air, it powerfully resists by 
 another property, possessed in a great degree 
 by bright metals, the entrance of heat radiated 
 towards it by the grass beneath. Besides ; the 
 grass under the metal possesses now less heat, 
 
ON DEW, &c. 231 
 
 tlian when this substance was in contact with 
 it, partly from having a small oblique aspect of 
 the sky, and partly from receiving air, which 
 has been cooled by passing over other grass 
 fully exposed to the heavens. 
 
 When a piece of metal, having closely applied 
 to its under surface a substance of some thick- 
 ness, which attracts dew powerfully, and, there- 
 fore, imbibes readily heat that is radiated to it, 
 is exposed to the sky at night, the heat supplied 
 by the attached substance, both from its own 
 original store, and from what it has acquired 
 through the radiation of the ground to it during 
 the exposure, will enable this piece to resist 
 longer, than a bare piece, the formation of dew, 
 or even than another piece, which has only a thin 
 coat of matter considerably attractive of dew 
 attached to its underside. The experiment 
 with the wooden cross, covered with gilt paper, 
 affords an example of the latter fact. 
 
 A very small metallic plate, suspended in the 
 air, is less readily dewed than a large one, 
 similarly situated, as it receives, in proportion 
 to its size, more heat from the atmosphere. 
 On the other hand, a very small plate laid 
 upon grass, rendered cold by radiation, will be 
 sooner dewed than a larger one in the same 
 situation, from presenting a greater propor- 
 tional circumference to the surrounding grass, 
 and therefore losing more quickly its heat by 
 
232 ESSAY 
 
 conduction. It will be also sooner dewed than 
 another very small plate suspended in the air; 
 since the latter, like other small bodies similarly 
 placed, must be continually acquiring more heat 
 than the former, in the manner described above 
 in this Essay *. 
 
 A piece of metal, applied to different por- 
 tions of cold grass in succession, will sooner be- 
 come cold itself, than another piece, which is 
 suffered to remain constantly upon one portion 
 of the same grass, and will in consequence be 
 sooner dewed. 
 
 If the bare side of a piece of metalled paper 
 be exposed to a clear and calm sky at night, it 
 will become cold, by radiation, and receive, by 
 conduction, the heat of the inferior metallic 
 surface ; whence, if this surface be afterwards 
 made the upper one, it will sooner acquire dew 
 than a similar metallic surface, which has been 
 exposed to the sky during the whole of the ex- 
 periment. 
 
 When a metal covers, in part only, the upper 
 surface of a piece of glass, the uncovered por- 
 tion of the glass quickly becomes cold by radia- 
 tion, on exposure to $ serene sky in a still night, 
 and then, by deriving to itself a part of the heat 
 of the metal, occasions this body to be more 
 readily dewed, than if the whole of the exposed 
 
 * Page 213. 
 
ON DEW, &c. 233 
 
 surface had been metallic. In this experiment, 
 the outer edge of the metallic surface, from 
 being nearest to the colder glass, will be the 
 first and the most dewed, while the parts of the 
 uncovered glass, which are contiguous to the 
 warmer metal, will be the last and the least 
 dewed, of their respective substances. 
 
 A piece of glass, covered on one side with a 
 metal, being placed on grass, with this side 
 down, its upper surface attracts dew as readily 
 as if no metal were attached to it ; since the 
 metal, in this situation, has no power to lessen 
 the radiation of heat from the upper surface of 
 the glass. I conclude, however, from general 
 principles, for I have not made the trial, that if 
 the same piece of glass, having its metallic side 
 still undermost, were raised in the air a little 
 above the grass, it would be more readily dewed 
 on its upper surface, than if it had been without 
 a metallic coating on the lower, as this coating 
 must resist the introduction of heat radiated by 
 the warmer grass, and thus preserve nearly un- 
 diminished the cold acquired, from radiation 
 of heat to the sky, by the bare upper surface. 
 
 The preceding remarks apply to the whole 
 class of metals ; but the discoveries of Mr. 
 Leslie, respecting the difference in the capa- 
 cities of these bodies to radiate heat, furnish 
 an explanation of a diversity among themselves, 
 in regard to attraction for dew, which was 
 
234 ESSAY 
 
 noted in the foregoing part of this Essay. Gold, 
 silver, copper and tin, are there said to resist 
 the formation of dew more strongly, than other 
 substances of the same class ; but these metals, 
 according to Mr. Leslie, radiate heat the most 
 sparingly. -On the other hand, lead, iron and 
 steel, which, according to the same author, 
 radiate heat more copiously than the former 
 metals, were found by me to acquire dew more 
 readily. I do not know, if the radiating power 
 of platina has been ascertained by direct ex- 
 periments ; but, as its conducting power is 
 small, its radiation must be great, since these 
 qualities exist always in opposite degrees in the 
 same substance ; and I have accordingly ob- 
 served it to be dewed, while the four first-men- 
 tioned metals were dry. I am ignorant both of 
 the radiating and the conducting power of zinc, 
 as determined by ordinary experiments ; but I 
 infer, from its being more easily dewed than 
 gold 'or silver, that it radiates heat more co- 
 piously than they do ; unless indeed, the pieces 
 which I used, from having had their surfaces 
 roughened by friction with sand, which was 
 employed to brighten them, had acquired a 
 radiating power, greater than that possessed by 
 polished pieces, agreeably to the results of some 
 of Mr. Leslie's experiments *. 
 
 * I once intended to subjoin here an explanation of some 
 very curious observations by Mr, Benedict Prevost on 
 
ON DEW, &c. '235 
 
 XI. Thinking it probable, that black bodies 
 might radiate more heat to the sky, at night, 
 than white, I placed upon grass, on five different 
 evenings, equal parcels of black and white wool. 
 On four of the succeeding mornings, the black 
 wool was found to have acquired a little more 
 dew than the white ; whence I inferred that it 
 had, in consequence of its colour, radiated a 
 little more heat. But I afterwards remarked, 
 that the white wool was somewhat coarser 
 than the black ; which circumstance alone was 
 
 which were published, first in the 44th volume of the French 
 Annals of Chemistry, and afterwards by Mr. Peter Prevost 
 of Geneva, in his Essay on Radiant Heat ; but fearing to be 
 very tedious, I have since given up the design. I will say, 
 however, that, if to what is now generally known on the 
 different modes, in which heat is communicated from one 
 body to another, be added the two following circumstances 5 
 that substances become colder, by radiation, than the air, 
 before they attract dew ; and that bright metals, when ex- 
 posed to a clear sky at night, become colder than the air 
 much less readily than other bodies j the whole of the ap- 
 pearances observed by Mr. Prevost may be easily accounted 
 for. 
 
 Note to second edition."] I found, shortly after the publica- 
 tion of the former edition of this Essay, that the learned Dr. 
 Young had, several years before, in his great work on Na- 
 tural Philosophy, employed the principle of the radiation of 
 heat to account for several of the facts observed by Mr. B. 
 Prevost. On the subject of Dr. Young's explanation, I have 
 spoken somewhat fully in the 28th number of Dr. Thomson's 
 Annals of Philosophy. 
 
236 ESSAY 
 
 sufficient to occasion a difference in their quan- 
 tities of moisture. Another night, I laid on 
 the raised board a piece of pasteboard covered 
 with white paper, and close to this a second 
 piece similar to the former in every respect, 
 except that it was covered with paper black- 
 ened with ink. At daylight, I saw hoarfrost 
 upon both pieces ; but the black seemed to have 
 a greater quantity than the white. A doubt, 
 however, afterwards arose upon the accuracy 
 of this experiment likewise ; for, as the light 
 was faint, when I viewed the two surfaces, the 
 quantity of hoarfrost, though equal on both, 
 might have appeared greater on the black, than 
 on the white, from the contrast of its colour 
 with that of the former surface. But trials of 
 this kind, as Mr. Leslie* has observed, never 
 afford firm conclusions ; since a black body 
 must always differ from a white in one or more 
 chemical properties, and this difference may of 
 itself be competent to produce a diversity in 
 their powers to radiate heat. 
 
 With the view to render the subject less com- 
 plicated, I have hitherto treated of dew, as if it 
 
 * On Heat, p. 95. 
 
ON DEW, &c. 237 
 
 were altogether derived from watery vapour pre- 
 viously diffused through the atmosphere ; this 
 appearing to me to be by far its most considera- 
 ble source, and none of my conclusions of any 
 importance being liable to be affected, even by 
 the establishment of a contrary opinion. Other 
 writers, however, have regarded dew as being 
 entirely the product of vapour emitted, during 
 the night, by the earth and plants upon it. 
 According to this theory, dew is said to rise. 
 
 The first trace, which I have found of the 
 opinion, that dew rises from the earth at night, 
 occurs in the History of the Academy of 
 Sciences for 1687. It is mentioned there briefly 
 and obscurely, and was, probably, shortly for- 
 gotten ; for Gersten, who advanced it anew in 
 1733, held himself to be its author. Muss- 
 chenbroek and Dufay embraced it immediately 
 after Gersten ; but the former soon admitted, 
 that dew sometimes falls. As far as I have 
 learned, no writer upon dew has since ascribed 
 its total production to vapour, emitted by the 
 earth at night, except Mr. Webster of New 
 England*. But this opinion is frequently ad- 
 vanced in conversation by persons, not much 
 accustomed to philosophical pursuits, chiefly, I 
 think, because it contradicts a popular belief. 
 
 * Mem. of American Acad. vol. III. 
 
238 ESSAY 
 
 The only argument used by the French aca- 
 demicians, in support of their opinion, is, if I 
 understand it rightly, that as much dew is ob- 
 served under an inverted glass-bell, as in any 
 other situation. But admitting, for a moment, 
 this to be true, they would not thus prove, 
 that the ground is the only source of that 
 fluid. 
 
 Gersten was led to think, that dew rises from 
 the earth, by often finding grass, and low shrubs, 
 moistened with it, while trees were dry. Re- 
 specting this fact, I shall add nothing to what 
 I have lately said upon it. But his chief argu- 
 ment is derived from another fact related in the 
 first part of this Essay, which is, that a plate of 
 metal, laid upon bare earth on a dewy night, 
 will remain dry on its upper surface, while it 
 becomes moist on the lower. This also is easily 
 explicable by what has already been mentioned 
 by me. For the lower side of the metal, in con- 
 sequence of the upper being in contact with the 
 air and being exposed to a clear sky, is colder 
 than the earth a little below the surface, and 
 therefore condenses the vapour, which strikes 
 against its bottom ; while the upper side, from 
 being frequently warmer, and never more than 
 a little colder than the air, is for the most part 
 unable to condense the watery vapour of the 
 
ON DEW, &c. 239 
 
 atmosphere*. Gersten, moreover, describes 
 several appearances himself, which refute his 
 opinion. He mentions, for example, that the 
 higher parts of shrubs are more dewed than the 
 lower j that metallic plates, placed horizontally 
 in the air, are as much dewed on their superior, 
 as on their inferior surfaces ; and that convex 
 and cylindrical bodies, suspended in the air, the 
 latter having a position parallel to the horizon, 
 are dewed only on their upper parts. 
 
 The principal reason given by Dufay for the 
 rising of dew is, that it appears more early on 
 bodies near to the earth, than on those which 
 are at a, greater height. But this fact readily 
 admits of an explanation on other grounds, that 
 have already been mentioned. 1. The lower 
 air, on a clear and calm evening, is colder than 
 the upper, and will, therefore, be sooner in a 
 condition to deposit a part of its moisture. 
 
 2. It is less liable to agitation than the upper. 
 
 3. It contains more moisture than the upper, 
 from receiving the last which has risen from 
 the earth, in addition to what it had previously 
 
 * I have, in like manner, observed, on a cloudy night, a 
 piece of glass, laid over an earthen pan containing water and 
 placed upon the ground, to be wet on its lower side, while 
 the upper was dry j the glass being, in this situation, suf- 
 ficiently cold to condense the vapour of water heated by the 
 earth, but not enough so to condense the watery vapour of 
 the atmosphere, 
 
240 ESSAY 
 
 possessed, in common with other parts of the 
 atmosphere. Dufay attempted to strengthen 
 his argument, by exposing, on three dewy 
 nights, similar substances at different heights 
 from the ground, expecting that the lower 
 would always acquire more moisture than the 
 upper ; but, upon all the nights, some one of 
 the lower substances acquired less moisture, 
 than some one of the higher. 
 
 Mr. Webster has advanced no new fact in 
 favour of the opinion, of which I am speaking. 
 
 Enough having been said to prove, that dew 
 is not entirely the product of vapour rising from 
 the earth at night, I shall next show, that it 
 often occurs, when this cause can have little or 
 no operation. 
 
 1. It appears from Hasselquist and Bruce, 
 that in Egypt, shortly before the rising of the 
 Nile, and consequently when the ground there 
 is in its driest state, dew becomes exceedingly 
 plentiful, though little or none had formed be- 
 fore, while the earth was somewhat less dry. 
 The cause evidently is, as was formerly men- 
 tioned, the moist air brought from the Me- 
 diterranean by the north wind, which then 
 prevails. 
 
 52. Mr. Webster, speaking of hoarfrost, which 
 he properly regards as frozen dew, candidly 
 says, though it overthrows his opinion : " This 
 
ON DEW, &c. 241 
 
 frost appears, when the surface of the earth is 
 sealed with frost, and of course the vapour of 
 which it is formed, cannot at the time, perspire 
 from the earth." 
 
 3. I have myself, at all seasons of the year, 
 frequently observed wool, upon the middle of 
 the raised board, and therefore out of the way 
 of vapour rising from the ground, to acquire 
 more dew, than wool laid upon the grassplat. 
 
 4. The bodies, that condense the rising va- 
 pour, must necessarily be colder than it ; but, 
 as they are likewise, according to the opinion 
 under view, of the same temperature with the 
 air surrounding them, this also should condense 
 the rising vapour. Dew, therefore, should never 
 appear in any considerable quantity, without 
 being accompanied with fog or mist. Now I 
 can assert after much attention to this point, 
 that the formation of the most abundant dew 
 is consistent with a pellucid state of the atmo- 
 sphere. Hasselquist makes a similar observa- 
 tion, with regard to Egypt j where, during the 
 season remarkable for the most profuse dews, 
 " the nights," he says, " are as resplendent with 
 stars, in the midst of summer, as the lightest 
 and clearest winter nights in the north." 
 
 But, although these facts prove, that copious 
 dews may occur with little or no contribution 
 by vapour immediately rising from the earth, it 
 
242 ESSAY 
 
 must yet be admitted, that some of the moisture, 
 which forms during clear and still weather, on 
 bodies situated upon or near its surface, is in 
 most cases to be attributed to this source ; 
 since, in my experiments, substances on the 
 raised board became much later moist than 
 others on the ground, though equally cold with 
 them. The quantity from this cause, however, 
 can never be great. For in the first place, until 
 the air be cooled by the substances attractive 
 of dew, with which it comes in contact, below 
 its point of repletion with moisture, it will be 
 always in a condition to take up that which has 
 been deposited upon grass, or other low bodies, 
 by warm vapour emitted by the earth ; just as 
 the moisture formed upon a mirror by our breath 
 is, in temperate weather, almost immediately 
 carried away by the surrounding air. Accord- 
 ingly ; I have sometimes, in serene and still 
 weather, observed dew to appear sparingly 
 upon grass in the shade, several hours before 
 sunset, and to continue in nearly the same 
 quantity till about sunset, when it would in- 
 crease considerably, at the time that the same 
 fluid began to show itself on the raised board. 
 In the second place; though bodies situated 
 on the ground, after they have been made suf- 
 ficiently cold, by radiation, to condense the 
 vapour of the atmosphere, will be able to retain 
 
ON DEW, &c. 243 
 
 the moisture, which they acquire by condensing 
 the vapour of the earth ; yet, before this hap- 
 pens, the rising vapour must have been greatly 
 diminished, by the surface of the ground having 
 become much colder. These considerations, 
 added to the fact, that substances on the raised 
 board attracted rather more dew, throughout 
 the night, than similar substances lying on the 
 grass, warrant me to conclude, that on nights, 
 favourable to the production of dew, only a very 
 small part of what occurs is owing to vapour 
 rising from the earth ; though I am acquainted 
 with no means of determining the proportion 
 of this part to the whole. On the other hand, 
 however, in a cloudy night, all the dew that 
 appears upon grass may sometimes be attributed 
 to a condensation of the earth's vapour ; since 
 I have several times, in such nights, remarked 
 the raised board to be dry, while the grass was 
 moist. These nights were calm, and evapora- 
 tion from the grass consequently not copious. 
 When evaporation on cloudy nights was assisted 
 by wind, dew has never, as was mentioned in 
 the first Part of this Essay, been any where 
 observed by me*. 
 
 * The interval between the first appearance of dew in the 
 afternoon on grass, in shaded places, and sunset, was formerly 
 said by me., on the authority, however, of only a few ob- 
 servations, to be considerably greater, than that between 
 
244 ESSAY 
 
 Agreeably to another opinion, the dew found 
 upon growing vegetables is the condensed va- 
 pour of the very plants, on which it appears. 
 But this also seems to me erroneous for several 
 reasons. 1. Dew forms as copiously upon dead 
 as upon living vegetable substances, 2. The 
 transpired humour of plants will be carried 
 away by the air which passes over them, when 
 they are not sufficiently cold to condense the 
 watery vapour contained in it ; unless, which is 
 almost never the case if mist does not already 
 exist, the general mass of the atmosphere be 
 incapable of receiving moisture in a pellucid 
 form. Accordingly, on cloudy nights, when 
 the air, consequently, can never be cooled more 
 than a little below the point of repletion with 
 
 sunrise, and the ceasing of the formation of dew upon grass 
 in the morning. These observations were made on spots 
 exposed during the greater part of the day to the sun. In 
 such places, the heat acquired, from the sun, by the upper- 
 most layer of earth, will be longer retained, than that ac- 
 quired by the grass, which will, therefore, be sufficiently 
 cool, soon after the heat of the day has declined, to condense 
 a part of the vapour then copiously rising from the earth; 
 whereas in the morning, both less vapour will rise, the sur- 
 face of the earth having now lost a great part of its heat, and 
 a less proportion of that which does rise will be condensed 
 by the grass, as the temperature of this body now more 
 nearly approaches that of the ground, from first receiving 
 the heat of the sun reflected from the atmosphere and other 
 substances. 
 
ON DEW, &c. 245 
 
 moisture, by bodies in contact with it, dew is 
 never observed upon any plants, that are ele- 
 vated a few feet above the ground. 3. If a 
 plant has become, by radiating its heat to the 
 heavens, so cold, as to be enabled to bring the 
 air in contact with it below the point of reple- 
 tion with moisture, that which forms upon it, 
 from its own transpiration, will not then, in- 
 deed, evaporate. But other moisture will, at 
 the same time, be communicated to it by the 
 atmosphere ; and when the difference in the 
 copiousness of these two sources is considered, 
 it may, I think, be safely concluded, that almost 
 the whole of the dew, which will afterwards 
 form on the plant, must be derived from the 
 air ; more especially when the coldness of a 
 clear night, and the general inactivity of plants 
 in the absence of light, both lessening their 
 transpiration, are taken into account. 
 
 An experiment, however, has been appealed 
 to in proof, that the dew of plants actually does 
 originate from fluid transpired by them ; that 
 namely, in which a plant, shut up in an air-tight 
 case, becomes covered with moisture. But this 
 experiment, if attentively examined, will be 
 found to have little weight. First ; the in- 
 closed plant, being exempt from the cold, which 
 its own radiation would have produced in its na- 
 tural situation, on a dewy night, will transpire 
 
246 ESSAY ON DEW, &c. 
 
 a greater quantity of fluid, than a similar plant 
 exposed at the same time to the open air. 
 Again ; the small quantity of air, contained in 
 the case, must soon be replete with moisture, 
 after which, the whole of what is further 
 emitted by the plant will necessarily assume 
 the form of a fluid, whatever may be the con- 
 dition of the external atmosphere ; whereas, 
 during even the clearest night, only a part of 
 the smaller quantity of moisture, emitted by 
 the exposed plant, will be condensed on its 
 surface. In the last place ; notwithstanding 
 the circumstances, which favour the appearance 
 of moisture upon inclosed plants from their own 
 transpiration, still the quantity observed on 
 them is said to be, for I have made no experi- 
 ment myself respecting this matter, much less 
 considerable, than what is seen upon plants of 
 the same kind, exposed to the air for the same 
 time, during a calm and serene night. 
 
PART III. 
 
 OF SEVERAL APPEARANCES CON- 
 NECTED WITH DEW. 
 
 1HERE are various occurrences in nature, 
 which seem to me strictly allied to dew, though 
 their relation to it be not always at first sight 
 perceivable. The statement and explanation 
 of several of these will form the concluding part 
 of the present Essay. 
 
 I. I observed one morning, in winter, that 
 the insides of the panes of glass in the windows 
 of my bedchamber were all of them moist, but 
 that those, which had been covered by an inside 
 shutter, during the night, were much more so, 
 than others which had been uncovered. Sup- 
 posing, that this diversity of appearance de- 
 pended upon a difference of temperature, I ap- 
 plied the naked bulbs of two delicate thermo- 
 meters to a covered and uncovered pane 5 on 
 which I found, that the former was 3 colder 
 than the latter. The air of the chamber, though 
 no fire was kept in it, was at this time 11 5 
 
248 ESSAY 
 
 warmer than that without. Similar experiments 
 were made on many other mornings, the results 
 of which were ; that, when the warmth of the 
 internal air exceeded that of the external, from 
 8 to 1 8, the temperature of the covered panes 
 would be from 1 to 5 less than that of the 
 uncovered ; that the covered were sometimes 
 dewed, while the uncovered were dry ; that at 
 other times both were free from moisture ; that 
 the outsides of the covered and uncovered panes 
 had similar differences with respect to heat, 
 though not so great as those of the inner sur- 
 faces ; and that no variation in the quantity of 
 these differences was occasioned by the wea- 
 ther's being cloudy or fair, provided the heat 
 of the internal air exceeded that of the ex- 
 ternal equally in both of those states of the 
 atmosphere. 
 
 The remote reason of these differences did 
 not immediately present itself. I soon, how- 
 ever, saw, that the closed shutter shielded the 
 glass, which it covered, from the heat, that was 
 radiated to the windows by the walls and furni- 
 ture of the room, and thus kept it nearer to the 
 temperature of the external air, than those parts 
 could be, which, from being uncovered, received 
 the heat emitted to them by the bodies just 
 mentioned. 
 
 In making these experiments, I seldom 
 
ON DEW, &c. 249 
 
 observed the inside of any pane to be more than 
 a little damped, though it might be from 8 to 
 12 colder than the general mass of the air in 
 the room ; while, in the open air, I had often 
 found a great dew to form on substances, only 
 3 or ,4 colder than the atmosphere. This at 
 first surprised me ; but the cause now seems 
 plain. The air of the chamber had once been 
 a portion of the external atmosphere, and had 
 afterwards been heated, when it could receive 
 little accession to its original moisture. It con- 
 sequently required being cooled considerably, 
 before it was even brought back to its former 
 nearness to repletion with water ; whereas the 
 whole external air is commonly, at night, nearly 
 replete with moisture, and therefore readily 
 precipitates dew, on bodies only a little colder 
 than itself. 
 
 When the air of a room is warmer than the 
 external atmosphere, the effect of an outside 
 shutter, on the temperature of the glass of the 
 window, will be directly opposite to what has 
 been just stated; since it must prevent the 
 radiation, into the atmosphere, of the heat of 
 the chamber transmitted through the glass. 
 
 II. Count Rumford* appears to have rightly 
 conjectured, that the inhabitants of certain hot 
 
 * Phil. Trans. 1804. p. 182. 
 
250 ESSAY 
 
 countries, who sleep at nights on the tops of 
 their houses, are cooled, during this exposure, 
 by the radiation of their heat to the sky ; or, 
 according to his manner of expression, by re. 
 ceiving frigorific rays from the heavens. An- 
 other fact of this kind seems to be the greater 
 chill, which we often experience upon passing, 
 at night, from the cover of a house into the 
 open air, than might have been expected from 
 the cold of the external atmosphere. The 
 cause, indeed, is said to be the quickness of 
 transition from one situation to another. But, 
 if this were the whole reason, an equal chill 
 would be felt in the day, when the difference, 
 in point of heat, between the internal and ex- 
 ternal air, was the same as at night, which is 
 not the case. Besides ; if I can trust my own 
 observation, the feeling of cold from this cause 
 is more remarkable in a clear than in a cloudy 
 night, and in the country, than in towns. The 
 following appears to be the manner, in which 
 these things are chiefly to be explained. 
 
 During the day, our bodies while in the open 
 air, although not immediately exposed to the 
 sun's rays, are yet constantly deriving heat from 
 them, by means of the reflection of the atmo- 
 sphere. This heat, though it produces little 
 change on the temperature of the air which 
 it traverses, affords us some compensation for 
 
ON DEW, &c. 251 
 
 what we radiate to the heavens. At night also, 
 if the sky be overcast, some compensation will 
 be made to us, both in towns and in the 
 country, though in a less degree than during 
 the day, as the clouds will remit towards the 
 earth no inconsiderable quantity of heat. But 
 on a clear night, in an open part of the country, 
 nothing almost can be returned to us from 
 above, in place of the heat which we radiate 
 upwards. In towns, however, some compensa- 
 tion will be afforded, even on the clearest 
 nights, for the heat which we lose in the open 
 air, by that which .is radiated to us by the sur- 
 rounding buildings. 
 
 To our loss of heat by radiation, at times 
 that we derive little compensation from the ra- 
 diation of other bodies, is probably to be at- 
 tributed a great part of the hurtful effects of 
 the night air. Descartes * says that these are 
 not owing to dew, as was the common opinion 
 of his cotemporaries, but to the descent of cer- 
 tain noxious vapours, which having been ex- 
 haled from the earth during the heat of the 
 day, are afterwards condensed by the cold of a 
 serene night. The effects in question certainly 
 cannot be occasioned by dew, since that fluid 
 
 * Meteorolog. c. vi. 
 
252 ESSAY 
 
 does not form upon a healthy human body, in 
 temperate climates ; but they may, notwith- 
 standing, arise from the same cause, that pro- 
 duces dew on those substances, which do not, 
 like the human body, possess the power of ge- 
 nerating heat, for the supply of what they lose 
 by radiation or any other means. 
 
 III. I had often, in the pride of half know- 
 ledge, smiled at the means frequently employed 
 by gardeners, to protect tender plants from 
 cold, as it appeared to me impossible, that a 
 thin mat, or any such flimsy substance, could 
 prevent them from attaining the temperature 
 of the atmosphere, by which alone I thought 
 them liable to be injured. But, when I had 
 learned, that bodies on the surface of the 
 earth become, during a still and serene night, 
 colder than the atmosphere, by radiating their 
 heat to the heavens, I perceived immediately a 
 just reason for the practice, which I had before 
 deemed useless. Being desirous, however, of 
 acquiring some precise information on this sub- 
 ject, I fixed, perpendicularly, in the earth of a 
 grassplat, 4 small sticks, and over their upper 
 extremities, which were 6 inches above the 
 grass, and formed the corners of a square, the 
 sides of which were 2 feet long, drew tightly 
 a very thin cambric handkerchief. In this 
 
ON DEW, &c. 253 
 
 disposition of things, therefore, nothing existed 
 to prevent the free passage of air from the ex- 
 posed grass, to that which was sheltered, except 
 the 4 small sticks, and there was no substance 
 to radiate heat downwards to the latter grass, 
 except the cambric handkerchief. The tem- 
 perature of the grass, which was thus shielded 
 from the sky, was upon many nights afterwards 
 examined by me, and was always found higher 
 than that of neighbouring grass which was un- 
 covered, if this was colder than the air. When 
 the difference in temperature, between the air 
 several feet above ihe ground and the un- 
 sheltered grass, did not exceed 5, the sheltered 
 grass was about as warm as the air. If that 
 difference, however, exceeded 5, the air was 
 found to be somewhat warmer than the shel- 
 tered grass. Thus, upon one night, when fully 
 exposed grass was 11 colder than the air, the 
 latter was 3 warmer than the sheltered grass j 
 and the same difference existed on another 
 night, when the air was 14 warmer than the 
 exposed grass. One reason for this difference, 
 no doubt, was that the air, which passed from 
 the exposed grass, by which it had been very 
 much cooled, to that under the handkerchief, 
 had deprived the latter of part of its heatj an- 
 other, that the handkerchief, from being made 
 colder than the atmosphere by the radiation of 
 
254 ESSAY 
 
 its upper surface to the heavens, would remit 
 somewhat less heat to the grass beneath, than 
 what it received from that substance. But still, 
 as the sheltered grass, notwithstanding these 
 drawbacks, was upon one night, as may be col- 
 lected from the preceding relation, 8, and upon 
 another 1 1, warmer than grass fully exposed to 
 the sky, a sufficient reason was now obtained 
 for the utility of a very slight shelter to plants, 
 in averting or lessening injury from cold, on a 
 still and serene night. 
 
 In the next place ; in order to learn whether 
 any difference would arise from placing the 
 sheltering substance at a much greater distance 
 from the ground, I had 4 slender posts driven 
 perpendicularly into the soil of a grass field, 
 and had them so disposed in other respects, 
 that their upper ends were 6 feet above the 
 surface, and formed the angular points of a 
 square having sides 8 feet in length. Lastly ; 
 over the tops of the posts was thrown an old 
 ship flag of a very loose texture. Concerning 
 the experiments made by means of this arrange- 
 ment of things, I shall only say, that they led 
 to the conclusion, as far as the events of dif- 
 ferent nights could rightly be compared, that 
 the higher shelter had the same efficacy with 
 the lower, in preventing the occurrence of a 
 cold upon the ground, in a clear night, greater 
 
ON DEW, &c. 255 
 
 than that of the atmosphere, provided the 
 oblique aspect of the sky was equally excluded 
 from the spots on which my thermometers were 
 laid. 
 
 On the other hand ; a difference in tem- 
 perature, of some magnitude, was always ob- 
 served on still and serene nights, between bodies 
 sheltered from the sky by substances touching 
 them, and similar bodies, which were sheltered 
 by a substance a little above them. I found, 
 for example, upon one night, that the warmth 
 of grass, sheltered by a cambric handkerchief 
 raised a few inches in the air, was 5 greater, 
 than that of a neighbouring piece of grass, 
 which was sheltered by a similar handkerchief 
 actually in contact with it. On another night, 
 the difference between the temperatures of two 
 portions of grass, shielded in the same manner, 
 as the two above-mentioned, from the influence 
 of the sky, was 4. Possibly, experience has 
 long ago taught gardeners the superior advan- 
 tage of defending tender vegetables, from the 
 cold of clear and calm nights, by means of sub- 
 stances not directly touching them ; though I 
 do not recollect ever having seen any con- 
 trivance for keeping mats, or such like bodies, 
 at a distance from the plants, which they were 
 meant to protect. 
 
256 ESSAY 
 
 Walls, I believe, as far as warmth is con- 
 cerned, are regarded as useful, during a cold 
 night, to the plants which touch them, or are 
 near to them, only in two ways ; first, by the 
 mechanical shelter which they afford against 
 cold winds, and secondly, by giving out the 
 heat which they had acquired during the day. 
 It appearing to me, however, that, on clear and 
 calm nights, those on which plants frequently 
 receive much injury from cold, walls must be 
 beneficial in a third way, namely, by prevent- 
 ing, in part, the loss of heat, which they would 
 sustain from radiation, if they were fully ex- 
 posed to the sky, the following experiment was 
 made for the purpose of determining the just- 
 ness of this opinion. 
 
 A cambric handkerchief having been placed, 
 by means of two upright sticks, perpendicularly 
 to a grassplat, and at right angles to the course 
 of the air, a thermometer was laid upon the 
 grass close to the lower edge of the handker- 
 chief, on its windward side. The thermometer 
 thus situated was several nights compared with 
 another lying on the same grassplat, but on a 
 part of it fully exposed to the sky. On two of 
 these nights, the air being clear and calm, the 
 grass close to the handkerchief was found to be 
 4 warmer, than the fully exposed grass. On a 
 
ON DEW, &c. 257 
 
 third, the difference was 6. An analogous 
 fact is mentioned by Gersten, who says, that an 
 horizontal surface is more abundantly dewed, 
 than one which is perpendicular to the ground. 
 IV. The covering of snow, which countries 
 in high latitudes enjoy during the winter, has 
 been very commonly thought to be beneficial to 
 vegetable substances on the surface of the earth, 
 as far as their temperature is concerned, solely 
 by protecting them from the cold of the atmo- 
 sphere. But were this supposition just, the 
 advantage of the covering would be greatly cir- 
 cumscribed ; since the upper parts of trees and 
 of tall shrubs are still exposed to the influence 
 of the air. Another reason, however, is fur- 
 nished for its usefulness, by what has been said 
 in this Essay ; which is, that it prevents the oc- 
 currence of the cold, which bodies on the earth 
 acquire, in addition to that of the atmosphere, 
 by the radiation of their heat to the heavens 
 during still and clear nights. The cause, in- 
 deed, of this additional cold, does not constantly 
 operate ; but its presence, during only a few 
 hours, might effectually destroy plants, which 
 now pass unhurt through the winter. Again ; 
 as things are, while low vegetable productions 
 are prevented, by their covering of snow, from 
 becoming colder than the atmosphere in con- 
 sequence of their own radiation, the parts of 
 
258 ESSAY 
 
 trees and tall shrubs, which rise above the snow, 
 are little affected by cold from this cause. For 
 their outermost twigs, now that they are de- 
 stitute of leaves, are much smaller than the 
 thermometers suspended by me in the air, which 
 in this situation very seldom became more than 
 Q colder than the atmosphere. The larger 
 branches too, which, if fully exposed to the sky, 
 would become colder than the extreme parts, 
 are, in a great degree, sheltered by them ; and, 
 in the last place, the trunks are sheltered both 
 by the smaller and the larger parts, not to men- 
 tion that the trunks must derive heat, by con- 
 duction through the roots, from the earth kept 
 warm by the snow *. 
 
 In a similar way is partly to be explained the 
 manner, in which a layer of earth or straw pre- 
 serves vegetable matters in our own fields, from 
 the injurious effects of cold in winter. 
 
 V. The bare mention of the subject of this 
 article will be apt to excite ridicule, it being an 
 attempt to show, in what way the exposure of 
 animal substances to the moon ? s light promotes 
 their putrefaction. I have no certain knowledge, 
 
 * It may be remarked here, however, that a thick covering 
 of snow, while it renders the surface of the earth warmer 
 that it would otherwise be, must occasion the lower atmo- 
 sphere to be colder, by preventing the passage of the heat of 
 the ground to the air, either by radiation or conduction. 
 
ON DEW, &c. 259 
 
 that such an opinion prevails any where, at pre- 
 sent, except in the^West Indies; but I con- 
 clude, from various circumstances, that it exists 
 -also in Africa, and that it was carried thence 
 by negro slaves to America. It was entertained, 
 however, by persons of considerable rank and 
 intelligence among the ancients ; for Pliny * 
 affirms it to be true, and Plutarch, after making 
 it a subject of discussion in one of his Sym- 
 posia f, admits it to be well founded. 
 
 As moonbeams communicate no sensible heat 
 to the bodies, on which they fall, it seems im- 
 possible, that they can directly promote putre- 
 faction. But still a reason, for ascribing such 
 a power to them, may be derived from their 
 being received by animal substances, at the 
 very time that a real, but generally unnoticed, 
 cause of putrefaction, in warm climates, (and 
 it is in these alone the opinion I am treating of 
 has ever prevailed) is taking place, which ceases 
 to act, as soon as the moon's light is excluded. 
 
 The nights, on which a steady moonshine 
 occurs, must necessarily be clear ; and nights, 
 which are clear, are almost always calmt. A 
 
 * Lib. ii. . civ. f Lib. iii. Prob. x. 
 
 I Mr. De Luc has remarked, that clouds frequently dis- 
 appear soon after sunset. Idees sur la Meteorologie, II. 8. 
 I have often observed this myself, and at the same time 
 another fact of which he takes no notice ; namely, that the 
 
 s 2 
 
260 ESSAY 
 
 moonshiny night, therefore, is one, on which 
 dew forms plentifully; hence the expressions 
 ' roscida* and * rorifera luna' employed by Virgil 
 and Statius ; and hence also an opinion, held, 
 as appears from Plutarch, even by philosophers 
 among the ancients, that the moon communi- 
 cates moisture to the bodies, which are exposed 
 to its light *. 
 
 Animal substances are among those, which 
 acquire dew in the greatest quantity. To do 
 this, indeed, they must previously become colder 
 than the atmosphere ; but, having acquired the 
 moisture of dew, in addition to their own, they 
 will, on the following day, be in that condition, 
 which is known, by experience, to favour putre- 
 faction most powerfully in hot climates* 
 
 The immediate cause assigned here, for the 
 quick putrefaction of animal substances, which 
 have been exposed to the moon's rays in a hot 
 country, is the same as that given by Pliny and 
 Plutarch ; but they attributed the origin of this 
 
 atmosphere is then calmer than it had been before sunset. 
 This calmness of the air very commonly, if not always, pre- 
 cedes the dissipation of th6 clouds. 
 
 * Akin to this opinion of the ancients respecting the 
 humefying quality of the moon, is one, which has been held, 
 by modern writers as well as ancient, upon that planet's 
 being a cause of cold to the bodies, which receive its rays . 
 though I know of no author who has taken notice of this 
 affinity. 
 
ON DEW, &c. 261 
 
 immediate cause, the additional moisture, to 
 the peculiar humefying quality, which they 
 supposed that luminary to possess. This false 
 theory has, probably, contributed to discredit, 
 with the moderns, the circumstance which it 
 was employed to explain. 
 
 VI. The last fact, of which I shall treat in 
 this Essay, is the formation of ice, during the 
 night in Bengal, while the temperature of the 
 air is above 32. 
 
 I have seen only two original descriptions 
 of this process, both of which are contained in 
 the Philosophical Transactions ; the first, by Sir 
 Robert Barker, in the 65th volume ; the other 
 in the 83rd, by Mr. Williams. 
 
 According to the method followed by Sir R. 
 Barker's ice-maker, square excavations, 2 feet 
 deep, and 30 wide, having been formed in a 
 large open plain, their bottoms are covered with 
 sugar-cane, or stems of Indian corn, dried, to 
 the thickness of 8 inches or 1 foot. On this 
 layer, are afterwards placed, in rows, near to 
 each other, small, unglazed earthen pans, i ,of 
 an inch thick, and 1 inch and -^ deep, filled 
 with boiled soft water. The pans are sufficiently 
 porous to allow their outer surface to appear 
 moist, after water has been poured into them. 
 Sir R. Barker adds ; that the nights, the most 
 favourable for the production of ice, are those, 
 
262 ESSAY 
 
 which are the calmest and most serene, and on 
 which very little dew appears after midnight ; 
 that clouds and frequent changes of wind, are 
 certain preventives of its formation; and that, 
 although ice is thus very readily procured by 
 art in Bengal, during the winter, it scarcely 
 ever occurs there naturally. 
 
 The process described by Mr. Williams must, 
 from its extent, 300 persons being employed in 
 it, have been carried on for profit, and would, 
 consequently, be conducted in the most eco- 
 nomical manner. A piece of ground, nearly 
 level, containing about 4 acres, was divided 
 into square plats, from 4 to 5 feet wide, which 
 were surrounded by little mounds of earth, 4 
 inches high. In these inclosures, previously 
 filled with dry straw, or sugar-cane haum, were 
 placed as many broad, shallow, unglazed earthen 
 pans, containing unboiled pump water, as they 
 could hold. The air was generally very still, 
 when much ice was formed ; wind prevented 
 its formation altogether. In the morning, be- 
 tween 5 and 6 h., at which time alone, Mr. Wil- 
 liams made his observations, a thermometer, 
 with its bulb naked, placed on the straw, amidst 
 the freezing vessels, was never found by him 
 lower than 35 ; and he has observed ice, when 
 a thermometer so placed was 42. Another ther^ 
 mometer, suspended 5j feet above the ground, 
 
ON DEW, &c. 263 
 
 was commonly 4 higher than that among the 
 pans. It is possible, therefore, that Mr. Wil- 
 liams may have seen ice, a little before 'sunrise, 
 when the temperature of the air was 46. But 
 granting this were the fact, it would not hence 
 follow, that the ice was formed, while the air 
 possessed that heat. For, although the air is 
 generally held to be in all countries colder 
 about sunrise, than at any other time, I know 
 from my own observations, that this is not 
 always the case in England ; and similar ex- 
 ceptions may occur in Bengal. Sir H. Davy 
 has said, in his Elements of Chemistry, that ice 
 will form in Bengal, when the temperature of 
 the air is not below 50 ; but he has given no 
 authority for this assertion. 
 
 The formation of ice, in the circumstances 
 which have been just mentioned, was attributed 
 by Sir R. Barker altogether, and by Mr. Wil- 
 liams in great measure, to cold produced by 
 evaporation. Sir R. Barker's opinion has since 
 been adopted by some of our most distinguished 
 writers on Natural Philosophy, as Watson, 
 Thompson, Young, Davy and Leslie, apparently, 
 however, without their having fully considered 
 it, as I shall now attempt to show. 
 
 1. It is necessary for the complete success of 
 the process, that the air should be very still ; 
 wind, which so greatly promotes evaporation, 
 
264 ESSAY 
 
 prevents the freezing altogether. Sir R. Barker 
 admits, that the excavations in the earth are 
 made to increase the stillness of the air in con- 
 tact with the water in the pans ; but, with the 
 view to explain the utility of this stillness he 
 supposes, in opposition to all experience, that 
 water kept very quiet freezes more readily, 
 when other circumstances are the same, than if 
 it were a little agitated. 
 
 2. No proof is given, that evaporation from 
 the pans actually does occur, at the times which 
 are the most favourable for the appearance of 
 ice. At any rate it cannot be considerable ; 
 since, agreeably to what is mentioned by Sir R. 
 Barker, dew forms in a greater or less degree 
 during the whole of the nights, the most pro- 
 ductive of ice ; and it is not to be thought, as 
 was said upon a former occasion, that one por- 
 tion of air will be depositing moisture, from 
 possessing a superabundance of it, while another 
 in the immediate vicinity is receiving moisture 
 in great quantity, in the state of pellucid vapour; 
 as the latter fact can exist only when the air 
 is far removed from a state of repletion with 
 water. 
 
 3. If evaporation produced the cold under 
 consideration, the wetting of the straw or other 
 matter, upon which the pans are placed, would 
 tend to increase it; and, accordingly, Sir H. 
 
ON DEW, &c. 265 
 
 Davy affirms this to be the case. But Mr. Wil- 
 liams, who must here be regarded as the better 
 authority, says, that it is necessary to the suc- 
 cess of the process that the straw be dry ; in 
 proof of which he mentions, that when the 
 straw becomes wet, by accident, it is replaced ; 
 and that when he purposely wetted it in some 
 of the inclosures, the formation of ice there 
 was always prevented. The reasons are clear. 
 The water, by softening the straw, renders it 
 easily compressible by the weight of the pans, 
 and at the same time fills up what would other- 
 wise be vacant spaces among its parts. The 
 straw, therefore, in this condensed state, must 
 afford a ready passage to heat from the earth to 
 the pans, the hindrance of which is allowed by 
 every person to be the use of it, in this process, 
 when dry. Again ; the moisture, which passes 
 through the straw to the earth it covers, will 
 rise afterwards in the form of vapour, having 
 the same temperature with the warm ground, 
 and will communicate heat to the pans. In the 
 last place ; a part of this vapour will be con- 
 densed into water by the pans, in consequence 
 of which heat must be extricated. 
 
 4. It is mentioned both by Sir R. Barker and 
 Mr. Williams, in support of their opinions, that 
 the pans, when new, are so porous, that they 
 readily permit water to transude them 5 and 
 
266 ESSAY 
 
 that old pans, which permit this in a less de- 
 gree, are less fit for the making of ice. But the 
 argument, which is hence derived by them, is 
 completely refuted by a fact related by Mr. 
 Williams himself; for he says, that the pans are 
 greased before they are used, to prevent the 
 adhesion of the ice to their sides ; since, if this 
 purpose be answered, the water can never be in 
 contact with the pans, and therefore can never 
 pass through them. 
 
 The real reason of the less fitness of old pans 
 for the making of ice is perhaps the following. 
 The production of the cold, which occurs in 
 this process, must take place in the water; 
 since neither the straw upon which the pans 
 are placed, nor the air above them, was ever 
 found by Mr. Williams of so low a temperature 
 as 32. l ' Whatever, therefore, obstructs the 
 passage of heat from the straw to the water, 
 must favour the freezing of the latter. But 
 this will be less effectually done by an old than 
 by a new pan, as the density of the former is 
 greater, from the grease forced into it by rub- 
 bing, and from the slime and sand that will 
 enter with the water into its pores, when these 
 are not entirely closed by the grease ; which 
 must often happen, as the smearing is performed 
 only once in three or four days. The difference, 
 however, in effect betwixt old and new pans 
 
ON DEW, &c. 267 
 
 must be very small ; as it does not appear that 
 the old are ever laid aside on account of their 
 unfitness. 
 
 In a like way may be explained, without the 
 aid of cold produced by the evaporation of 
 moisture from the outsides of the pans, another 
 fact mentioned by Mr. Williams, that ice was 
 often found by him in those vessels, while water 
 contained in a china plate, surrounded by them, 
 had none ; since the thin and dense substance 
 of the plate must have transmitted more readily, 
 than the thick and rare substance of the pans, 
 the heat of the straw to the water. 
 
 5. In accounting for the making of ice in 
 Bengal, it is requisite to show, not only how the 
 first film is produced, but also, in what way the 
 thickness of this film is afterwards increased. 
 If evaporation be the cause of this increase, it 
 follows, that a plate of ice in the night-time, 
 and in the stillest air, both unfavourable to that 
 process, must yet emit as much moisture, as is 
 necessary for the production of a cold, accord- 
 ing to Mr. Williams, of at least 14, and accord- 
 ing to Sir H. Davy of at least 18; a conclu- 
 sion, as it appears to me, of itself sufficient to 
 destroy the credit of the theory, from which it 
 is drawn. 
 
 While attending to this subject, I became 
 desirous of acquiring some knowledge of the 
 
268 ESSAY 
 
 degree of cold, which might be produced by 
 evaporation from water contained in a shallow 
 vessel. With this view, I placed on a feather- 
 bed, situated between the door and window of 
 a room in my house in London, two china plates, 
 into one of which as much water was poured, 
 as covered its bottom to the depth of ^ of an 
 inch. The other plate was kept dry. The 
 bulb of a small thermometer being then applied 
 to the inside of the bottom of each plate, I ob- 
 served upon many days, in various seasons of 
 the year, the difference between these instru- 
 ments while the door and window were open. 
 I found, in consequence, that when the tem- 
 perature of the air in the room was 75, the 
 highest at which any experiment was made, the 
 thermometer in the plate, containing water, was 
 between 6 and 7 degrees lower than the one in 
 the dry plate ; that the difference between these 
 thermometers diminished gradually as the air 
 became colder ; and that when the temperature 
 of the air was 40, the lowest for which I have 
 any observation, the difference was only lj. 
 At 32, therefore, it would have been very 
 small, and at a few degrees below 32 it would 
 probably have vanished. This supposition agrees 
 with an observation made by Mr. Wilson of 
 Glasgow, who found, that no cold was pro- 
 duced by evaporation from snow possessing a 
 
ON DEW, &c. 269 
 
 temperature of 27, though the air in the im- 
 mediate neighbourhood was purposely much 
 agitated by him. 
 
 The conclusions here given by me, respecting 
 the cold produced by the evaporation of water, 
 were drawn from experiments made in the day, 
 while the sky was clear, the air very calm, and 
 the temperature of the atmosphere stationary. 
 At night, and during a cloudy day, the differ- 
 ences were less. On the other hand, if there 
 was any perceptible motion in the air, they 
 were greater. They were also greater if the 
 heat of the atmosphere was increasing; but 
 less, if this was decreasing. 
 
 Having thus, I think, placed beyond doubt, 
 that the formation of ice in Bengal is not occa- 
 sioned by evaporation, I shall now state several 
 reasons, which have induced me to believe, that 
 it depends upon the radiation of heat to the 
 heavens. 
 
 1. This cause not only exists, but exists in a 
 degree, sufficient for the production of the effect, 
 which I attribute to it. For Mr. Wilson found 
 the surface of snow, during a clear and calm 
 night, to be 16 colder than air feet above it, 
 the temperature of the latter being taken by a 
 naked thermometer ; whereas the greatest heat 
 of the atmosphere ever observed by Mr, Wil- 
 liams, at the distance of 5j feet from the ground, 
 
270 ESSAY 
 
 during the time that he supposed ice to be form- 
 ing, was only 14 higher than the freezing point 
 of water. I need say nothing of the difference 
 of 18 related by Sir H. Davy, as he does not 
 speak from his own observation, and as he gives 
 no authority for what he advances ; though even 
 this difference is considerably less, than what I 
 have attempted to show must sometimes occur, 
 from the radiation of heat at night, between 
 the temperature of air, a few feet above the 
 earth, and that of bodies placed on its surface. 
 
 It is to be mentioned here also, that, accord- 
 ing to Mr. Leslie*, the power of water to ra- 
 diate heat, exceeds, perhaps, that of all other 
 substances. 
 
 2. Ice is chiefly formed in Bengal during the 
 clearest and calmest nights ; and it is on such 
 nights that the greatest cold, from radiation, is 
 observed on the surface of the earth. In Sir R. 
 Barker's more refined mode of conducting the 
 process, an unusual stillness of the air, in con- 
 tact with the water to be frozen, is procured, 
 by placing the pans containing it a little below 
 the level of the ground ; in which situation, it 
 was formerly shown, bodies must grow colder 
 from radiation to the heavens at night, than in 
 any other. 
 
 * On Heat, p. 80. 
 
ON DEW, &c. 271 
 
 3. The cold, by means of which ice is pro- 
 duced in Bengal, appears, as I think may be 
 inferred from what is said by Sir R. Barker, in 
 its greatest degree, like cold from radiation 
 in other substances, on those still and serene 
 nights, during which little dew is deposited by 
 the atmosphere. 
 
 4. Clouds and wind prevent the formation of 
 ice in Bengal ; and the same states of the atmo- 
 sphere either prevent, or considerably diminish, 
 the occurrence of cold from the radiation of 
 heat at night by bodies on the ground. 
 
 I shall close this subject, by giving some ac- 
 count of a few attempts to procure the freezing 
 of water at night, in this country, by exposing 
 it to air of a temperature, higher than that of 
 32. These were made by me in 1812, at my 
 usual place of experiment, which was formerly 
 stated to be not well adapted for the appearance 
 of a great cold from radiation, and on nights 
 not among the most favourable to such an un- 
 dertaking, even of those which occur in this 
 country. It is proper also to mention, that I 
 was then less able to conduct such experiments, 
 and to make use of them, than I afterwards be- 
 came, from a longer attention to similar objects. 
 
 ' 
 
272 ESSAY 
 
 EXPERIMENT 1st. 
 
 With a view to imitate the method of making 
 ice described by Sir R. Barker, I had a pit dug, 
 on the evening of the 3rd of May, in the middle 
 of the garden so often spoken of, 4^ feet long, 
 3 wide and 2 deep. It consequently had the 
 same depth as the excavations mentioned by 
 that gentleman, but was considerably less in its 
 other dimensions. Clean dry straw was then 
 strewed, to the height of a foot, over the bottom 
 of the pit. On the straw were next laid a num- 
 ber of small shallow earthen pans, a part of 
 which were glazed, and a part unglazed. In 
 the last place ; all the pans were filled with soft 
 water, which had been boiled on the same even- 
 ing. Contrary to my expectation, the unglazed 
 pans remained as dry on the outside, after water 
 had been poured into them, as those which were 
 glazed. I conclude, therefore, that the former 
 were more dense in their substance, than the 
 unglazed pans used in India; and that their 
 density was probably the reason, why ice did 
 not afterwards form in them, sooner than in 
 the glazed pans, which were employed by me. 
 
 Two pans, containing boiled water, were set 
 upon the grassplat, at a little distance from the 
 pit. A watch-glass filled with boiled water was 
 
ON DEW, &c. 273 
 
 also placed upon the grassplat, and another was 
 laid upon the raised board, which had been 
 thinly covered with sand. All these arrange- 
 ments were not completed before lOh. at night. 
 At 1 h. in the morning, ice appeared in the 
 watch-glasses on the grassplat and raised board ; 
 the heat of the air, as measured by a naked 
 thermometer, being then, at 4 feet above the 
 ground, 39^, and at 7 feet, 40 J. At 2h. ice 
 was observed in the pans in the pit, while a 
 thermometer in the air, 2j feet above the 
 ground, was 36J . Shortly afterwards, ice 
 began also to form in the pans upon the grass- 
 plat. The temperature of grass, fully exposed 
 to the sky, was at the same time 30, while that 
 of the earth an inch below the bottom of the 
 grass was 45. During the time of these ob- 
 servations dew formed copiously. 
 
 EXPERIMENT 
 
 My next attempt was in the manner men- 
 tioned by Mr. Williams. 
 
 On the evening of the 22nd of May, I en- 
 compassed a square piece of level ground, the 
 sides of which were 3 feet long, with a border 
 of earth 4 inches high, and filled the area with 
 dry straw. On this were placed several of the 
 earthen pans, which had been formerly used, 
 
 T 
 
274 ESSAY 
 
 and a few smaller vessels, all containing unboiled 
 water. After an exposure of little more than 
 an hour, water in a watch-glass upon the straw 
 was found frozen, the temperature of the air, 
 feet above the straw, being then 37. In 
 half an hour more, ice began to appear in the 
 earthen pans, while a thermometer 5^ feet 
 above them, this being the height at which Mr. 
 Williams used to suspend his instrument, was 
 36. The air soon after became colder; but 
 its temperature was never less than 33, though 
 taken by a naked thermometer, which, as was 
 before said, upon a clear and calm night, 
 occasions the air to seem about 2 colder than 
 it really is. 
 
 It might be inferred, from what is mentioned 
 by Mr. Williams, that the temperature of the 
 straw beds, on which the ice-pans were set at 
 Benares, was always found by him above the 
 freezing point, for this reason, that the straw, 
 from containing no moisture, could not, like 
 the water, grow cold by evaporation. I had, 
 therefore, been surprised, during the first ex- 
 periment, for I had then but little acquaintance 
 with the phenomena of cold observed with dew, 
 that a thermometer, laid upon an exposed part 
 of the straw, was always below the freezing 
 point, after ice had bgun to form in the pans. 
 On reading, however, his account of the process 
 
ON DEW, &c. 275 
 
 a second time, with increased attention, my 
 wonder ceased. For, as the pans he speaks of 
 were large, and touched one another, and as all 
 the pans employed in India, for the making of 
 ice, widen as they rise from the bottom, like 
 our milk-pans, the thermometer, placed by him 
 on the straw, must have been secluded from all 
 view of the sky, and would therefore mark a 
 temperature much higher, than if it had been 
 laid, as in my experiment, upon straw fully 
 exposed to the heavens. On this, the second 
 night, therefore, I placed a thermometer under 
 the edge of one of the pans lying on the straw 
 bed, and found it some time afterwards 6 higher, 
 than a similar instrument upon a part of the 
 straw bed which was uncovered. Generally, 
 however, the difference was not so great. If 
 my pans had been large, like those of Mr. Wil- 
 liams, I should, no doubt, have observed more 
 considerable differences ; for, in consequence 
 of their smallness, I could not lay a thermo- 
 meter on the straw bed, so as to be fully screened 
 from the sky by the edge of any of them, with- 
 out its being almost in contact with the vessel, 
 every part of which was always colder than the 
 sheltered straw. 
 
 Much dew formed in the course of this night. 
 The greatest difference remarked by me, during 
 it, between the temperatures of grass and of air, 
 
 T 2 
 
276 ESSAY 
 
 was 6, and between those of air and a fully 
 exposed part of the straw bed 9. 
 
 EXPERIMENT 3rd. 
 
 This was begun on the evening of the 16th 
 of October, and was likewise made agreeably 
 to the method related by Mr. Williams, 
 
 Ice appeared in the pans, when the tempera- 
 ture of the air, at the height of 5J feet, was, 
 according to a naked thermometer, 37. 
 
 On this night, I placed upon the straw bed a 
 dry earthen pan, among those which contained 
 water, and found the inside of its bottom to be 
 as much colder than the air, as the water was 
 in the other pans, before ice appeared in them. 
 After the water had begun to freeze, no proper 
 comparison could be made between its tem- 
 perature and that of the empty pan. This pan, 
 in the course of the night, attracted moisture, 
 which was afterwards converted into a film of 
 ice. 
 
 But the chief fact established by the present 
 experiment was, that water may freeze at night, 
 in air of a temperature higher than 32, not 
 only without any loss of weight from evapora- 
 tion, but with a gain of weight from an opposite 
 process. 
 
 I had observed that water, exposed early in 
 
ON DEW, &c. 277 
 
 the evening in the open air to the sky, lost a 
 little weight, in the course of a ..clear night. 
 This I imputed to evaporation taking place, 
 before the water had been cooled enough to 
 condense the vapour of the atmosphere, and to 
 the weight gained afterwards being insufficient 
 to compensate the previous loss. I exposed, 
 therefore, on this night, water to the influence 
 of the sky, until it was cooled to 34. Of this 
 I put 2 ounces into each of two china saucers? 
 which had also been exposed to the air, and 
 then placed the saucers upon the straw bed. 
 In the morning, a thin cake of ice was found in 
 both saucers, one of which had gained 2^, and 
 the other 3 grains, in weight. Dew was also 
 copious on this night. At one time, grass was 
 9j, and the exposed part of the straw bed 12, 
 colder than the air*. 
 
 It must be evident to every person, that the 
 formation of ice, in the three preceding experi- 
 ments, was the effect of a natural operation, 
 similar to that by which the same substance is 
 produced in Bengal. These two facts must, 
 therefore, have a common cause, and this has 
 
 * The greater cold, observed in this and the preceding 
 experiment, upon straw than upon grass, is to be referred 
 to the shortness of the latter, by reason of which heat was 
 readily communicated to its upper parts by the earth. 
 
278 ESSAY 
 
 been shown, by the last experiment, independ- 
 ently of what; was said before in this Essay, not 
 to be evaporation. It is also clear, that the 
 cold, induced on the water in those experi- 
 ments, had a common cause with that observed, 
 at the same time, upon the grass and the straw ; 
 which latter cold must, in consequence of proofs 
 formerly given, be admitted to have arisen from 
 the radiation of the heat of those substances to 
 the heavens. A necessary inference, therefore, 
 appears to be, that the formation of ice in Ben- 
 gal, in the circumstances described by Sir R. 
 Barker and Mr. Williams, must likewise be 
 attributed, in by far the greater measure, if 
 not altogether, to a loss of heat, which the 
 water suffers by its own radiation, while situated 
 in such a manner, that it can receive little heat 
 from other bodies, either by radiation or con- 
 duction*. 
 
 * On the evenings preceding the nights, during which ice 
 is produced in Bengal, the temperature of the water exposed 
 in the pans is, probably, often 60 or more. But water of 
 the heat of 60, if exposed in a shallow earthen vessel to air 
 of the same temperature, during the day, while the weather 
 is calm and clear, will lose about 3 of heat by evaporation. 
 A cold from this cause may, therefore, concur with that from 
 radiation, and, consequently, may, in Bengal, accelerate 
 somewhat the formation of ice. The influence, however, of 
 evaporation there, in this respect, should the state of the air 
 
. ON DEW, &c. 279 
 
 with regard to moisture still permit it, which must often not 
 be the case while dew is forming, will, as the night proceeds, 
 gradually diminish, and at length almost disappear, before 
 the freezing of the water commences; since I have lately 
 shown, that evaporation from water of 32 produces very 
 little cold, even in the day-time. Indeed, it seems to me 
 much more probable, that on a clear and calm night, though 
 in a dry winter of Bengal, water at the temperature of 3$t 
 will acquire warmth from the formation of dew upon it, than 
 that it will become cold from evaporation. 
 
280 ESSAY 
 
 CONCLUSION. 
 
 i 
 
 The experiments which were made by me 
 on dew, and other subjects treated of in the 
 preceding Essay, were unavoidably attended 
 with many inconveniences, which were the 
 more felt, as my health had long been feeble, 
 and as my professional duties obliged me often 
 to return to London in the morning, without 
 having previously taken rest, after the whole' 
 of a night had been spent in attending to the 
 objects of my pursuit. The inconveniences 
 here alluded to were, indeed, so great, that I 
 was twice or thrice obliged to intermit my 
 labours for several months together, and at 
 length found it necessary to cease from them 
 entirely, before I had nearly completed the 
 plan, which I had formed. I take the liberty 
 of mentioning these things, to excuse, in part, 
 the imperfections, which will be observed in 
 what I have written, as some of them would, 
 no doubt, have been removed by a further in- 
 terrogation of Nature *. 
 
 London, September 25, 1815. 
 
 * Of the experiments related in the beginning of the second 
 Part of this Essay, with the view of proving, that the forma- 
 tion of dew is an effect of previous cold in the substances on 
 
ON DEW, &c. 281 
 
 which it appears, those of only one evening were remarkable 
 for the greatness of their results, the weather upon the other 
 evenings not having favoured much my purpose. I took 
 advantage, therefore, of being in the country, at the distance 
 of a few miles from London, on the 2 1st of the present 
 month, the last day but one of an unusually long tract of 
 dry weather, to expose to the sky, 28 minutes before sunset, 
 weighed parcels of wool and swandown, upon a smooth, 
 unpainted, and perfectly dry fir table, 5 feet long, 3 broad, 
 and nearly 3 in height, which had been placed an hour 
 before, in the sunshine, in a large level grass-field. At this 
 time, and throughout my experiments, the air was very still, 
 and the sky very serene. The atmosphere, too, in all pro- 
 bability, contained but little moisture, in consequence of the 
 long absence of rain ; and the surface of the ground appa- 
 rently contained none. The wool, 12 minutes after sunset, 
 was found to be 14 colder than the air, the temperature of 
 the latter being measured by a naked thermometer suspended 
 4 feet above the ground, and to have acquired no weight. 
 The swandown, the quantity of which was much greater 
 than that of the wool, was at the same time 13 colder than 
 the air, and was also without any additional weight. In 2O 
 minutes more, the swandown was 144- colder than the 
 neighbouring air, and was still without any increase of its 
 weight. My experiments now ceased from a failure of 
 daylight. 
 
 In my former experiments of this kind, the greatest cold 
 observed by me from radiation, without the appearance of 
 dew, was only 9^. 
 
 While making the experiments on wool and swandown, 
 I attended frequently to the temperature of the grass, and 
 found it at one time 15 colder than that of the air 4 feet 
 above the ground. This difference is 1 greater, than any 
 I had ever before seen between the temperatures of the same 
 
282 ESSAY ON DEW, &c. 
 
 substances, and is equal to the greatest which I had ever 
 known to occur, between those of the atmosphere and of 
 swandown lying upon grass. I had this evening placed no 
 swandown upon grass. 
 
 These experiments were not made till nearly the whole of 
 the present edition of my Essay was printed, and could not, 
 therefore, be mentioned in their proper place. 
 
A 
 
 LETTER 
 
 TO 
 
 THE RIGHT HONOURABLE 
 
 LLOYD, LORD KENYON, 
 
 RELATIVE TO SOME CONDUCT OF THE 
 
 COLLEGE OF PHYSICIANS OF LONDON, 
 
 POSTERIOR TO THE 
 
 DECISION OF THE COURT OF KING'S BENCH 
 
 IN THE 
 
 CASE OF DR. ST ANGER; 
 
 AND CONTAINING 
 
 OBSERVATIONS ON A PRINCIPAL GROUND OF THAT 
 DECISION. 
 

TO 
 
 DAVID HUME, ESQ. 
 
 ADVOCATE, PROFESSOR OF SCOTCH LAW IN THE 
 UNIVERSITY OF EDINBURGH, &C. 
 
 MY DEAR FRIEND, 
 
 When you requested some months ago, 
 that I would proceed no farther in the letter, which I 
 had informed you I was writing to the Chief Justice of 
 the Court of King's Bench, it appeared to me that you 
 had, in a great measure, mistaken its object. I therefore 
 considered myself entitled to continue my undertaking, 
 and have accordingly now brought it to a conclusion. I 
 readily admit, that, in one point of view, I may have been 
 imprudent ; 
 
 fe <rrsra.i f nfi$ ?' aur^a-iv dtyeoc, 
 
 But I trust that, as far as the more important parts of 
 moral character are concerned, you will find nothing in 
 what I have done unworthy of the friendship between 
 us, which, from its commencement, now nearly thirty 
 years ago, when our boyish fancies gilded every prospect 
 before us, has been ever my pride, and often, in the storms 
 of life, the chief anchor of my hope. 
 
 As a piece of composition, my letter will no doubt be 
 deemed faulty by you in many respects. You will per- 
 ceive, for instance, a considerable want of unity in the 
 execution, should indeed the great rules of criticism be 
 
286 
 
 thought applicable to such a trifle as the present, from 
 the introduction of circumstances, which must seem both 
 trifling and irrelevant, if the rank and character of the 
 person to whom they are communicated be considered. 
 My excuse for part of them is, that, it being one of my 
 intentions to give information to some of my own profes- 
 sion, I conceived it allowable to mention various things 
 for this purpose alone. 
 
 I shall anticipate only one other of your observations 
 regarding my letter, and this refers to the lateness of its 
 appearance. I confess myself much ashamed, that almost 
 a twelvemonth has passed away since the occurrence of 
 the event, which especially gave rise to it. But accidents, 
 which would appear ridiculous in narration, whatever 
 their eifects may have been, often interrupted my labour, 
 and indolence often pleaded, for a time, irresistibly against 
 the performance of an ungrateful task, which duty had 
 imposed. The delay, however, has necessarily tended to 
 diminish the probability of there being many considerable 
 errors in what I have advanced. 
 
 Forgive me for employing this mode of conveying my 
 sentiments to you, and accept my warmest wishes for 
 your welfare. 
 
 I remain, 
 Your most affectionate friend, 
 
 WILLIAM CHARLES WELLS. 
 
 London, 
 September 1, 1799. 
 
LETTER 
 
 TO 
 
 THE RIGHT HONOURABLE 
 
 LLOYD, LORD KENYON, &c. 
 
 La fede unqua non deve esser corrotta, 
 O data a un solo, o data insieme a mille ; 
 E cosi in una selva, in una grotta, 
 Lontan dalle cittadi, et dalle ville; 
 Come dinanzi a tribunal!, in frotta 
 Di testiraon, di scritti, e di postille ; 
 Senza giurare, o segno altro piu espresso, 
 Basti una volta, che s'abbia proraesso. 
 
 MY LORD, 
 
 IF confidence can be placed in the accuracy 
 of the well-known writer of short-hand, Mr. 
 Gurney, the decision of your Lordship, and the 
 other Judges of the Court of King's Bench, in 
 the case of Dr. Stanger, rested principally upon 
 this ground that he might readily obtain by a 
 direct application to the College of Physicians, 
 what he then prayed the court to enjoin that 
 body to grant. -Every person, your Lordship 
 said, has already a right io address himself to 
 the honourable feelings pf those breasts, to 
 
288 LETTER TO 
 
 whicli Dr. Stanger must at last address him- 
 self, if the mandamus were issued. The same 
 sentiment was immediately after expressed by 
 you a second time ; " if any one proposes him" 
 I venture to repeat your Lordship's words 
 " the question is submitted to a majority. It 
 goes then to that tribunal, which, 1 hope and 
 believe, is the sanctuary of honour and good 
 faith, and he may as well address himself to 
 them now, as if this mandamus went." I am, 
 my Lord, one of those persons, whom you thus 
 declared to have a right to address themselves 
 to the honourable feelings of the College of 
 Physicians of London. I have exercised that 
 right. I have applied to the sanctuary of honour 
 and good faith, for a completion of those as- 
 surances, which your Liordship regarded so 
 deeply imbedded in truth, that you erected 
 upon them a decision, wbich was to affect the 
 reputation and fortunes of many of your fellow- 
 subjects, of no mean rank in society, and from 
 which there could be no appeal. Of the suc- 
 cess of this application f now think it my duty 
 to inform you, as it originated in your counsel. 
 The counsel was given in open court ; the nar- 
 ration of its consequences ought, therefore, in 
 my opinion, to be made with equal notoriety, 
 if my feebleness would permit ; and this con- 
 sideration will, I hope, induce your Lordship 
 
LORD KENYON. 289 
 
 to pardon the unusual liberty which I take in 
 addressing you thus publicly. 
 
 But it seems to me proper, before entering 
 upon this narration, that I should speak at 
 greater length of the case of Dr. Stanger. 
 Your Lordship's attention must have been so 
 much occupied by the many important affairs, 
 in which you have been engaged since its oc- 
 currence, that the traces left in your memory 
 by some of its circumstances, the knowledge of 
 which is necessary to the right understanding 
 of what I have to say respecting myself, are 
 now perhaps nearly, if not altogether effaced. 
 
 On the 26th of January, 1796, Dr. Christopher 
 Stanger, a physician of eminence in this metro- 
 polis, made oath in the Court of King's Bench, 
 that he had shortly before applied to the College 
 of Physicians, to be examined for admission 
 into their order of candidates; and that this 
 examination had been refused to him in con- 
 sequence of a by-law, which he conceived con- 
 trary to the intention of the charter and acts 
 of Parliament, by which their corporation had 
 been established. The next day, a rule was 
 granted by the court for the college to show 
 cause, why a mandamus should not issue to 
 compel them to examine that gentleman. In 
 the beginning of the following April, Sir George 
 Baker, president of the college, and Mr. Roberts, 
 
290 LETTER TO 
 
 their attorney, made each of them an affidavit, 
 to justify the refusal to admit Dr. Stanger to 
 the examination he required. In these affidavits 
 it was stated, that the college, in pursuance of 
 a power granted by their charter, had from 
 time to time prescribed certain qualifications 
 and conditions, as requisite for the admission 
 of persons into the commonalty or fellowship, 
 and into the order of candidates ; that, by one 
 of their statutes then in force, no person could 
 be admitted into that order, unless he were a 
 doctor in medicine of Oxford or Cambridge j 
 that Dr. Stanger was not a graduate of either 
 of those universities ; and that there were two 
 by-laws of the college, by which licentiates of 
 certain descriptions might be received into the 
 fellowship, without their previously entering 
 into the order of candidates. Such were the 
 general grounds on which the refusal of the 
 college to examine Dr. Stanger was to be de- 
 fended. It was, however, clearly seen from Sir 
 George Baker's affidavit, that if the reasons 
 hitherto alleged should be found insufficient, 
 an attempt would be made to show, that the 
 applicant was unworthy of reception into any 
 society, from having violated the faith which, 
 it was said, he had solemnly pledged to the col- 
 lege, upon being admitted a licentiate. 
 
 The by-law, which restricted admission into 
 
LORD KENYON. 291 
 
 the order of candidates to the graduates of Ox- 
 ford and Cambridge, had been decided by Lord 
 Mansfield to be bad ; and according to the con- 
 fession of the counsef of the college, the two 
 by-laws, which allowed licentiates to enter the 
 fellowship, had been framed in consequence of 
 the censure passed by that judge upon the 
 former system of admission, and of his recom- 
 mendation that a more liberal one should be 
 adopted. Their real defence, therefore, as 
 having regard to the possible applications of 
 persons in whom they could not pretend to find 
 the smallest appearance of blame, rested en- 
 tirely upon the two last-mentioned by-laws. 
 
 These by-laws were recited at length in the 
 affidavit of Mr. Roberts. By one of them, the 
 president was allowed once in two years, but 
 not oftener, to propose a licentiate often years 
 standing, to be admitted into the college without 
 examination of his fitness. If he chose, how- 
 ever, to omit the exercise of this privilege*, as 
 the present president has repeatedly done, it was 
 not to devolve upon any other person. But 
 when Lord Mansfield condemned the whole of 
 the former system of admission, there existed a 
 much more liberal statute for the reception of 
 licentiates, through favour ; for according to it 
 every licentiate of three years standing, who 
 had taken the degree of doctor in medicine, 
 
292 LETTER TO 
 
 after studying four years in any university, 
 might in this way be admitted a member of the 
 college : one at least, therefore, of the new 
 by-laws, certainly afforded no corrective to the 
 evil, of which that great man complained. 
 
 The remaining by-law was consequently the 
 only source, from which such a corrective could 
 be expected. It declared, that licentiates of 
 seven years standing, and who had completed 
 the thirty-sixth year of their age, might be ad- 
 mitted into the fellowship of the college, should 
 they be found fit upon examination. I shall not 
 trouble your Lordship, at this time, with any 
 observations upon the numerous fetters, by 
 which the action of the pretended principle of 
 this by-law was impeded. I have at present 
 nothing in view but to show, that this was the 
 only measure of any importance the college had 
 adopted for the purpose of removing the re- 
 proach, which had been thrown upon them by 
 Lo*d Mansfield ; and that it therefore afforded 
 the only good ground for their resisting the 
 issue of the mandamus which Dr. Stanger so- 
 licited. 
 
 Accordingly, when the question of the man- 
 damus came to be argued before the Court of 
 King's Bench, on the 23d of April, 1796, Mr. 
 Erskine, the leading counsel of the college, was 
 found to derive from this by-law his chief 
 
LORD KENYON. 293 
 
 reasons against the proceeding of that writ. It 
 can scarcely be thought, that so ingenious and 
 eloquent an advocate would confine the defence 
 of his clients to any single point. It was not 
 surprising, therefore, that he should make a 
 show of resistance at various parts. But still 
 it was evident, that this by-law was regarded 
 by him as his only secure position. How could 
 he indeed act otherwise, consistently with the 
 deference which was due to the opinion of 
 Lord Mansfield? Dr. Stanger had applied for 
 a mandamus to the Court of King's Bench, be- 
 cause the College of Physicians refused to ex- 
 amine him for admission into their order of 
 candidates. But the by-law, in which they 
 grounded this refusal, had been decided to be 
 bad by the late chief judge of that court. No 
 other defence then could well be offered there 
 for such conduct, than that, in compliance with 
 the advice with which his censure was accom- 
 panied, a new by-law for the admission of 
 members had been framed, which so qualified 
 the former, as to take away from it all appear- 
 ance of illiberality ; and that if Dr. Stanger 
 chose to apply under the new statute, he would 
 readily be received into the college. 
 
 This appears to me a just summary of the 
 chief, if not the only argument, of Mr. Erskine 
 upon that occasion. But to avoid all suspicion 
 
294 LETTER TO 
 
 of error, I shall now take the liberty of showing 
 in what manner Mr. Erskine represented hi^ 
 own argument, and what assistance he expected 
 to gain from it, in his attempt to prevent the 
 issue of the mandamus. I shall at least prove 
 by this procedure, that I can have no intention 
 to deceive. 
 
 EXTRACTS* from Mr. ErskmJs Speech in the Court 
 of Kings Bench, April 23, 1796, ii\ the Case of Dr. 
 Stanger against the College of Physicians. 
 
 \ " Subsequently to the time when Dr. Fother- 
 gill's case was before the court, there was a re- 
 vision of the statutes of this learned body, who 
 took the very best and the most eminent advice 
 which this kingdom could furnish them." 
 
 " They made two by-laws in which there 
 is a power given for any fellow at the ordinary 
 comitia majora, after Michaelmas, to propose a 
 licentiate of seven years standing, who is thirty- 
 six years of age, for examination, who, if ap- 
 proved of by the majority of the fellows then 
 present, is to be examined at the three next 
 comitias, and then, if approved, to be admitted 
 
 * From Mr. Gurney's Report, taken in short-hand. 
 
LORD KENYON. 295 
 
 a candidate, though he has not studied at either 
 of the English universities. 
 
 " Your Lordship will observe, that Dr. Stanger 
 could not have this mandamus under this by- 
 law* and therefore I admit I must support the 
 by-law Sir George Baker sets forth in his affi- 
 davit, because no person, except he be of one 
 university or the other, can possibly be ex- 
 amined, but upon the proposition of one of the 
 fellows that he should be examined ; and upon 
 the proposition of one of the fellows, if he be a 
 licentiate of seven years standing, and thirty- 
 six years of age, though he has not that qualifica- 
 tion which is required in the by-law set forth in 
 Sir George Baker's affidavit, yet this door is open 
 to him. And can it be supposed, or will any 
 gentleman stand up and say it is consistent 
 with probability, that a man of eminent learn- 
 ing and high qualifications, who, notwithstand- 
 ing he has not had that species of education, 
 which I will show from the time of the charter 
 to this day has been constantly adopted; yet, if 
 he be a person who has undoubtedly qualifica- 
 tions for it, is it to be supposed that there is not 
 one fellow of the whole college who would pro- 
 pose such a person ?" 
 
 " In the by-law which I have just stated, any 
 one fellow may propose the examination of an 
 
296 LETTER TO 
 
 individual, though such individual could not, 
 according to the ordinary by-laws of the col- 
 lege, be admitted to examination. But to leave 
 the door open, and to prevent the observations 
 that were made in the case of Dr. Fothergill, 
 and under the auspices of the most learned 
 men of the profession, this by-law was made ; 
 therefore, I think, I have gone the length of 
 showing, that the present by-laws can be at- 
 tended with no possible inconvenience." 
 
 " I will only ask my friends, by-and-by, to 
 explain to your Lordship, how it is consistent 
 with reason or common sense, to say, that the 
 public can suffer, or this learned profession be 
 affected in its dignity or advantages, if no per- 
 son should have an opportunity to force him- 
 self into their college, unless he comes within 
 the scope of their by-laws, sanctioned from all 
 antiquity, and comes within the sense of these 
 by-laws; although no door is shut against them 
 at all, but any one fellow of the college may, 
 notwithstanding that statute, propose them for 
 examination, &c. It is not easy to conceive, 
 that a man can be entitled to so much favour, 
 because of his eminent qualifications, as that 
 he can supersede all the rules and provisions 
 of the country, and yet shall not be able to 
 find one person within the walls of a college, 
 
LORD KENYON. 297 
 
 consisting of near a hundred members, to pro- 
 pose him, although such a man would add dig- 
 nity and lustre to the college." 
 
 " Will any man say that these things are at- 
 tended with any inconvenience to the public ? 
 They are not at all ; for, in the first place, if 
 the gentleman who proposes himself to ex- 
 amination has studied at either of the English 
 universities, then this does not apply ; if he has 
 not studied at either of the universities, and 
 can find one fellow in the college who knows 
 any thing of him, and thinks him a fit person to 
 be proposed, then this by-law does not stand 
 in his way." 
 
 Such, my Lord, was the use which Mr. 
 Erskine made of this by-law, in resisting the 
 issuing of the mandamus. The pleadings ceased 
 almost immediately after he had finished his 
 speech, and the rule was discharged, in con- 
 sequence of an error which was discovered in 
 the mode of Dr. Stanger's application to the 
 college. While it was in doubt, however, whe- 
 ther this error was of sufficient importance to 
 put a stop to the proceedings, some conversa- 
 tion took place between the judges and Dr. 
 
298 LETTER TO 
 
 Stanger's counsel, an exact relation of a part of 
 which will demonstrate more strongly than I 
 can possibly do, that the court uniformly re- 
 garded the conditions, which were required by 
 the by-law for admitting licentiates into the 
 college, merely as cautionary measures against 
 the entrance of improper persons into their 
 body ; and constantly supposed, that if any 
 licentiate of good character, and possessing the 
 qualifications marked by the statute, could pre- 
 vail upon a fellow to propose him, no obstacle 
 would afterwards exist to his admission. How 
 far these opinions were well founded, will here* 
 after appear to your 'Lordship. 
 
 Extract from Mr. Gurnets Report. 
 
 Mr. Justice Lawrence. " Where is the diffi- 
 culty of a gentleman's getting some one fellow 
 of the college to propose him ?" 
 
 Mr. Law. "There has been no person ad-, 
 mitted there have been many trials, but no- 
 body has ever got through that wicket, nor 
 ever will.*' 
 
 Mr. Justice Lawrejice. " Do you imagine, if 
 they think Dr. Stanger, or any other physician, 
 is a fit person, that they will not propose him?" 
 
 Lord Kenyan. " There is a wicket of that 
 
LORD KENYON. 299 
 
 kind put in our own profession for, as I un- 
 derstand, all the four inns of court have for 
 some time insisted, that one of their body shall 
 propose a gentleman to be called to -the bar, 
 and that precaution has been attended with ex- 
 tremely good consequences. I am sorry, from 
 what one hears, that it has not been quite a 
 sufficient guard now and then, through a little 
 inadvertence or misinformation ; but certainly 
 it is attended with good consequences." 
 
 Mr. Law. " That is a delegation of the 
 power of inquiry to one whose special business 
 it is, and I believe every gentleman upon whom 
 that delegation falls, discharges his duty pro- 
 perly, and makes that inquiry," 
 
 Mr. Justice Grose. " But why should not 
 this supposed duty be as honourably and as 
 well executed by them, as in our profession*?" 
 
 * Students of law, as the author is informed, are admitted 
 to the bar by the benchers of the inns of courts, who, for the 
 following reasons, may be supposed to execute their trust 
 with fairness and impartiality. 1. As they are few in number, 
 each of them must feel himself responsible for the acts of the 
 whole. 2. They are either of advanced age, and little con- 
 nected with the practice of their profession, or of considerable 
 rank in it. None of them, therefore, can well be jealous of 
 any person who may apply for admission. 3. The applicants 
 for admission are, for the most part, very young men, who 
 for this reason cannot have exhibited such talents as are 
 
300 LETTER TO 
 
 The mode of Dr. Stanger's first application 
 to the college having been determined to be 
 
 likely to excite jealousy in persons much their seniors, were 
 these even liable to be affected with that passion. 4. The 
 profession of law includes so many individuals, that the ac- 
 cession of one more to it can scarcely excite fear in any former 
 member, that his profits may hence be diminished. 5. Since 
 none are allowed to practise as advocates before admission at 
 an inn of court, an applicant cannot, in the previous exercise 
 of his profession, have given umbrage to any of those who 
 are to decide upon his fitness. 6. So many gentlemen of 
 great figure and independent fortune embrace the profession 
 of law, either with the view of preparing themselves for the 
 discharge of various duties incident to their rank in society, 
 or in the expectation of obtaining some high office in the 
 state, that it is natural to infer that great liberality exists in 
 its government. Stronger reasons may no doubt be given 
 by persons better acquainted with the subject, but these seem 
 to the author sufficient to explain the fact, that every person, 
 who possesses the prescribed qualifications, is morally certain, 
 upon application to any of the inns of court, of being admitted 
 to the bar. 
 
 Similar reasons cannot be given, why fairness and impar- 
 tiality should be found in the decisions of the College of 
 Physicians, upon the applications of licentiates for admission 
 into their body. For, 1 . Not a few of the fellows, but all of 
 them indiscriminately, determine the fate of every such ap- 
 plication. 2. The greater part of the voters are consequently 
 not of such a rank in their profession as to be above the 
 reach of jealousy. The proportion of such persons at the 
 meetings of the college is further increased by their having 
 little to do elsewhere. 3. As the seven years of the applicant's 
 licentiateship will, in all probability, have been spent in the 
 metropolis, it is surely not unlikely, that some of the voters 
 
LORD KENYON. 301 
 
 wrong, in the June following he presented him- 
 self a second time to them, requesting permis- 
 sion to undergo any examination which might 
 ascertain his fitness to be & fellow of their body. 
 An examination was again refused. Dr. Stanger 
 having made oath of this, a new rule was ob- 
 tained on the 26th of November, from the Court 
 of King's Bench, for the college to show cause 
 why a mandamus should not issue against them. 
 
 may have become jealous of his talents or success. 4. The 
 members of the college are very few in number when com- 
 pared with the barristers belonging to all the different inns 
 of court. In the list for 1798, there are only twenty-seven 
 fellows who exercise their profession in London, and some of 
 these are very aged, and take little concern in practice. Any 
 advantage, therefore, to be derived by a licentiate from being 
 admitted into the college, will probably be regarded by some 
 of the former members as tending to diminish, or prevent 
 the increase of their own emoluments. 5. In the course of 
 seven years passed in the exercise of a profession, in which, 
 above all others, misunderstandings are apt to arise among 
 its different members, it is almost impossible that a licentiate 
 should not have given umbrage to some of those who are to 
 decide upon this application. 6. Physicians in this country 
 are almost universally taken from the middle ranks of men. 
 They cannot therefore be expected to conduct themselves, as 
 a body, in the same liberal manner as the members of a pro- 
 fession, which contains a considerable number of persons of 
 high birth and large hereditary fortunes. Other circum- 
 stances, tending in like manner to produce unfair and partial 
 decisions in the College of Physicians, when licentiates apply 
 to them to be examined, will be mentioned hereafter. 
 
302 LETTER TO 
 
 On the 23d of January, 1797, Dr. Gisboriie* 
 then president of the college, made an affidavit 
 in answer to Dr. Stanger's, the purport of 
 which was the same as that of the affidavits of 
 Sir George Baker and Mr. Roberts, in the 
 former cause. In the new trial which followed 
 on the llth of May, 1797, the leading counsel 
 of the college, as in the preceding one, was 
 Mr. Erskine, who quickly abandoning all weak 
 points, again fixed upon the by-law for the 
 admission of licentiates, after an examination 
 of their fitness, as the only ground which was 
 fit to bear his works of defence. To prove 
 that I am here also justifiable in attributing 
 such conduct to him, I proceed to insert several 
 passages from his speech upon this second oc- 
 casion. 
 
 EXTRACTS from Mr. ErsJcine's Speech in the Court 
 of King's Bencft, May 11, 1797, in the Case of Dr. 
 Stanger against the College of Physicians. 
 
 " Your Lordship will take it that this last 
 statute which I have read, [that restricting 
 admission into the order of candidates to gra< 
 duates of Oxford and Cambridge] and which 
 still is in existence, and which is qualified by 
 others I am about to state, was the last in exist- 
 ence at the time when the cases of the King v. 
 
LORD KENYON. 303 
 
 Dr. Askew and Dr. Fothergill, and those other 
 cases came before the Court of King's Bench, 
 as reported in Sir James Burrow. Since that 
 time your Lordship will find that other by-laws 
 have been introduced, greatly qualifying those 
 previous by-laws, and as I have been given to 
 understand, framed under the advice, and with 
 the assistance of some of the most eminent and 
 learned persons in this kingdom, in the profes- 
 sion of the law." [Mr. Erskine then recited 
 the by-law, by which licentiates might enter 
 the college upon being examined in regard to 
 their fitness, and afterwards proceeded thus :] 
 " Your Lordship observes then, that by the last 
 by-law which I have just now stated, though 
 a man had never seen either of the universities, 
 yet if he can find out of the whole college of 
 physicians, any one person who is a fellow of 
 the college, to usher him in for an examination, 
 he is, notwithstanding the other statute, of 
 which this statute, your Lordship observes, is 
 a great qualification, entitled to undergo the 
 ceremonies which the college has thought fit 
 to prescribe ; and which I will show your Lord- 
 ship, by and by, it has, and it cannot be denied 
 that it has, a right to prescribe for its own go- 
 vernment ; he may be admitted." 
 
304 LETTER TO 
 
 " Then what are we assembled here upon ? 
 Why upon this grave and notable question 
 whether the by-laws which I have read to your 
 Lordship, taken altogether as one body ; those 
 that are subsequent qualifying, restraining, and 
 modifying those that are antecedent ; whether all 
 these taken together constitute a reaso able body 
 of' by-laws^ within the meaning of the charter^ 
 granted by the king, and confirmed by an act 
 of the legislature. Orl, whether these by-laws 
 shut out any persons who had a right by some 
 privilege inherent in them as British subjects, 
 under this charter, and this act of parliament, 
 from becoming members of this grave and 
 learned body.*' 
 
 " Would a mandamus go to the Bishop of 
 London if he refused ordination to a person 
 who set forth his learning, but had not been at 
 either of the universities, and was therefore re- 
 jected ? But have the college of physicians done 
 that ? No ; they have done no more than this^ 
 if you have been at the university, and have 
 acquired a degree and testimonials, without 
 dispensation, we presume that you are learned 
 from the place from whence you came, and the 
 'discipline you have been engaged in, and we 
 examine you at once j but if you have not, do 
 
LORD KEXYtfN. 305 
 
 rce reject you? No\ but we require that you 
 should be introduced for examination by some one 
 of the fellows of the. college, and then we will exa- 
 mine you. Is it consistent with common sense 
 to say, that there is any thing unreasonable in 
 that?" 
 
 " Your Lordships have the same authority, 
 assembled in your judicial capacity, as judges 
 over our voluntary societies, as you have over 
 a college by mandamus. I apprehend, if a 
 person were to apply to your Lordships, and 
 say, I have been rejected at Lincoln* s-Inn ; 
 why ? because I could find nobody who would 
 give in my name to the benchers to be called 
 to the bar ; you would reject such petition with 
 indignation. You would say, that those learned 
 bodies, who have a jurisdiction exactly similar, 
 only that it is directed and referred to a different 
 profession, in the regulation, and in the learn- 
 ing and integrity of the members of which, the 
 public have a similar interest, inasmuch as they 
 exercise a profession very important in every 
 view of it ; your Lordships would say, that he 
 ought not to be admitted, who could not find 
 one person to propose him as fit to be examined ; 
 (and that is all that we are contending for] be- 
 cause if a man can find any one fellow of the 
 
 x 
 
306 LETTER TO 
 
 college to propose him, he may be admitted, 
 provided they think him fit." 
 
 " Now I will consent to the learned Serjeant 
 making this rule absolute, if he can prove that 
 this by-law is unreasonable ; for we are here 
 upon the reasonableness of the by-law. I read 
 that part of the charter which gives them au- 
 thority to make by-laws, and I defy the wit or 
 imagination of man to put another question 
 upon the court here, than Whether this class 
 of by-laws, taken altogether, be unreasonable. 
 I consent to the rule being made absolute, if 
 any one of my friends, or all of them together, 
 can, in their imagination ; I do not appeal to 
 any experience they can bring ; but if they can 
 in their imaginations, however fertile they may 
 be, figure to themselves an inconvenience that 
 may arise from them. They may say, Oh, there 
 may be a conspiracy which may exclude a vir- 
 tuous and enlightened man ! Setting aside the 
 main improbability, that members of a learned 
 body could league themselves in a conspiracy so 
 base and so scandalous, as to refuse to examine 
 a man proposed to them by one of their own 
 order, under their own laws, from a professional 
 jealousy, lest they should be eclipsed by that 
 person, &c." 
 
LORD KENYON. 307 
 
 " And yet what is the argument, that, when 
 bowing to the great learning and ability of Lord 
 Mansfield upon that occasion, when the college 
 having no other end and object in the world ; 
 and what other end and object can they have, 
 than the regulation of a profession, which I will 
 say and let Dr. Stanger take part of the honour 
 if he pleases is a profession which not only 
 preserves the health of our relations and friends, 
 and gives greater security to human life, but 
 which, I say, also gives us a class of men who 
 are an ornament to society and to this country, 
 with a knowledge of the languages and the 
 various branches of philosophy, which gives 
 that insight into nature and its works which are 
 acquired in the learned institutions, which now 
 are to be broken down, and all sort of persons 
 are to be suffered to do What ? Not to practise 
 physic, for they practise it already, but they 
 are to be let in for the purpose of governing 
 one of the wisest and the most learned bodies ; 
 of governing men who, one and all of them, 
 almost, are deeply skilled in every thing that 
 learning and science have brought forth in any 
 age ; and yet, forsooth, it is to be considered, 
 as if the charter and acts of parliament were 
 likely to suffer, because a man has kept his learn- 
 ing so much to himself, that nobody could everjind 
 it out) so as to be able to think it was wise or decent 
 
308 LETTER TO 
 
 to propose him\ or else, that he is such a pheno- 
 menon in human shape, that there must be a 
 conspiracy among them to keep him out, lest 
 he should eclipse them all. I am sure Dr. 
 Stanger does not wish to represent himself as 
 such a person ; but I am certain that if Dr. 
 Stanger would have applied to the college, as men 
 of the first learning in every age have applied 
 to it, he would have been admitted. 39 
 
 These were the arguments, which the by- 
 law for admitting licentiates to examination 
 furnished to Mr. Erskine, against the issuing 
 of the mandamus, upon Dr. Stanger's second 
 application. That they were the only argu- 
 ments, upon which he placed the least depend- 
 ance in preventing that writ from going forth, 
 is demonstrated by the concluding sentence of 
 his speech, in which he collects to a single point 
 the scattered tendencies of all that he had 
 before advanced. " My proposition is," said 
 Mr. Erskine, " that it is reasonable the college 
 should say ; if you are of the universities we 
 will examine you at once ; if not of the universi- 
 ties^ we do not refuse to examine you, but we 
 consider it reasonable to point out the mode 
 in which that examination should go forward ; 
 otherwise we must examine all the world : and 
 
LORD KENYON. 309 
 
 we conceive that the regulation which we have 
 imposed, in order to prevent frivolous examina- 
 tions, is not inconsistent with the reasonable 
 exercise of discretion ; and which, therefore, is 
 warranted by the charter, which entitles us to 
 make these statutes." 
 
 Having already occupied so much of your 
 Lordship's time in citing passages from Mr. 
 Erskine's speeches in the case of Dr. Stanger, 
 I feel averse to give extracts from those of the 
 remaining counsel of the college upon the same 
 occasion. I shall, therefore, only recal to your 
 Lordship's tecollection, that two of them, Mr. 
 Gibbs and Mr. Dampier, made use of the same 
 by-law to convince the court, that the manda- 
 mus ought not to proceed. 
 
 I know not, my Lord, exactly in what light 
 the declarations of counsel in a court of law are 
 to be regarded, or how far they may be thought 
 binding upon the persons in whose behalf they 
 are made ; but if they are ever held to impose 
 an obligation upon a client, and without admit- 
 ting that they do, it is difficult to conceive how 
 the business of a court of judicature can be 
 carried on, those in Dr. Stanger's case ought 
 to have produced this effect. The principal 
 advocate, of a body termed by your Lordship, 
 the sanctuary of honour and good faith, declares 
 
310 LETTER TO 
 
 to the Court of King's Bench, that his clients 
 are willing to examine Dr. Stanger, or any other 
 person of the same description, should applica- 
 tion be made to them under a particular by- 
 law which he recites. This pledge, for so I 
 must call Mr. Erskine's declaration, was given 
 in April, 1796. The same cause was tried a 
 second time in May, 1797. If, therefore, Mr. 
 Erskine had gone beyond his instructions in 
 giving that pledge, sufficient time had surely 
 intervened, to have allowed the college to make 
 the discovery, and to warn him against commit- 
 ting the same error a second time. Did this 
 happen ? His subsequent conduct proves that 
 it did not ; for in his second speech he repeats 
 the pledge, in language still stronger than that 
 which was formerly employed by him. But 
 it may be said, that inveterate obstinacy, or 
 unconquerable ignorance on the part of Mr. 
 Erskine, might occasion the repetition. Some 
 of a committee of the college appointed to 
 conduct the law-suit hear Mr. Erskine's second 
 speech. Do they then caution the remaining 
 counsel to avoid the rock, which had twice en- 
 dangered the safety of the vessel committed to 
 his care ? We can here also only judge frpm the 
 event. The two who speak next, vouch, like 
 Mr. Erskine, for the willingness of the college 
 
LORD KEN YON. 311 
 
 to examine Dr. Stanger, or any other person of 
 similar qualifications, for admission into their 
 body. 
 
 But it seems superfluous to offer proof, that 
 the college were bound by .the repeated and 
 unchecked declarations of their counsel, to a 
 prompt and honourable execution of the statute 
 for the admission of licentiates to examination, 
 when it is considered in what light that statute 
 was regarded by the court. For in delivering 
 your opinion on Dr. Stanger's case, your Lord- 
 ship, after speaking of the by-law which had 
 formerly restricted admission into the college 
 to the graduates of Oxford and Cambridge, was 
 pleased to express yourself in the following 
 manner : 
 
 " If it, [the restrictive statute] had been a 
 sine qua non, if it had controlled the parties 
 who are to form their judgment, and taken 
 from them all power of decision upon candi- 
 dates, it would have had that seed of death in it, 
 which Lord Mansfield found in that by-law* which 
 he decided to be bad. But this is not so ; here 
 
 * The only difference between the present restrictive sta- 
 tute, and that which was in existence in the time of Lord 
 Mansfield, is, that foreigners, who have taken degrees at 
 Oxford or Cambridge, are not now prevented from entering 
 the college : but it is evident that tl^is relaxation can affect 
 very few persons, perhaps not more than one in a century. 
 
312 LETTER TO 
 
 every person has a right to address himself to the 
 honourable feelings of those breasts, to which Dr. 
 Stanger must at last have addressed himself, if 
 this mandamus went. If they find him to be, 
 (as I am inclined to believe he is from what I 
 hear of him) possessed of all the requisites of 
 medical learning and moral character, he will 
 address as powerful arguments to those gentle- 
 men, every individual of whom is called upon to 
 exercise his opinion upon the subject. He is not 
 to wait to be seconded; the by-law does not 
 require that ; if any one proposes him the ques- 
 tion is submitted to a majority. It goes then 
 to that tribunal, which, I hope and believe, is 
 the sanctuary of honour and good faith, and he 
 may as 'well address himself to them now as if this 
 mandamus went ; they are not bound to admit, 
 all they are bound to do is to examine." 
 
 One of your brethren on the bench, Mr. 
 Justice Grose, refused the mandamus on the 
 same ground as your Lordship. Another, Mr. 
 Justice Lawrence, had several times, in the 
 course of the two trials, declared his confidence 
 in the readiness of the college to admit any 
 licentiate, in the situation of Dr. Stanger, to 
 an examination, and for this reason probably 
 thought it unnecessary to repeat the same opi- 
 nion, when he gave his reasons for refusing the 
 mandamus. The remaining judge, Sir William 
 
LORD KENYON. 313 
 
 Ashurst, was the only one who did not, at some 
 period or other of Dr. Stanger's applications to 
 the court, approve of the by-law for the admis- 
 sion of licentiates into the college upon exa- 
 mination, and express his belief that it would 
 be carried into execution, whenever an applica- 
 tion should be made in consequence of it. What 
 he said, however, at the close of the trial, 
 afforded no reason to suppose, that he enter- 
 tained sentiments on these subjects, different 
 from those of his brethren. 
 
 I have now, my Lord, finished the relation of 
 those parts of Dr. Stanger's case, which seem to 
 me to form a proper introduction to what I shall 
 say concerning myself. I may have been tire- 
 some by minuteness of detail ; but if I have 
 been at the same time accurate, as I believe I 
 have, I trust that I / shall readily receive your 
 Lordship's forgiveness ; more especially .when 
 it is considered with what view that statement 
 has been given. It is to point out, in a manner 
 not to be questioned, what conduct the college 
 were bound to pursue upon the application of 
 a licentiate for examination, before 1 describe 
 the conduct which they actually did pursue, 
 when such an application was made. It is to 
 exhibit a picture from the masterly hand of 
 
314 LETTER TO 
 
 your Lordship, guided rather by the suggestions 
 of a warm and virtuous fancy, than by an ac- 
 curate knowledge of the object to be repre- 
 sented, before I produce another picture of the 
 same object copied from nature, by an artist, 
 rude indeed and unskilful, but whose diligence 
 and fidelity may have compensated his want of 
 genius and taste. 
 
 Before the decision of the Court of King's 
 Bench, in the case of Dr. Stanger, I had with 
 many others believed, that the fellows of ih& 
 college never meant to admit any licentiate to 
 an examination. But when I had become ac- 
 quainted with the declarations of their counsel, 
 respecting the by-law for examining licentiates, 
 and with your Lordship's opinion, that it fur- 
 nished a remedy for the evil in the former system 
 of admission, equal to that which even a man- 
 damus could afford, I concluded with some 
 firmness, that although my belief had been 
 originally well founded, still they would scarcely 
 be hardy enough to refuse to examine a licen- 
 tiate, while the circumstances of Dr. Stanger's 
 cause were recent in the memory of every one. 
 Not having been in court myself during the 
 trial of that cause, my first knowledge of the 
 proceedings in it was derived from verbal re- 
 ports. Fearing, however, that these might be 
 incorrect, I thought it prudent not to form any 
 
LORD KENYON. 315 
 
 plan in consequence of what had passed there, 
 before I should see an account of the proceed- 
 ings, which Mr. Gurney was then preparing 
 from his notes in short-hand. When I had 
 perused that account which from various cir- 
 cumstances I did not receive until several 
 months after the trial, I hastened to inquire, 
 whether any licentiate, w r ho came within the 
 conditions of the by-law, meant to avail himself 
 of it : but, finding that there was none, I deter- 
 mined to apply f >r an examination of my own 
 fitness to be a fellow of the college, whatever 
 reason I might have for being fearful of its 
 issue, rather than allow the grounds of the de- 
 cision to run any hazard of being forgotten, 
 from want of an early appeal to them. I men- 
 tioned this intention to two of my friends among 
 the fellows, Dr. David Pitcairn, and Dr. Mat- 
 thew Baillie, who, by immediately offering to 
 propose me, removed the first, and in the opi- 
 nion of the judges of the Court of King's 
 Bench, the only difficulty in the way of obtain- 
 ing my object. On the 29th of September, 
 1797, a motion was accordingly made at the 
 college by Dr. Pitcairn, and seconded by Dr. 
 Baillie, not that I should be admitted a fellow, 
 but merely that I should be examined concern- 
 ing my fitness to become one hereafter. If 
 your Lordship's surprise would have been 
 
316 LETTER TO 
 
 excited, as surely it must, by any opposition 
 whatever to this proposal, to what height will 
 it be carried, when you learn the ground of that 
 which was actually made ? Could your Lordship 
 have even imagined, that a by-law of the college 
 of Physicians, which, by the declaration of their 
 counsel in the Court of King's Bench, had been 
 framed in 1778, with the best legal advice this 
 country could afford, for the express purpose of 
 removing the blame which had been thrown 
 upon them by Lord Mansfield ; that a by-law, 
 which, if before forgotten, had been recalled to 
 their recollection in 1789, by an application 
 under it from Dr. Sims ; that a by-law, to whose 
 existence they had twice sworn before your 
 Lordship, once in April 1796, and again in 
 January 1797; that a by-law, upon which they 
 had rested their chief defence in a recent trial 
 before the Court of King's Bench ; that a by- 
 law, to the beneficial operation of which Dr. 
 Stanger had, in the course of that trial, been 
 advised by one of the judges upon the Bench 
 to trust implicitly, instead of applying to the 
 court for a mandamus ; and, lastly, that a by- 
 law, which your Lordship had expressly said 
 bound them to examine every person who ap- 
 plied under it, should in September 1797, be 
 declared a dormant by-law, the propriety of 
 who'se revival formed a question of very great 
 
LORD KENYON. 317 
 
 concern, and was consequently not to be de- 
 cided upon before it had undergone much se- 
 rious consideration ? In the midst of your in- 
 dignation against such conduct in a body of 
 men, formerly styled by your Lordship, the 
 sanctuary of honour and good faith, it must yet 
 afford you some consolation to know, that many 
 of the members were free from its guilt ; and 
 that when a motion was made to get rid of Dr, 
 Pitcairn's proposal, by what is termed the pre- 
 vious question, out of twenty-three, the whole 
 number at the meeting, ten voted against it. 
 
 Few men are so lost to shame, as not to de- 
 sire that their most iniquitous acts should wear 
 an appearance of justice. It is not, therefore, 
 wonderful, that the college of Physicians should 
 have attempted to palliate the conduct which 
 has been mentioned. With this view they 
 maintained, that proper notice had not been 
 given of the intended proposal by Dr. Pitcairn. 
 But no such notice was either required by the 
 by-law which authorised that proposal, or had 
 been established by custom. What end indeed 
 would the giving of notice in the case before 
 them have served? Not surely to afford time 
 for their considering, whether they were to re- 
 ceive what they were bound to receive. If it 
 were to have relation to the character of the 
 person to be proposed, all that could be well 
 
318 * LETTER TO 
 
 known of it was already in their possession ^ 
 since, during the nine years of his being a licen- 
 tiate, he had never absented himself from Lon- 
 don an entire day, and had in the same time 
 associated more with fellows of the college than 
 with licentiates. Besides, the mere admission 
 to an examination did not prevent the free ex- 
 ercise of their judgment at any one of the four 
 other ballots which were to take place, before 
 he could be received into their body ; and as 
 the last of those ballots was not to be held till 
 twelve months after the admission to be ex- 
 amined, those of the tenderest consciences were 
 allowed sufficient time for the most scrupulous 
 inquiries respecting him. 
 
 But not to dwell longer upon this mode of 
 answer to their pretext, I proceed ito assert, 
 that notice was given to the college of Dr. Pit- 
 cairn's intended proposal. If the proper person 
 for receiving it was absent from his duty, the 
 fault lay with him. Among the many illiberal 
 circumstances of the by-law for admitting licen- 
 tiates to an examination, is this ; that no person 
 can be proposed under it, except upon one day 
 in the year ; namely, at the general meeting of 
 the college, immediately after Michaelmas. I 
 had not been able before the 20th of September, 
 to ascertain whether or not I could be proposed 
 in 1797. Two days after this, and seven days 
 
LORD KENYON. . 319 
 
 before the meeting of the college, I went to the 
 president's house in London, to inform him of 
 what was intended, being desirous that, although 
 such a notification was not required, it should 
 not afterwards be said, that an attempt had 
 been made to surprise the judgment of the col- 
 lege. I was told there that he was in the 
 country, at a considerable distance from Lon- 
 don, but that he was expected to return in a 
 day or two. Upon this, I wrote a letter at his 
 house, which I left there, to acquaint him with 
 the object of my visit. Three days after, how- 
 ever, I learned that he was still out of town, 
 and probably would not come to it till the day 
 preceding the meeting of the college. In con- 
 sequence of this information, I immediately sent 
 a letter to him in the country, to make known 
 what was meant to be done. On the same day 
 I called upon the officer of the college, whose 
 business it is to summon the fellows to their 
 meetings, and authorised him to acquaint those 
 whom he should see, that I was to be proposed 
 for examination. I gave the same information 
 myself to one fellow, my colleague, Dr. George 
 Fordyce. If I did not give it to more, this was 
 from fear, lest the doing so might be regarded 
 as an indirect solicitation of votes. Yet, not- 
 withstanding all this supererogatory care to ap- 
 prize the president and fellows of the college 
 
320 LETTER TO 
 
 of what was intended by Dr. Pitcairn, they 
 were bold enough to refuse even to allow his 
 proposal to proceed to a ballot, on this pre- 
 tence, among others, that it had not been pro- 
 perly notified to them. 
 
 Amongst the voters against a ballot on the 
 proposal of Dr. Pitcairn, was Dr. John Burges*, 
 whose conduct in this matter seems worthy of 
 particular notice, as he had himself only a few 
 years before made a similar motion regarding 
 another licentiate. I dispute not here the claim, 
 which that gentleman makes to ancient faith, 
 and purity of manners, and most exemplary 
 zeal for the honour of the college : but as an 
 humble inquirer into the principles of human 
 
 * I here, and perhaps shall elsewhere, venture to say, upon 
 which side of a question a particular member of the college 
 has voted, though it be the custom of tjiat body to collect 
 suffrages by ballot. But ballots are so little adapted to the 
 freedom of Englishmen, that they are seldom in this country 
 attended with the concealment, which is probably derived 
 from them among the crafty and dissembling Italians. When 
 votes are collected here, in this way, many of those who give 
 them openly mention the side which they support; others, 
 though they do not make a direct confession, yet by the 
 tenour of their conversation, leave little doubt upon the 
 same point ; and by these means, as tittle doubt at length 
 remains in regard to the few, \vho have endeavoured to in- 
 volve their conduct in mystery. I shall be very ready, how- 
 ever, to correct any mistake which I may fall into upon this 
 subject. 
 
LORD KENYON. 
 
 nature, I think myself entitled to say a few 
 words upon his acting so differently at different 
 times, in circumstances apparently the same. 
 
 Travellers inform us, that many of the in- 
 habitants of the isthmus, which connects the 
 two great continents of America, labour under 
 a most strange depravation of sight. When 
 the sun has arisen above the horizon, and has 
 enabled other men by its light to pursue their 
 ordinary occupations, these people become blind, 
 and retire into caverns and dark woods, there to 
 pass the day in quiet and repose. But, as soon 
 as night has descended upon the earth, and the 
 face of nature is to other eyes covered with 
 darkness, their sight is restored, and they then 
 come forth from their hiding-places, to exercise 
 the labours, and enjoy the pleasures of life. 
 I know not whether a similar infirmity has hi- 
 therto been observed in the mental perceptions 
 of man ; if it has not, I announce the existence 
 of an undescribed disease, and produce the con* 
 duct of Dr. Burges in proof of my discovery. 
 This gentleman, some years ago, saw so clearly 
 the propriety of carrying into execution the 
 by-law of the college, for admitting licentiates 
 to examination, that he proposed Dr. James 
 Sims, as a candidate under it. To the other 
 fellows, however, the propriety of the measure 
 was then involved in so great darkness, that no 
 
 Y 
 
322 LETTER TO 
 
 one of them could be induced to second his 
 motion. In 1797, the arguments of the ad- 
 vocates of the college, and the speech of your 
 lordship, diffused such light over this subject, 
 that when another licentiate is proposed for ex- 
 amination, ten fellows, without the smallest so- 
 licitation from any person, and in direct oppo- 
 sition to the suggestions of ancient prejudice, 
 declare their opinion, that it ought to be granted. 
 But the light which now enables men of healthy 
 minds to discern merit in a measure, in which 
 they formerly could see none, overpowers by its 
 excess the infirm perception of Dr. Burges. In 
 this distress, groping in darkness, he begs for 
 time to consider, whether the by-law for the 
 examination of licentiates, ought even in any 
 instance to be carried into effect. 
 
 At the time that I was made acquainted with 
 the fate of Dr. Pitcairn's proposal, I was also 
 told, that since it had not been put to a vote, it 
 could not be said to have been rejected, but was 
 rather to be supposed still lying on the table 
 of the college. Hence I concluded, that if a 
 charge were now brought against them of dis- 
 regard to the decision of your Lordship, they 
 might attempt to evade it by maintaining, that 
 the consideration of Dr. Pitcairn's motion had 
 
LORD KENYON. 323 
 
 been only suspended. I resolved, therefore, to 1 
 bring their sincerity to trial afresh, by having 
 myself proposed a second time for examination. 
 Upon mentioning this determination to Dr. Pit- 
 cairn, he offered his aid in accomplishing it, by 
 repeating his former motion in September 1798, 
 before which, in consequence of what has al- 
 ready been observed, it could not be received. 
 
 During that interval, the college proceeded 
 to impose a new restriction upon the admission 
 of licentiates into their body, as if to demon- 
 strate the truth of the allegation against them, 
 which had been so scornfully repelled by the 
 judges of the Court of King's Bench, that their 
 by-law for the examination of persons of that 
 class was altogether illusory, and had been 
 framed with the intention, that no one should 
 ever be received by it. -The new restriction 
 was, that whoever meant to propose a licentiate 
 for examination, should give notice of this at 
 a preceding quarterly meeting of the college. 
 Its professed object was to allow r time to the 
 fellows for inquiry into the character of the 
 person to be proposed. The pledge to be given 
 by a fellow upon proposing a licentiate, the 
 candidate's residence for at least seven years in 
 the midst of them, and the interval of a twelve- 
 month between the first and last ballots upon 
 his fitness, were consequently declared to be 
 
S24 LETTER TOT 
 
 insufficient barriers against the entrance of un- 
 worthy persons into the corporation: But it 
 seems to me more difficult to admit that this 
 was in truth their opinion, than to believe, that 
 the real object of the new regulation was very 
 remote from the one exhibited; and what I 
 shall immediately say, will probably incline 
 your Lordship to form a similar conclusion. 
 
 In the end of June 1798, Dr. Pitcairn, though 
 much debilitated by a dangerous illness, under 
 which he had lately laboured, attended at the 
 college to give notice, that he should in the 
 following September again propose me for ex- 
 amination. To this notice he premised, that he 
 conceived it to be unnecessary, since the merits 
 of his first proposal had not yet been considered. 
 But unfortunately for mankind and himself, he 
 was shortly after again taken ill, and was in 
 consequence obliged to leave London for the re- 
 covery of his health, a few days before the time 
 arrived for making his motion. Previously to 
 his departure, however, he wrote a letter to 
 Dr. Baillie, in which, after stating his own in- 
 ability to propose me, he delegated that office 
 to him. Accordingly, Dr. Baillie produced this 
 letter at the meeting of the college in Sep- 
 tember, and then proceeded to execute his 
 trust. This was resisted by the same men, who 
 had opposed the former motion for my being 
 
LORD KENYON. 325 
 
 ^examined. It was urged -by them, that die new 
 by-law required the proposal to be made 'by the 
 vary person who had given notice of it. To 
 this it was answered, that as the avowed object 
 of the notice was to allow time for inquiry into 
 the character of the person to be proposed, the 
 spirit of the by-law prescribing it had, in the 
 present ^case, been completely satisfied. And 
 it was asked, whether a delegation had never 
 formerly been received, when he who had de- 
 clared his intention of bringing forward any 
 measure was prevented 'by illness, or the un- 
 avoidable duties of his profession, from attend- 
 ing .at the college to propose it. No reply was 
 made ; *but a question was immediately put, 
 whether the present delegation should be ad- 
 mitted. A ballot being taken, twelve votes 
 were found against the delegation, and nine in 
 favour of it. 
 
 An attempt was then made to bring in a dif- 
 ferent way before the college the original ques- 
 tion of examination. It was maintained, that 
 the first proposal by Dr. Pitcairn was still upon 
 their table, as it had never been decided upon, 
 and that it ought now to receive their de- 
 termination. The minutes of the meeting in 
 September 1797, were in consequence called 
 for and read, upon which it was declared, that 
 Dr. Pitcairn's proposal had -then been finally 
 
326 LETTER TO 
 
 disposed of and rejected. No cloud, therefore, 
 now hangs over the conduct of the college ; 
 nothing now intervenes to alter its natural co- 
 lours, or to distort the light by which it is seen, 
 
 Your Lordship, perhaps, notwithstanding the 
 facts which have been described in the fore-' 
 going narrative, will scarcely think it possible, 
 that the college of Physicians should have in- 
 tentionally violated their engagement, or have 
 advisedly acted in contempt of the grounds of 
 a decision in the Court of King's Bench ; and 
 hence you may imagine, that they were in^ 
 fluenced by some well-founded objection to the 
 person proposed for examination, though from 
 ignorance or inadvertence, they might have 
 given to their conduct the appearance of a de- 
 sertion of principles, which they were bound to 
 maintain. Lest, therefore, you should be in^ 
 duced by your ancient respect for the college, 
 to form an opinion so unfavourable to my cause, 
 I will now attempt to prove, that no well- 
 founded objection did exist to my being ex- 
 amined by them ; I mean, no objection, which 
 any of those who resisted the proposal for an 
 examination, would venture publicly to avow. 
 In such an undertaking I must necessarily 
 speak much of myself ; but for this I hope I 
 
LORD KENYON. 327 
 
 shall readily be pardoned, since I stand now 
 before your Lordship in the situation of one 
 accused, and hence acquire a right of pro- 
 ducing whatever testimony I can collect in my 
 favour. 
 
 By the charter of the college, the qualifica- 
 tions required for its members, are learning and 
 good character. In addition to these, a by-law 
 demands from licentiates that they be of seven 
 years standing, and thirty-six years of age, be- 
 fore they can be proposed for admission by 
 means of an examination. Since, therefore, I 
 possessed, without dispute, the latter requisites, 
 all the avowable objections, which, in ordinary 
 times, could possibly be brought against me by 
 the college, are reducible to two kinds ; one 
 containing those which have any relation to my 
 learning; the other, such as are connected with 
 my moral reputation. 
 
 When I was proposed at the college by Dr. 
 Pitcairn, all that was asked was, that they would 
 examine whether I possessed the proper degree 
 of knowledge for a fellow. Nothing more, 
 therefore, on this head, could reasonably be re- 
 quired by them, before the trial, than a strong 
 presumption of my being able to undergo it. 
 Now this presumption was manifested to them 
 in various ways. Their own advocates had as- 
 serted, in the case of Dr. Stanger, that the 
 
328 LETTER TO 
 
 charter of incorporation, though it divided the 
 physicians of London into two classes, members 
 of the corporation and licentiates, demanded 
 however the same learning from both ; and that 
 the college would act contrary to their duty, if 
 they gave equal liberty to practise medicine to 
 descriptions of men possessing unequal degrees 
 of ability*. But, nine years previously to my 
 being proposed by Dr. Pitcairn, I had under- 
 gone the trials of fitness, to which licentiates 
 are subjected before admission to practise, and 
 if I may venture to credit what was said by Sir 
 
 * This is a dictate of common sense ; but though found 
 by the counsel of the college, in the charter which was 
 granted to them nearly three hundred years ago, its justness 
 was not acknowledged when the late Dr. Fothergill became 
 a licentiate ; for he was permitted to exercise his profession in 
 London, under a by-law which declared, that one reason for 
 constituting a class of licentiates was, that many persons who 
 were fit to practise medicine, had not, however, sufficient 
 learning to be fellows. But there is reason to believe, that 
 the late admission, on the part of the college, of equality in 
 point of learning between the fellows and licentiates, was 
 merely to serve a particular purpose during the trial of Dr. 
 Stanger's cause. For in the testimonials of fitness to prac- 
 tise, which they give to licentiates, they still refuse to style 
 them doctors of physic, though they constantly bestow that 
 title on fellows j and it was, I suppose, in consequence of 
 this distinction, that a president of the college had the 
 effrontery to tell a learned professor of Gottingen, when upon 
 a visit to this country a few years ago, that the licentiates of 
 the college were not proper physicians. 
 
LORD KENYON. 329 
 
 George Baker, and the censors who examined 
 me, I had passed through those trials with more 
 than ordinary ease. In the interval, I had be- 
 come a member of the Royal Society, the cer- 
 tificate of my fitness for which was signed by 
 the late and present presidents of the college, 
 Sir George Baker, and Dr. Gisborne, and by 
 four others of the present fellows of that body. 
 During the same interval, I had endeavoured 
 to extend the boundaries of our knowledge in 
 various parts of natural philosophy; and two of 
 my attempts of this kind, certainly not the most 
 considerable, had been recorded in the printed 
 transactions of the Royal Society. As I had 
 thus demonstrated industry at least, in the cul- 
 tivation of sciences collateral to medicine, it is 
 not probable that I had been inattentive to the 
 study of my own profession, since my peace 
 of mind necessarily depended upon my under- 
 standing it. Nor had my opportunities of gain- 
 ing experience in it been very small ; for I had 
 been eight years a physician to an extensive 
 establishment for the relief of the sick poor, 
 and I had also been physician for some time to 
 another institution of the same kind, but still 
 more considerable. From all these circum- 
 stances, I think it will readily be allowed by 
 your Lordship, that it was not likely I had be- 
 come less learned since passing the trials of a 
 
330 LETTER TO 
 
 licentiate, and that consequently there was a 
 s.trong presumption of my being sufficiently 
 learned to be admitted to undergo the addi- 
 tional tests of knowledge, if there be any such, 
 which the statutes of the college demand from 
 those who desire to be fellows. This will be 
 the more readily granted, when it is considered, 
 that though the college contains at present 
 many learned men, and will no doubt continue 
 to contain many such, as long as the inhabitants 
 of this country are sufficiently rich to reward 
 liberally the professional labours of physicians, 
 yet the degree of knowledge which is just suf- 
 ficient to enable any person to enter their body, 
 cannot be regarded, even by themselves, as very 
 high : For, 
 
 First, among the forty-three members who 
 have undergone the required examinations, how- 
 ever they may have differed in original talents, 
 industry, opportunities of studying their pro- 
 fession and in modesty, there is only one, whose 
 learning is said to have been declared insuf- 
 ficient upon his first application for admission : 
 
 And secondly, the three physicians, who to 
 my poor apprehension have appeared to have 
 the weakest understandings and the smallest ex- 
 tent of knowledge, of all those with whom I have 
 happened to converse, either in this or any other 
 country, are fellows of the college of London. 
 
LORD KENYON. 331 
 
 I come now to the objections which might be 
 brought against me on account of my moral re- 
 putation. 
 
 How far my previous life had entitled me to 
 a reputation for good morals, it does not be- 
 come me to say ; and I am for many reasons 
 unwilling to exhibit the direct testimony of my 
 friends upon this part of my character. Such 
 a step, indeed, seems on the present occasion 
 altogether unnecessary, as I think I can easily 
 prove, that the majority of those who formed 
 the meeting of the college, when Dr. Pitcairn 
 proposed me for examination, did not conceive 
 me unfit to be received into their corporation, 
 by reason of my immorality. 
 
 It will, I suppose, be readily granted, that as 
 many, at least, as voted for receiving Dr. Pit- 
 cairn's proposal, entertained no objection to 
 me, on the ground which has just been men- 
 tioned. Now, the numbers on the different 
 sides of the question, when his proposal was 
 rejected, having been thirteen and ten, if I can 
 only show, that two of the thirteen had shortly 
 before manifested their satisfaction with my 
 character for morals, the object at present in 
 view must, in my opinion, be looked upon as 
 gained. 
 
 Dr. Gisborne, the president of the college, 
 who I venture to maintain voted for the rejec- 
 tion of Dr. Pitcairn's proposal, some years ago, 
 
332 LETTER TO 
 
 as has already been mentioned, signed a certi- 
 ficate of my fitness for being a fellow of the 
 Royal Society. Now, as a good moral reputa- 
 tion is always esteemed there a necessary ingre- 
 dient of fitness, he must certainly have then 
 believed me to be possessed of that qualification. 
 That he professed a similar belief, only a few 
 months before Dr. Pitcairn proposed me, I can 
 assert upon the authority of Dr. James Robert- 
 son, a fellow of the college, at present with his 
 Majesty's forces in Minorca ; and I take upon 
 myself to say, that nothing happened in that 
 short interval, which ought to have lessened it. 
 Sir Lucas Pepys was another of the fellows 
 who voted for the rejection of Dr. Pitcairn's 
 proposition. In the beginning of the year 1797, 
 I appeared before the Board of Censors of the 
 college, to complain of irregular conduct in an 
 apothecary, who was also present to answer to 
 my charge. Sir Lucas Pepys, then sitting as 
 president of a court, the members of which are 
 sworn to do justice, addressed the delinquent in 
 a grave and solemn speech, in the course of 
 which he delivered these words : " Dr. Wells 
 is no mean person; he is well known to the 
 world both as a gentleman and a scholar." 
 Whether this opinion be just or not, is at pre- 
 sent no matter of inquiry. But in tenderness 
 to Sir Lucas Pepys, acting as a judge, under 
 the solemn obligation of an oath, it must be 
 
LORD KENYON. 333 
 
 supposed that he really entertained it. In like 
 manner as when I spoke of Dr. Gisborne, I 
 venture here to affirm, that nothing occurred 
 in my conduct from that time to the 29th of 
 September in the same year, which should have 
 induced Sir Lucas Pepys to alter his opinion of 
 me as a gentleman. 
 
 I might proceed to show, my Lord, that other 
 fellows of the college refused to receive Dr. 
 Pitcairn's proposal, upon grounds that had no 
 connexion with my moral reputation. But, as 
 what] I have already said appears sufficient for 
 attaining the end I proposed, I quit with joy a 
 subject so distasteful, and betake myself to one 
 more congenial to your Lordship's feelings, the 
 consideration of the support which was given to 
 my fitness for being received at the college, by 
 the characters of him who made, and of him 
 who seconded the proposal for my being exa- 
 mined. 
 
 One of those gentlemen must already be well 
 known to your Lordship. I cannot, however, 
 refrain from saying respecting him, that the son 
 of the gallant Major John Pitcairn, who died 
 the glorious and enviable death of a soldier, 
 fighting for his country, and the adopted son 
 of the high-minded, upright, and generous Dr. 
 William Pitcairn, must have every title to the 
 
334 LETTER TO 
 
 strictest honour, which inheritance, education, 
 and domestic example can bestow. But why 
 do I speak of titles, after his countrymen had 
 acknowledged his complete possession of that 
 most invaluable property, and had in conse- 
 quence, as well as from their high opinion of 
 his learning and skill, placed him at the head of 
 the profession of medicine, in the metropolis of 
 Great Britain*? 
 
 He who seconded the proposal, Dr. Matthew 
 Baillie, is more upon a level with myself, in re- 
 gard both to age, and length of residence in 
 London. Somewhat, therefore, of the obscurity 
 which involves almost every young physician, 
 may have hitherto concealed him from your 
 Lordship's notice. But that obscurity is fast 
 dissipating, and he must soon, my Lord, very 
 soon, appear to your view, with all the just 
 
 * Two circumstances must concur to place a physician at 
 the head of his profession in London ; 1. Great employment, 
 which alone, is certainly not sufficient for that purpose, as it 
 is often possessed by persons of no considerable ability. 2. 
 Respect from other physicians, indicated by their frequently 
 requesting his aid in their practice. This can arise only from 
 a high opinion of his honour and skill, of which qualities in 
 a physician, scarcely any but those of his own profession have 
 either opportunities or capacity to judge rightly. Dr. Piteairn, 
 from the death of Dr. Warren to his own unfortunate illness, 
 was indisputably the physician in London, in whom those 
 circumstances existed together in the greatest degree. 
 
LORD KENYON. 335 
 
 proportions and accurate lineaments of a man 
 of integrity, learning, and great professional 
 skill. 
 
 Can it be conceived then, my Lord, that such 
 men were ignorant of the character of one, with 
 whom they had been acquainted for many years ; 
 or, that believing it to be unfit for mixture with 
 the college, they would yet pledge their own 
 honour upon its pureness ? None scarcely are 
 so depraved as to do wrong for its own sake ; 
 temptation is for the most part necessary to 
 induce the most abandoned villain to add to 
 his crimes. Supposing now for a moment, that 
 Dr. Pitcairn and Dr. Baillie were capable of 
 being actuated by unworthy motives, they could 
 not have possibly gained aught by proposing 
 me. They could not desire to get rid of im- 
 portunity, for what they did was of their own 
 free motion ; or to repay favours which had 
 been received by them, for on the score of good 
 offices I was already greatly in their debt. On 
 the other hand, they knew well, that what they 
 were doing was highly disagreeable to the go- 
 verning members of the corporation. These 
 men they were obliged to meet frequently, 
 either in ordinary society, or in the exercise of 
 their profession, or at the comitia of the college. 
 It was, therefore, of importance to their ease 
 and comfort at least not to offend them. Since, 
 
336 LETTER TO 
 
 however, they did offend them, without deriving 
 the smallest advantage to themselves from their 
 conduct, they must necessarily have had the 
 firmest conviction of its rectitude ; and in this 
 conviction I find the strongest proof I can offer, 
 that in point both of learning and moral reputa- 
 tion, I was not unfit to be examined for admis- 
 sion into the College of Physicians of London. 
 I have now, my Lord, considered the two 
 grounds, upon which the college, consistently 
 with their charter, might possibly have regarded 
 me as unfit for admission into their body. But 
 perhaps it will be said, that they drew their 
 objections to me from a source different from 
 either of those which have been mentioned ; 
 that they believed me infected with the mad- 
 ness of the present times, and desirous of enter- 
 ing their corporation, for the purpose of assist- 
 ing more effectually to destroy it, along with 
 every other ancient establishment in this coun- 
 try. Such at least were the principles of con- 
 duct attributed by many of the fellows of the 
 college to those licentiates, who had engaged 
 in the scheme of opening the corporation to 
 every physician of learning and honourable 
 character. Even after Dr. Stanger's cause was 
 determined, when apparently no object to be 
 gained by calumny existed, one of the fellows, 
 Dr. Robert Bourne of Oxford, a gentleman, as 
 
LORD KENYGN. 337 
 
 1 have since known, of great prudence, and of 
 the mildest manners, and who was then proba- 
 bly not acquainted with any one of those licen- 
 tiates, placed notwithstanding a revolutionary 
 spirit among the reasons which were assigned 
 by him, in a public oration, for their attempt 
 to gain admittance into the college. Nothing 
 can more strongly demonstrate the pains, which 
 had been taken to propagate such slander, than 
 its having been received, credited, and still fur- 
 ther spread by Dr. Bourne. 
 
 Opinions, leading to the overthrow of the 
 monarchical part of our constitution, have long 
 existed in this country, in a greater or less de- 
 gree ; but since the termination of the grand 
 rebellion, they have been, till very lately, almost 
 entirely confined to a few speculative men, who 
 have shown little desire to gain proselytes, or 
 in any other way to attempt a completion of 
 their fanciful projects. -Neither therefore the 
 college of Physicians, nor, I believe, any other 
 of our corporations, ever formerly refused to 
 admit a person among them, merely on account 
 of his notions of government, provided he had 
 complied with the forms which were prescribed 
 by the laws of the country, or their own private 
 regulations. But the modern holders of repub- 
 lican principles, if indeed the workers of confu- 
 sion can be said to possess principles, and if 
 tends to the misery of the whole can be 
 
LETTER TO 
 
 denominated republican, follow a far different 
 course. They labour with an apostolic zeal to 
 impress their tenets upon others. No fancy is 
 so wild as to be refused admittance into their 
 minds ; and whatever exists there is regarded 
 by them as a legitimate cause of action. To 
 employ the influence which they derive from 
 places of trust under an ancient government, 
 as a means of subverting it, is with them a 
 duty ; their great ambition is to show, that they 
 are ready to sacrifice friends, family, and coun- 
 try, to obtain their beloved object, the destruc- 
 tion of order. It appears*, therefore, highly 
 proper, that the guardians of the different public 
 establishments^ to whom any discretion is in this 
 respect allowed bylaw, should resist the entrance 
 of every person, who notoriously holds opinions 
 unfriendly to their existence. But though this 
 be granted, it surely ought not to follow, that 
 a vague surmise, an unauthorised suspicion of 
 disloyalty, should operate to the exclusion of 
 any one from a situation of honour or profit, to 
 which he is otherwise legally entitled. Envy 
 and malice in their native forms have considera- 
 ble influence over human affairs j if permitted 
 to assume the shape of patriotism, their power 
 must be irresistible. 
 
 Upon what ground the college charged the 
 licentiates with being disaffected to the consti- 
 tution of their country, I know riot. It was 
 
LORD KENYON. 339 
 
 clearly no proof of their being so, that they 
 appealed to the Court of King's Bench, for a 
 rigid execution of a charter, which had issued 
 from the most tyrannic prince of the despotic 
 house of Tudor; or that they founded their 
 title, to what they prayed the court to grant, 
 upon -the interpretation given to that charter 
 by your Lordship's immediate predecessor, 
 Earl Mansfield, certainly no friend to levelling 
 principles, or to seditious combinations of men. 
 Perhaps the proof was derived from this cir- 
 cumstance, that no oiTe of the licentiates who 
 signed the address to the college, in which they 
 set forth their right to be examined for admis- 
 sion into the corporation, either enjoyed, or 
 expected to enjoy, any professional honour or 
 advantage directly connected with the present 
 government of the country. " Is it probable 
 that these men," the fellows of the college 
 might say, " who are attached by nothing spe- 
 cial to the existing constitution, can desire its ; 
 continuance? Our own bosoms declare that 
 they cannot ; they must, therefore, be labour-* 
 ing to subvert it." But the pampered Rich 
 basely deserted his master in the hour of dis- 
 tress, while thousands of our countrymen, bound 
 to their sovereign by no other tie than their 
 allegiance as Englishmen, fought and died in 
 his defence. From whom did the expiring 
 
340 LETTER TO 
 
 cause of royalty in France receive its last sup- 
 port ? Not from the pensioned courtiers of Ver- 
 sailles ; but from a Stoflet, and a Charette, men 
 before unknown, but whom the occasion that 
 called for their talents formed into heroes; 
 from the plain and simple inhabitants of Brit- 
 tany, actuated by no motives but what arose 
 from attachment to the ancient government of 
 .their country, and reverence for the religion of 
 their fathers. 
 
 Leaving, however, to more able advocates, 
 what further defence may be deemed proper 
 for the other licentiates, who have been charged 
 with disloyalty by the members of the college, 
 I shall now confine myself to a special vindica- 
 tion of my own character from so atrocious a 
 calumny. If, my Lord, I speak with warmth 
 upon this subject, I trust that I shall find an 
 excuse in the energy of your own feelings. He 
 that is wealthy may be robbed, without knowing 
 that he has experienced an injury. But the 
 poor man's all is often included in a single ob- 
 ject, which, though to other eyes worthless and 
 contemptible, may be to him the sole spring of 
 joy and hope. Any attack' upon it excites his 
 utmost powers of resistance ; its loss leaves him 
 without bond to the world, or interest in its 
 concerns. When we read of a rich man's de- 
 spoiling a poor neighbour of his only property, 
 
LORD KENYON. 341 
 
 cc 
 
 one little ewe-lamb which lay in his bosom, 
 and was unto him as a daughter," our sympathy 
 with the sufferer is nearly as great, as if he had 
 been a monarch unjustly expelled from his do- 
 minions. I may well then be allowed to feel 
 acutely the attempt which has been made to 
 strip me of almost my only possession, to which 
 my title is founded upon paternal discipline and 
 personal suffering, and has been illustrated by 
 the whole tenor of my life. 
 
 I was born, my Lord, in Charlestown, in 
 South Carolina, but my parents were from Scot- 
 land. My father, who was a man of observation 
 and a scholar, though a tradesman, had carried 
 with him those opinions respecting the kingly 
 branch of the British constitution, which in the 
 former state of our parties constituted Toryism ; 
 and the resistance they met with in a country, 
 the inhabitants of which were, from their situa- 
 tion, always somewhat inclined to republicanism, 
 served only to strengthen them. These opinions 
 he early endeavoured to impress upon myself* 
 To remove, however, every fear of my being 
 infected, from my companions, with the factious 
 and disloyal principles, which had very generally 
 pervaded the British Colonies in America, from 
 the conclusion of the peace of Paris, in 1763, 
 and to give me at the same time an opportunity 
 
342 LETTER TO 
 
 of receiving the elements of a sounder edu- 
 cation, in other respects, than Carolina could 
 afford,, he sent me while yet a boy to this 
 kingdom. 
 
 In one of his views he was not disappointed. 
 For some time after I had returned to Carolina, 
 to pass a part of my youth under his immediate 
 care, a paper, called AN ASSOCIATION, having 
 been offered for signature to all the male in- 
 habitants of jCharlestown above sixteen years 
 of age, the subscribers to which bound them- 
 selves to obey implicitly certain authorities 
 unconnected with the former government of 
 the country, I was one of a very few who re- 
 fused to put their names to it. Those who had 
 now a legal controul over my conduct, my father 
 having shortly before fled from Chariest own to 
 avoid persecution, strongly urged my com- 
 pliance. They stated, among other things, 
 that many persons of the most undoubted 
 loyalty had signed the ASSOCIATION, and that 
 a continuance in my refusal would expose me 
 to the resentment of the populace. My answer 
 was, that men of established reputation might 
 conceive themselves entitled to a certain lati- 
 tude of conduct, to which I could not pretend, 
 who had yet a character to gain ; and that I 
 was therefore determined, whatever mightf be 
 
LORD KENYON. 343 
 
 like event, that my entrance into manhood 
 should not be marked by what appeared to me 
 an act of treason and rebellion. I was conse- 
 quently obliged to leave Carolina, altogether 
 uncertain of the future means of subsistence; 
 but I found them here, in the exertions of a 
 father, who, to supply me with what was neces- 
 sary for the prosecution of my studies, sub- 
 mitted to privations ill befitting his age, and 
 former habits of life. I was in this way enabled 
 to take the degree of doctor of Physic, at Edin- 
 burgh, in 1780. Charlestown was now in the 
 possession of his Majesty's forces, and I returned 
 to it for the purpose of collecting the scattered 
 remains of my father's fortune. While there, 
 though exempted by my profession from mili- 
 tary calls, I made an offer of my personal ser- 
 vices to the commandant of the town, the pre- 
 sent lieutenant-general Nesbitt Balfour, and 
 was appointed by him an officer in a body of 
 volunteers, who, by performing a part of the 
 duty of the garrison, enabled a greater number 
 of the regular troops to take the field, than 
 could have done so, without such aid. When 
 Charlestown was abandoned by the king's 
 forces, I went to East Florida. Shortly after 
 my arrival there, apprehensions being enter- 
 tained for the safety of the province, I requested 
 permission from governor, now general Tonjn, 
 
344 LETTER TO 
 
 to assist in its defence, and received from him, 
 in consequence, the command of a company of 
 volunteers, who were to serve without pay. 
 This company I raised, and kept together as 
 long as the fears continued, on account of which 
 it had been formed. 
 
 I have thus mentioned, my Lord, some of the 
 facts which I possess in proof, that my conduct 
 at least was not, formerly, disloyal. They hap- 
 pened at a time of life, from the age of eighteen 
 years to that of twenty-six, when actions are not 
 often discordant with internal feelings ; when 
 the veil of hypocrisy is seldom worn, and, if 
 ever assumed, is soon blown aside by the tem- 
 pests of passion, which so frequently arise in 
 that season of human existence. I shall, how* 
 ever, exhibit more direct testimony that my 
 conduct and principles were in unison. I shall 
 produce to your Lordship a profession of attach- 
 ment to my country and its constitution, which 
 was 'made by me in the midst of enemies, from 
 an unwholesome prison, and while threatened 
 with assassination on account of that attach- 
 ment. For, going to Charlestown, in 1783, 
 upon some family concerns, I was arrested there 
 and thrown into gaol, a few days after my arrival, 
 in violation of a flag of truce with which I had 
 entered the country. Such, at least, was the 
 opinion of governor Tonyn, who had given that 
 
LORD KENYON. 345 
 
 flag ; for as soon as my arrest was known in 
 Florida, he sent a commissioner to Carolina, 
 Mr. Wyllie, the present chief justice of the 
 Bahama Islands, to demand my release. In the 
 mean time, a publication appeared respecting 
 me, signed by the gaoler in whose custody I 
 had been placed, which began thus ; " William 
 Charles Wells, a political sinner of the first 
 magnitude in this land, and now suffering but 
 a very small proportion of those pains and pe- 
 nalties which his high crimes and misdemeanours 
 have so justly deserved, in the common gaol 
 of this metropolis," &c. Nature had not formed, 
 nor had education trained me, to submit with 
 silence to oppression. By means of money, I 
 got a letter inserted in one of the Charlestown 
 newspapers, the following extracts from which 
 will show to your Lordship, whether my senti- 
 ments then partook of disloyalty. 
 
 Charlestown, in Gaol^ July 17, 1783. 
 
 " I left this place in August, 1775; pur- 
 posely to avoid signing a paper, at that time 
 handed about under the title of AN ASSOCIATION. 
 I returned to it in January, 1781, when in pos- 
 session of the British army, and left it again 
 with those troops in December, 1782. I am. 
 
346 LETTER TO 
 
 I ever was, and I ever shall be, a subject of 
 Great Britain. 
 
 "In what respect, therefore, I can be a ' po- 
 litical sinner of the first magnitude in this land,' 
 and what are those ' high crimes and misde- 
 meanours' which I have committed, I cannot 
 
 well conceive. If indeed to wish well to my 
 
 country while contending with other powers, 
 and to be ready at all times to lay down my 
 life in support of its honour and interests, be 
 a crime, I cheerfully plead guilty to the charge." 
 
 " For a freeman to be deprived of his liberty, 
 and lodged in a common gaol ; to be kept con- 
 stantly locked up in a room, whose ceiling is in 
 that condition that the rain pervades it in every 
 shower, sometimes in such quantity that it must 
 be carried out in pails, and whose only window 
 looks to the north, a quarter of the heavens 
 from which the wind never blows when the 
 weather is most sultry, and which not being 
 glazed, obliges him to exclude the cheerful 
 light of day, at the same time that he shuts out 
 the storm * ; lastly, to be without the conversa- 
 tion of his friends, whom the dread of popular 
 
 * Thunder-storms occur almost daily in South Carolina, 
 in the months of July and August, and almost always pro- 
 ceed from the north or north-west. 
 
LORD KENYON. 347 
 
 resentment prevents from visiting him*; if 
 these sufferings are but a small portion of what 
 
 * However unconnected it may appear with the subject of 
 this letter, I cannot forbear mentioning the conduct of two 
 of my friends in Carolina., Mr. John Harleston, and his wife, 
 Mrs. Elizabeth Harleston, persons of rank and fortune in that 
 country. I had received many civilities from them during 
 my stay in Charlestown, while it was a British garrison, and 
 had on my part, done them some small service. But small 
 as this was, it sunk deep into their noble natures, and con- 
 stituted a debt, unused as they were to receive obligations, 
 which seemed to them inextinguishable. On my return to 
 Charlestown, with the flag of truce, they insisted upon my 
 staying at their house j but it was during my imprisonment 
 that the energy of their friendship was chiefly conspicuous. 
 No one day of the three months which it lasted passed away, 
 without my receiving from them repeated instances of kind- 
 ness, such as I could have expected only from those, who 
 were bound to me by the closest ties of blood. This conduct 
 would at any time have merited my warmest gratitude 5 but 
 when I consider the circumstances under which it occurred, 
 my feelings altogether unman me. Mr. Harleston's estate 
 had been heavily amerced by the legislature of South Caro- 
 lina j and at that period, when the affairs of the state were 
 regulated by the narrow principles of a petty corporation., 
 nothing could tend more to frustrate his hope, that the fine 
 would be taken off, than his showing attention to any one in 
 my situation. The reins of government also were then so 
 feebly held, that the populace almost daily wreaked their ven- 
 geance upon such as had fallen under their displeasure. One 
 night, during this anarchy, a mob surrounded Mr. Harleston's 
 house, threatening to destroy it on account of his behaviour 
 to me. He was from home $ but his wife, with the spirit and 
 dignity of a Roman matron, went out to the rioters, and told 
 
34$ ETtER TO 
 
 he is to bear, he can look forward to nothing 
 but DEATH, as the full expiation of his crimes. 
 Grant him but the choice of the mode, and he 
 will thank Heaven for the opportunity of de- 
 monstrating his attachment to his sovereign: 
 let but thousands witness that his last prayers 
 were for his country's prosperity, and it will 
 afford him more exquisite happiness in the ex- 
 treme moments of his life, than good men enjoy 
 when angels sing requiems to their departing 
 souls/* 
 
 them, that her husband and herself had done nothing towards 
 me but their duty, and that they should not be prevented 
 from continuing to perform it, by any menace whatsoever. 
 One of those persons is since dead ; the other still exists an 
 ornament to her sex. Excellent woman! enjoying in af- 
 fluence, in the midst of thy children, and their children, the 
 calm evening of a well spent life, and looking forward with 
 a firm hope, inspired by our holy religion, to another and a 
 better state, though thou seemest already to possess as much 
 of happiness, as is compatible with the infirmity of our pre- 
 sent natures ; it may yet afford thee some momentary satis- 
 faction to know, that neither distance of place, nor interven* 
 tion of time, hath lessened my sense of thine unspeakable 
 goodness - } and that, at this moment, my bosom heaves and 
 my eyes drop tears, while I reflect, that without thy tender 
 cares concerning me, when sick and in prison, and far re- 
 moved from those, whose duty it was to render me service 
 under such distress, I might long ago have been numbered 
 with the dead. 
 
LORD KENYON. 349 
 
 The smallest drop of blood may become visible 
 on the surface of an animal body, and may serve 
 there some special and useful purpose; sent 
 back to the heart, it is mixed with such a mul- 
 titude of similar particles, that all marks of it 
 as an individual are lost. In like manner, 
 having returned from the frontiers of the Bri- 
 tish empire to its capital, I naturally sunk back 
 into the obscurity, which was suitable to my 
 condition in life, rendered now still more low 
 by the poverty, which had been brought upon 
 my family, by their adherence to a great public 
 cause. In more happy times, therefore, than 
 those which have since followed, I could scarcely 
 have expected an opportunity of demonstrating 
 a love for my country, otherwise than by a ready 
 obedience to its laws. In consequence, how- 
 ever, of the attempts which some men, incited 
 to deeds of parricide by the example of suc- 
 cessful crimes in a neighbouring state, have 
 made to overthrow our ancient constitution, 
 persons of every rank have within these few 
 years been called upon to declare their attach- 
 ment to it. I have gladly obeyed this call ; 
 and my name appears in the list of those inha- 
 bitants of London, who signed the declaration 
 at Merchant Taylors' Hall, in December, 1792 ; 
 and in that of the same description of persons 
 .who signed the declaration at Grocers' Hall, in 
 
350 LETTER TO 
 
 December, 179-5. More lately, when profes- 
 sions alone were deemed insufficient for the 
 public safety, and a demand was made upon 
 the lovers of their country for their services as 
 its armed defenders, I obtained the honour of 
 being enrolled in a body of men, perhaps not 
 unknown to your Lordship, THE TEMPLE ASSO- 
 CIATION, and since I have belonged to it, my 
 exertions to fit myself by a knowledge of mili- 
 tary exercises, for the great object of its institu- 
 tion, have not been less than those of many- 
 members, younger than myself, and probably 
 not more engaged in other serious pursuits. 
 
 It may now appear to your Lordship, that I 
 have spoken of every possible personal objection 
 to my being examined for admission into the 
 college of Physicians. But as pretexts are 
 never wanting to those who wander from the 
 path of honour in search of them, I shall take 
 the liberty of mentioning still another ground, 
 which I have been told they affected to have, 
 for their refusing to inquire into my qualifica- 
 tions. For, Dr. Pitcairn informed me, in the 
 course of last summer, when it could not be 
 foreseen, that he would be unable in the ensuing 
 September to propose me a second time for exa- 
 mination, that, contrary to his former opinion, 
 
LORD KENYON. 351 
 
 he now believed that his intended motion would 
 be opposed, on this among other accounts, as 
 he understood, that I had been active in the 
 late dispute between the fellows and licen- 
 tiates. 
 
 That an individual should lose his title to a 
 privilege which had been adjudged by a court 
 of law to belong to a body of men, of which he 
 was a member, merely because he had lent his 
 aid towards obtaining that adjudication, may be 
 perfectly consistent with the notions of right 
 entertained by the college of Physicians, but 
 is certainly not so with those of your Lordship. 
 For if any person had been pre-eminently active 
 in the dispute alluded to, it was surely Dr. 
 Stanger, who, by his applications to the Court 
 of King's Bench, had subjected the college to 
 considerable trouble, expence, and obloquy 5 
 and yet your Lordship expressly declared your 
 conviction of his fitness to become a fellow of 
 that corporation. My share in the dispute may 
 be described in a very few words. When it was 
 proposed to me by some licentiates, with whom 
 the scheme originated, to assist in endeavouring 
 to obtain admission into the college by process 
 of law, if it could not otherwise be gained with 
 honour, I immediately consented. I was after- 
 wards appointed one of five to draw up an ad- 
 dress to the college, and this address Dr. Cooke, 
 
352 LETTER TO 
 
 Dr. Stanger, and myself, delivered to the pre- 
 sident. These were the only parts of my con- 
 duct, in that undertaking, which can be called 
 public, except this appellation should also be 
 given to the subscribing of a small sum of 
 money towards defraying its expence. My pri- 
 .vate conduct in it was studiously guarded ; for 
 as it very soon appeared to me, that the dispute 
 must be terminated by a court of law, I held 
 all private discussion of it with the fellows as 
 useless, and tending only to produce mutual 
 irritation of mind. I therefore constantly for- 
 bore to introduce it as a subject of conversation, 
 in the presence of a fellow. My reserve upon 
 this point was indeed so strict, that one of that 
 order, with whom I am more intimately con- 
 nected than with any other physician in London, 
 could not refrain from mentioning it to me, at 
 the same time that he compared my behaviour 
 in this respect with that of another licentiate of 
 his acquaintance, who made the dispute a topic 
 of conversation whenever they met. I mean, 
 however, only to state, not to extenuate my 
 conduct ; for had it been as active as that of 
 Dr. Stanger, I should for this very reason have 
 thought it entitled to considerable applause. 
 But I feel ashamed at occupying your Lord- 
 ship's attention with such trifles. Nothing in- 
 deed could have induced me to present them 
 
LORD KENYON. 353 
 
 to your notice, but the desire of affording you 
 the most ample grounds for reconsidering the 
 opinion, which you publicly gave of the col- 
 lege of Physicians ; and trifles often furnish the 
 most sure, because the most unguarded, avenue 
 to a knowledge of the characters of men. 
 
 I HAVE thus, my Lord, endeavoured to prove> 
 that the college of Physicians have not, by 
 their conduct since the decision of the Court 
 of King's Bench, in the case of Dr. Stanger, 
 shown themselves worthy of the high praise, 
 which you were then pleased to bestow upon 
 them. But it appears to me, that if your Lord- 
 ship had minutely examined the materials of 
 which that body is composed, or had been well 
 acquainted with its previous proceedings, you 
 would not have regarded the honour and good 
 faith of its members, as sufficient barriers 
 against their acting unjustly towards the licen- 
 tiates, who should apply for admission into their 
 corporation. 
 
 In this country, the glory of whose legis- 
 lators has been to view men as they are found 
 to be by experience, the honour and good faith 
 of no person are, I believe, ever esteemed by 
 
 A A 
 
354 LETTER TO 
 
 the law as adequate securities for his acting 
 justly, when he is tempted to act otherwise by 
 interest. The judges of our superior courts of 
 law are selected from a profession, the conduct 
 of whose members is more open to public in- 
 spection, and is consequently better known, than 
 that of the members of any other. No mistake, 
 therefore, can well occur with respect to the 
 characters they possessed before their elevation 
 to the Bench, more especially as few receive 
 that honour before they are past middle age; 
 and every one admits, that, in modern times at 
 least, they have been very generally, if not 
 always, chosen by the executive power with 
 the purest intentions. When they afterwards 
 appear to the world in the exercise of their pe- 
 culiar functions, the eyes of all men are fixed 
 upon them. Every part of their conduct is 
 scrutinized with the utmost care ; by some 
 whom education and habit have particularly 
 fitted for this purpose; by others, whose dearest 
 interests lead them to turn their whole atten- 
 tion to this single point, and whose disappointed 
 hopes naturally suggest some fault in those, 
 who have dissipated their gay dreams, and have 
 awakened them to poverty and disgrace. Yet 
 even these men, so formed to their stations, se- 
 parated by their retired life from many causes 
 of bias to human opinion, venerated by their 
 
LORD KENYON. 35,> 
 
 country if they act uprightly, detested if they 
 furnish the least suspicion of a contrary con- 
 duct, possessing their places by the most certain 
 tenure to persons of honour, receiving for their 
 labours a fixed and ample reward, and solemnly 
 sworn to administer justice impartially, are still 
 supposed liable to be influenced by improper 
 considerations, and are therefore forbidden to 
 try a great class of causes, when these occur in 
 the counties where they were born, or at pre- 
 sent reside. 
 
 If a situation can be conceived in which in- 
 terest could furnish no temptation to the aban- 
 doning of duty, or none which might not be 
 easily resisted, this would surely occur, when we 
 were charged with the preservation of the life 
 of some one connected with us by the closest 
 ties of consanguinity, who from tender years 
 or imbecility of mind, might be unable to pro- 
 tect himself. On one side, good faith, honour, 
 humanity, the claims of blood, would urge us 
 to the faithful execution of our trust ; on the 
 other, public execration, eternal remorse, and 
 disgraceful death, would necessarily present 
 themselves as consequences of its breach. Yet 
 our Saxon ancestors, perhaps not less virtuous 
 than any other .nation in the world, whether 
 ancient or modern, building their law upon ex^ 
 perience, and knowing hence how unfit men 
 
 A A 2 
 
356 LETTER TO 
 
 are to resist repeated attacks of interest, where 
 there is the smallest chance that their yielding 
 to them will be concealed, refused to commit 
 an orphan, or person of insane mind, to the care 
 of the next heir, though he were the nearest 
 relation. 
 
 It would, I think, be difficult, if not impos- 
 sible, to point out, in any part of the world, a 
 large body of men, who are more likely, in 
 their collective capacity, to regulate their con- 
 duct by the principles of honour and good 
 faith, than the Commons of the Parliament of 
 Great Britain ; and yet not many years have 
 elapsed, since they confessed by their proceed- 
 ings, that they had often corruptly exercised 
 the power of determining contested elections to 
 their House, and by a noble act of general jus- 
 tice, deprived themselves of the means in future 
 of violating the rules of right in detail. 
 
 Distrust of the virtue of mankind, seems in- 
 deed to be a leading principle of the con- 
 stitution of our country. The supreme power 
 of the state is vested in no one person, or set of 
 persons , but is broken down into various parts, 
 which are distributed among different descrip- 
 tions of men. Each of these, from the original 
 laws of human nature, aims at its own aggran- 
 dizement, which the others labour equally to 
 oppose. From this contention arises the most 
 
LORD KENYON. 357 
 
 lovely order ; our public happiness is thus bot- 
 tomed in our private infirmities, and the sta- 
 bility of our government is secured by the very 
 means, which to superficial observers appear 
 fraught with its destruction. 
 
 If therefore it cannot be inferred from the 
 common qualities of Englishmen, that the col- 
 iege of Physicians, when under no other con- 
 troul than that of honour and good faith, will 
 always act justly, it appears to me that, setting 
 aside actual experience, the only ground for ex- 
 pecting such conduct from them must be looked 
 for in the habits and principles, which physi- 
 cians acquire in the practice of their profession. 
 The probability of finding it there shall be my 
 next subject of inquiry. This perhaps will be 
 best conducted by considering, in the first 
 place, the state and estimation of medicine, 
 when exercised as a gainful art, in ages and 
 countries different from our own. 
 
 When men first begin in any country to prac- 
 tise the medical art for hire, their knowledge 
 of diseases, and of the proper modes of treating 
 them, is necessarily very small. To conceal, 
 therefore, their ignorance, they affect mystery, 
 and have recourse to various modes of decep- 
 jtion. Thus, in all rude nations, physicians 
 Jaave pretended to use supernatural means in 
 
358 LETTER TO 
 
 the cure of diseases ; among those nations in- 
 deed, the different trades of conjurer and phy- 
 sician are commonly exercised by the same 
 person. But such a course of life must debase 
 the character, in every respect, of him who 
 follows it. No one can promise to himself, 
 that he will stop at any certain point in villany. 
 Temptation solicits him to proceed, and his 
 powers of resistance diminish as he advances ; 
 till at length he arrives where honesty and 
 truth seem no more than scare-crows, set up 
 by designing men to prevent the weak and 
 timid from pursuing their own good. 
 
 As the knowledge of diseases and their re- 
 , medies increases, the obtaining of it becomes 
 more difficult, and from the general progress 
 of improvement, there are now men who can 
 estimate the value of the acquisition. Phy- 
 sicians are therefore less tempted either to con- 
 ceal their methods of cure, or to pretend to 
 derive assistance from supernatural agents. 
 Hence medicine, considered as a gainful pro- 
 fession, has for the most part been less despised 
 in civilized, than in barbarous nations. It ap- 
 pears, however, to have been held in very little 
 estimation, even by the most polished nations 
 of antiquity, of which we have any tolerably 
 well authenticated accounts. 
 
LORD KENYON. 359 
 
 In Egypt, a physician, who attempted to cure 
 a disease by means different from those which 
 were mentioned in the sacred books, forfeited 
 his own life, if his patient died. By the con- 
 fession of Hippocrates, medicine was regarded 
 by the Greeks as the lowest of the arts. The 
 oath which he exacted from his scholars, not to 
 commit some of the vilest crimes, and to keep 
 secret the knowledge which he should commu- 
 nicate to them, is a strong proof of the truth of 
 his observation. With the Greek comic writers, 
 " a son of Hippocrates/' was a term of derision. 
 So low indeed was the condition of physicians 
 in Greece, that Alexander the Great seems to 
 have been neither affected with remorse, nor 
 accused of cruelty, for crucifying Glaucus, the 
 physician of Hephasstion, though the death of 
 his favourite had been occasioned by his own 
 imprudence. Many learned men have shown 
 that, before Julius Caesar, the physicians in 
 Rome were, for the most part, if not alto- 
 gether, either freedmen or slaves. Afterwards, 
 medicine rose there somewhat in esteem, both 
 from the greater knowledge of its professors, 
 and the degradation of the former civil distinc- 
 tions in society ; but it was still attended with 
 so little respect, that even Galen was afraid 
 to prescribe some pepper in wine to Marcus 
 
360 LETTER TO 
 
 Aurelius, for a pain in his stomach, because it 
 was too strong a remedy for an emperor. 
 
 It forms no argument against the justness of 
 this statement, either that kings and princes 
 anciently exercised the medical art, or that 
 physicians were sometimes held in considerable 
 estimation by the great. For, in the first place, 
 there are many arts which adorn those who 
 cultivate them for their own use or amusement, 
 or for the benefit of others, but which degrade 
 the persons that practise them for money. Our 
 country gentlemen are very desirous of knowing 
 the diseases of horses, and their remedies : but 
 the trade of a farrier is with us a very low one. 
 The talent of singing is much prized by females 
 of the highest rank ; yet how meanly are those 
 persons thought of, who gain by it their liveli- 
 hood ? And secondly, eunuchs, and other men 
 confessedly of the vilest condition, have not un- 
 frequently been entrusted with the management 
 of empires. 
 
 Physicians have, in modern Europe, obtained 
 a higher rank in society, than they possessed 
 among the ancients, principally however, as it 
 appears to me, by means entirely unconnected 
 with the exercise of their profession. For, 
 upon the revival of a taste for letters in our 
 western parts of the world, spme persons applied 
 
LORD KENYON. 361 
 
 themselves to the study of the ancient writers 
 upon medicine, with the view of becoming 
 more successful practitioners of that art, than 
 those were, who had learned it in the ordinary 
 manner. But the same skill in languages, which 
 was necessary for this undertaking, fitted them 
 also for the acquisition of every other kind of 
 knowledge, which had been treated of by the 
 authors of Greece and Rome. They made use 
 of this advantage, and physicians became noted 
 for their proficiency in every branch of the 
 learning of antiquity. This erudition naturally 
 rendered those who possessed it respectable, 
 and, by an obvious association, raised their 
 profession in the esteem of the public. It 
 produced the same effect in another way. A 
 tedious and even expensive education was 
 henceforward deemed requisite for physicians, 
 which could be borne only by persons of some 
 fortune, and therefore, less likely to be guilty 
 of baseness and deceit, than men in the low 
 condition of the former practitioners of me- 
 dicine. 
 
 The operation of these causes was, in this 
 country, considerably assisted by the same cir- 
 cumstances, that have given our merchants and 
 manufacturers their present place in society ; 
 and by reason of this combination, its physicians 
 hold a much more elevated situation than those 
 
362 LETTER TO 
 
 of any other considerable nation in the world. 
 When an English physician travels upon the 
 continent of Europe, he frequently finds that 
 his profession, if known, is a bar to his recep- 
 tion into good company, and therefore very ge- 
 nerally conceals it. 
 
 But, my Lord, though the physicians in this 
 country have been thus freed from, what may 
 almost be termed, the necessity, which formerly 
 existed for using improper means to gain em- 
 ployment, they are still often strongly tempted 
 to do wrong in the same pursuit. They are, 
 indeed so often, and so strongly tempted to do 
 so, and are at the same time, from the nature of 
 their profession, so little liable to be prevented 
 from yielding, by that great guardian of virtue, 
 public censure, that it seems to me beyond a 
 doubt, that the body of physicians here must 
 contain a greater proportion of persons, who 
 have made undue sacrifices to their rise in the 
 world, than several other classes of English- 
 men ; than, for instance, the body of barristers, 
 with which alone, indeed, it can properly be 
 compared. What knowledge I have of this sub- 
 ject, is derived from my residence in London ; 
 the observations, therefore, which I shall make 
 upon it are, in strictness, only applicable to the 
 state of physicians in the capital. Your Lord- 
 ship, however, will not suppose it my intention 
 
LORD KENYON. 363 
 
 to insinuate, that I have not yielded to the 
 same temptations : Video meliora proboque ; de- 
 teriora sequor. A soldier may relate the defeats, 
 as well as the victories, in which he has borne a 
 share* 
 
 The young men, who apply to the study of 
 medicine in this country, are chiefly of small 
 original fortune, and the greater part of this is 
 commonly consumed in their education. Very 
 few physicians, therefore, when they come to 
 London to exercise their profession, which, if 
 they have graduated at either of the English 
 universities, they seldom do till they are nearly 
 thirty years old, have sufficient incomes for 
 living in the manner, which is thought here 
 becoming the rank of a gentleman. They are 
 consequently extremely desirous to supply this 
 deficiency in their private fortunes by the pro- 
 fits of practice, and their age strongly urges 
 them against every needless delay in attempting 
 to accomplish this end. Barristers, from enter- 
 ing more early into their profession, may witri 
 less inconvenience wait the gradual approach 
 of business. These too have frequently, soon 
 after they commence practice, opportunities of 
 appealing to the world, in the most honourable 
 manner, on their fitness to be employed. They 
 address themselves publicly to men well quali- 
 fied to judge of their abilities, and upon subjects 
 
364 LETTER TO 
 
 of which almost every person understands as 
 much, as renders him capable of determining, 
 whether or not they have been rightly con- 
 ducted. If the exhibition of talents has been 
 considerable, it is soon very generally known, 
 and is in a short time followed by an increase 
 of employment, from the desire of many to 
 benefit themselves by their assistance. A phy- 
 sician has no such opportunity of showing the 
 knowledge which he possesses ; he possesses 
 indeed, on beginning practice, much less know- 
 ledge capable of being turned to immediate use, 
 than a barrister of the same standing, and equal 
 application. His art is founded upon experi- 
 ment and observation, and the rules for exer- 
 cising it are always modified by external cir- 
 cumstances, which can never be accurately 
 known, except by one long conversant with 
 diseases, as they actually occur. Skill in me- 
 dicine is therefore not to be acquired by reading 
 alone : whereas law, being a collection of the 
 opinions and ordinances of men, is necessarily 
 studied in books ; and hence a considerable 
 knowledge of it may be obtained by those, who 
 have seen little 'of its application to particular 
 cases. Besides, a young barrister does not 
 appear in the management of any case, until 
 a considerable time has been spent by him in 
 preparation^ but the first calls upon a young 
 
LORD KENYON. 366 
 
 physician are frequently to oppose sudden at- 
 tacks of disease, which do not permit his think- 
 ing long, how this can best be done. For these 
 reasons, it seldom happens, that physicians 
 either merit much praise from their first efforts 
 Jto cure diseases, or quickly acquire a considera- 
 ble increase of practice from any single display 
 of great talents. They must consequently be 
 strongly disposed to adopt other means to raise 
 themselves to notice. 
 
 The present possession of practice being a 
 considerable recommendation of physicians to 
 further employment, every young physician 
 finds an advantage in having it thought, that 
 his business is greater than it actually is ; and 
 should he endeavour to impress the public with 
 such an opinion, the privacy with which the 
 medical profession is for the most part exer- 
 cised, prevents any flagrant discovery, that it is 
 not well founded. Many of them are therefore 
 induced, notwithstanding the smallness of their 
 incomes, to imitate the exterior expence of their 
 seniors, hoping that the world will hence believe, 
 that they enjoy a corresponding degree of em- 
 ployment. The business of a. barrister being, 
 on the contrary, chiefly conducted in open 
 courts, any attempt to make it appear greater, 
 than it is in reality, would soon expose him 
 to ridicule and disgrace* He lives, therefore., 
 
366 LETTER TO 
 
 except his private fortune be large, for man}' 
 years in Chambers, and goes to Westminster 
 Hall in a hackney-coach ; whereas a physician, 
 sometimes immediately upon coming to town, 
 very commonly only a year or two after, occu- 
 pies a whole house, and visits patients in his 
 own chariot. But this expence, though its 
 object should be ultimately attained, reacts in 
 the mean time upon the cause which gave rise 
 to it, and augments in him the necessity for 
 professional gains. 
 
 The female sex, it is well known, have great 
 influence on the extent of practice which phy- 
 sicians possess. But, for many reasons, they 
 are averse to communicate their own complaints 
 to any one who is unmarried, and they naturally 
 recommend to others the person whom they 
 consult themselves. Physicians, therefore, very 
 generally marry soon after they commence 
 practice. As they are then far from being 
 wealthy, if they marry women in other respects 
 equal to themselves, they seldom receive for- 
 tunes with them. In this case, the calls for 
 money increase, for some time at least, more 
 rapidly than the beneficial effects of their new 
 situation ; and hence, actions, which were 
 formerly regarded as contemptible, will now 
 perhaps seem even praiseworthy, from afford- 
 ing subsistence to the objects of their most 
 
LORD KENYON. 367 
 
 tender affections. If, on the other hand, they 
 marry rich women, these are commonly unequal 
 to them in some material circumstance, in age, 
 education, habits, or personal appearance. But 
 a sacrifice to interest, in so momentous a con- 
 cern, is surely no pledge, that they will not 
 make others of less importance, in the exercise 
 of their profession. Barristers are much less 
 exposed to this cause of ill conduct in the pur- 
 suit of employment. Marriage gives to them 
 no advantage in it ; and hence, they generally 
 either enter into that state later in life than 
 physicians, or remain single to the end of it. 
 
 What I have said, my Lord, seems sufficient 
 to show, that the physicians of London are often 
 placed in situations, in which temptations to do 
 mean things for money are known by experience 
 to act forcibly. But collections of men appear 
 to be more or less virtuous, nearly in proportion 
 to the number and greatness of the enticements 
 to vice, with which they are surrounded. The 
 principles of honour may, indeed, become more 
 firmly fixed in the bosoms of some few indivi- 
 duals of uncommon make, from the very at- 
 tempts which are made to loosen their hold ; 
 but- though gold is purified and brightened by 
 fire, common metals are by the same agent 
 turned into dross. According to the model of 
 prayer, which has been given to us by the divine 
 
368 LETTER TO 
 
 author of our religion, we are not to petition for 
 strength to resist temptation ; man's presump- 
 tuous confidence in his own powers might have 
 been heightened by such a permission : but we 
 are humbly to beg our heavenly father not to 
 lead us into it, hereby confessing our insuf- 
 ficiency for the contest, whenever it shall occur. 
 I do not, however, my Lord, wish to convey 
 an opinion, that physicians become dishonest in 
 the situations which I have described ; my de- 
 sign is fully answered, if I have rendered it pro- 
 bable, by stating the difficulties in which they 
 are frequently involved, that their temptations 
 to lay aside the character of men of high ho- 
 nour, are sometimes too great for resistance* 
 I now add, that proofs of their actually yielding 
 to those temptations are furnished by what we 
 daily hear of their needless visits to sick persons, 
 their rapacity with respect to fees, and their ser- 
 vility to apothecaries*. When these, or similar 
 
 * The present division of medical practice in this country, 
 between physicians and apothecaries, did not commence in 
 London, until some time after the separation of the latter 
 from the grocers, in l6l7 and was not firmly established, 
 before the great plague in 1 665, during which, by far the 
 greater part of the physicians having fled into the country, 
 the apothecaries were left with almost the entire care of the 
 sick. These facts were at least advanced in a controversy, 
 which existed about the end of the last century, respecting 
 the title of apothecaries to practise medicine, and were not 
 
LORD KENYON. 369 
 
 practices have been adopted, they are not often 
 afterwards abandoned, because the circumstances 
 
 then contradicted. To support them, it may be mentioned, 
 that according to a publication from the college, dated 1698, 
 the number of apothecaries in London and Westminster, 
 sixty years before, was not 100, but was then above 800 ; 
 and that in 1701, they were said to be nearly IOOO, partners 
 included. At the date of their charter, in 1617, the number 
 was 1 14 5 so that it must have decreased for the first 20 years 
 after their separation. This division, however, seems to 
 have begun more early in some other parts of the kingdom ; 
 for a physician of Salisbury speaks of it in P566 as being 
 lately introduced there. Its origin may, I think, be placed 
 in the greatness of the fees/ which English physicians have 
 always been accustomed to receive. I find many notices of 
 an angel, or ten shillings, being the usual fee to them, from 
 16(55, to the beginning of the present century ; and in 1670, 
 Dr. Goddard, a fellow of the college, and Gresham Professor 
 of Physic, asserted, that the fees then given were according 
 to the ordinary and accustomed rates, time out of mind in 
 England. Many persons, therefore, who wished to receive 
 benefit from medicine, but unable or unwilling to fee phy- 
 sicians so largely, and at the same time too proud to solicit 
 their gratuitous aid, would naturally apply to those, who 
 offered both advice and medicines at a cheap rate. This also 
 seems the chief reason, and not the greater credulity of the 
 people, why empirics formerly abounded here, more than in 
 any other country in Europe. For, since the complete esta- 
 blishment of apothecaries, as medical practitioners, the num- 
 ber of empirics has been considerably lessened ; the descrip- 
 tions of men, who on account of cheapness used to resort to 
 the latter, now applying to the former, for the cure of their 
 complaints. The existence then of a lower order of practi- 
 tioners of medicine appears necessary in this country ; and 
 
 B B 
 
370 LETTER TO 
 
 which gave them origin have ceased. The 
 pride and delicacy of a gentleman, if once 
 
 the attempts of the college to destroy it were as absurd and 
 unjust, as they were fruitless. 
 
 When the division first took place, one of its effects was 
 probably not foreseen. For apothecaries coming at length 
 to be employed by many persons who were sufficiently rich 
 to fee a physician ; when the assistance of one was desired 
 by these in dangerous disorders, the choice of the individual 
 was frequently left to the apothecary,, he being supposed 
 better qualified to make it, than the sick person or his family. 
 From this time, therefore, the friendship of apothecaries be- 
 came highly useful to physicians, and was often sought for, 
 and requited by them, in the most disgraceful manner. I 
 might bring many proofs of these points from authors of the 
 last century; but! shall content myself with one, the authen- 
 ticity of which is beyond doubt, as it is found in an account 
 of the proceedings of the college, in establishing a dispensary 
 for the relief of the sick poor, which was published by them- 
 selves, in 1697- They there say; " Several amongst them 
 [the apothecaries] set themselves by all the art and industry 
 they were capabk of to frustrate the whole design ; and find- 
 ing no method so promising, as to stir up a party among 
 ourselves, to oppose our proceedings, they fell to intriguing 
 with several of our own members, ivho were too easily lured 
 off to seme the apothecaries' interest, for their own private ad- 
 vantage. And from this cause, as we have too much reason 
 to believe, have chiefly sprung the unhappy differences, 
 which are still fomented among us. But notwithstanding 
 all the discouragements we met with from those of our own 
 members, who contrary to all the obligations of honour and 
 conscience,, constantly discovered to our adversaries what- 
 soever passed in the college relating to this design, and ex*, 
 posed to them the names of such as were promoters thereof,. 
 
LORD KENYON. 371 
 
 surrendered, are scarcely, I fear, ever fully re- 
 gained. No one, however, who does not com- 
 pletely possess them, is surely fit to constitute a 
 part of the sanctuary of honour and good faith. 
 
 \ 
 
 that they might be kept out, as far as in them lay, from all 
 patients where they should be proposed, and themselves 
 brought in," &c. The college of Physicians, therefore, a 
 hundred years ago, were surely not the sanctuary of honour 
 and good faith ; since one part of them were then declared 
 by their colleagues to have violated every obligation of ho- 
 nour and conscience in pursuit of their private interest j while 
 those, who had thus erected themselves into censors of mo- 
 rals, openly confessed, that they were afraid to have ft known 
 they were doing a right thing, lest they should not be called 
 in by apothecaries to see their patients. Physicians, in ge- 
 neral, have in the course of the present century become more 
 prudent, and, I believe, more honourable j but it is, not- 
 withstanding, very notorious, that many of them at present 
 cultivate the acquaintance of apothecaries, in ways very 
 disreputable to gentlemen. Barristers may be tempted, 
 though, I think in a less degree, for reasons already men- 
 tioned, to act similarly towards attornies ; but the restraints 
 upon their yielding, are much greater. Their frequent meet- 
 ings in courts, and upon circuits, afford many opportunities 
 of discovering defaulters, and of inflicting punishments, 
 which few are hardy enough to disregard ; whereas phy- 
 sicians, having little necessary intercourse with each other, 
 are consequently in a great measure without the salutary fear 
 of the reprehension of their equals. In what estimation 
 would a barrister be held, who should give frequent and 
 costly dinners to attornies ? But it is said, and I believe 
 truly, that physicians of great eminence have derived much 
 of their practice from giving such dinners to apothecaries. 
 
 B B 2 
 
372 LETTER TO 
 
 But there are various circumstances in the 
 practice of medicine, unconnected with its 
 profits, which tend to injure the character of 
 those who follow it. An action at law remains 
 at rest, except it be urged forward by human 
 force, and its termination is induced by means, 
 which we can easily comprehend. The value,* 
 therefore, of the talents employed by any one 
 in conducting it, may be tolerably well appre- 
 ciated, and the fame which hence arises to him, 
 is almost always proportioned to his merit. It 
 is far otherwise in medicine. Diseases proceed 
 by their own energy, and terminate sponta- 
 neously, for the most part, in health. Such a 
 termination, however, of a dangerous disease, if 
 a physician has been concerned in its manage- 
 ment, is very commonly attributed to his skill. 
 He may at first blush at undeserved praise. At 
 length, from frequent repetitions of it, he often 
 fancies himself really capable of producing the 
 eifects, which he hears attributed to his agency. 
 Again ; should a barrister have any natural ten- 
 dency to overrate his talents, the frequent mor- 
 tifications he must experience, in his daily con- 
 tests with others of his own class, before public 
 assemblies of men, will soon teach him to value 
 them more justly. The same corrective is not 
 applied to physicians. In the exercise of their 
 profession, they appear always as dictators of 
 
LORD KENYON. 373 
 
 rules to others, and the feeling of self-import- 
 ance, which this situation excites, in time often 
 diffuses itself over every part of their conduct. 
 Men too form insensibly an estimate of their 
 own worth, from secretly comparing themselves 
 with those whom they see most commonly. But 
 well-employed physicians spend much of their 
 time in the company of persons weakened in 
 mind by disease, and of the female attendants 
 of sick rooms ; it ought not then to seem strange, 
 if, like schoolmasters from conversing chiefly 
 with children, they should acquire an opinion 
 of their own talents, much higher than what 
 they merit. 
 
 I shall take notice of only one other source 
 of injury to the character of physicians. Those 
 among them of the greatest learning and ex- 
 perience know well, that the most unexpected 
 changes sometimes take place in diseases, and 
 are best acquainted with the difficulty of re- 
 ferring to their proper causes, the various events 
 that occur in so complicated a structure as the 
 human body. It might therefore be thought, 
 that such men would always be modest, cautious, 
 and even timid, in the practice of their art. But 
 this is not the conduct which recommends a 
 physician most. It suggests to a sick person, 
 what indeed may be true, that a doubt exists 
 respecting the nature of his complaints, than 
 
374 LETTER TO 
 
 which nothing can be more distressing to him. 
 He often, therefore, applies to one, who ac- 
 knowledges no difficulty in the treatment of 
 diseases, who pretends to see clearly what is 
 hidden from human beings, and who speaks of 
 uncertain events, as if they were entirely under 
 his command. In this way, the sick man is 
 gratified, but too frequently at the expence to 
 the physician of one of the most valuable parts 
 of the character of a gentleman, and faithful 
 observer of nature. The exquisite painting by 
 Moliere of the vanity, affectation, and pedantry 
 of the French physicians of his time, exhibits 
 a resemblance to the general character even of 
 English physicians of the present day, which is 
 sufficiently strong .to make it probable, that those 
 qualities are, in a greater or less degree, almost 
 inseparably connected with the exercise of the 
 medical profession. But he in whom they exist, 
 though he should have the most upright inten- 
 tions, will often decide as unjustly, when his 
 own interest or consequence in the world is 
 concerned, as if he had been actuated by the 
 vilest motives. Before men, who are not go- 
 verned by others, can do what is right, they 
 must first clearly perceive it, which nothing cer- 
 tainly more effectually prevents, in whatever 
 has relation to themselves, than a false or ex- 
 travagant opinion of their own worth. 
 
LORD KENYON. 375 
 
 Many of our physicians have no doubt re- 
 ceived little injury from the causes of the cor- 
 ruption of character, to which they have been 
 exposed ; and some few may have escaped their 
 influence altogether. One of these few, Dr. 
 William Heberden, I must conclude to have 
 been well known to your Lordship, from the 
 eulogy which you pronounced upon him, dur- 
 ing the trial of Dr. Stanger's cause. He was 
 probably, indeed, the only physician with whom 
 you were intimately acquainted, and hence, 
 from the natural error of attributing to a whole 
 species the properties of its only individual we 
 have seen, you might imagine, that he possessed 
 his many virtues in common with the rest of his 
 class. But Dr. Heberden, my Lord, stands, in 
 a manner, alone in his profession. No other 
 person, I believe, either in this or any other 
 country, has ever exercised the art of medicine 
 with the same dignity, or has contributed so 
 much to raise it in the estimation of mankind. 
 A contemplation of his excellencies therefore 
 can afford little help towards obtaining a just 
 notion of the general worth of physicians. In 
 speaking of a mole-hill, we would not employ 
 terms that had relation to the immensity of a 
 mountain. 
 
 Were I, my Lord, possessed of talents ade- 
 quate to the undertaking, I should here en- 
 deavour to describe at full length the character 
 
376 LETTER TO 
 
 ,of that illustrious man. In this attempt, I should 
 first mark his various and extensive learning, his 
 modesty in the use of it, and his philosophical 
 distrust of human opinions in science, however 
 sanctioned by time, or the authority of great 
 names. I should then exhibit him in the exer- 
 cise of his profession, without envy or jealousy j 
 too proud to court employment, yet underva- 
 luing his services after they were performed ; 
 unwearied, even when a veteran in his art, in 
 ascertaining the minutest circumstances of the 
 sick, who placed themselves under his care, 
 taking nothing in their situation for granted, 
 that might be learned by inquiry, and trusting 
 nothing of importance that concerned them to 
 his memory. To demonstrate his greatness of 
 mind, I should next mention his repeatedly de- 
 clining to accept those offices of honour and 
 profit at the British court, which are regarded 
 by other physicians as objects of their highest 
 ambition, and are therefore sought by them with 
 the utmost assiduity. I should afterwards take 
 notice of his simple yet dignified manners, his 
 piety to God, his love for his country, and his 
 exemplary discharge of the duties of all the 
 private relations in which he stood to society; 
 and I should conclude by observing, that his 
 whole life had been regulated by the most ex- 
 quisite prudence, by means of which his other 
 virtues were rendered more conspicuous and 
 
LORD KENYON. 377 
 
 useful, and whatever failings, he might as a 
 human being possess, were either shaded or 
 altogether concealed. After my description 
 was finished, I should think it proper to say, 
 that I had never been acquainted with Dr. 
 Heberden, and consequently could neither be 
 dazzled by the splendour of his virtues, from 
 approaching them too nearly, nor influenced in 
 my opinion concerning them, by benefits he had 
 already conferred upon me ; and that standing, 
 as he does, upon the verge of this state of exist- 
 ence, ready to wing his flight to another of 
 glory, his ear must now be closed to the voice 
 of flattery, had he ever listened to that siren, 
 or were I base enough to solicit her aid, in the 
 foolish expectation of receiving from him some 
 future reward. 
 
 I think, my Lord, it has now been shown, 
 that physicians, considered singly, cannot by 
 reason of the discipline of their profession, 
 claim exemption from the moral infirmities, to 
 which the other inhabitants of this country are 
 subject. Is it then to be supposed, that a body 
 of them will always be governed by the strictest 
 rules of justice ? Is it, my Lord, at all consistent 
 with the experience we have of human actions 
 to expect, that those, who may have individually 
 
LETTER TO 
 
 yielded to temptations of interest, will, when 
 exposed in a collected state to similar tempta- 
 tions, continue long to deserve the title of the 
 sanctuary of honour and good faith? 
 
 But perhaps it will be said. here : " Granting 
 that the college of Physicians, like other men, 
 are open to the influence of motives, which 
 pervert or corrupt the judgment, it is yet im- 
 possible not to believe, that their general con- 
 duct is agreeable to the common maxims of 
 prudence. Their reputations must surely be 
 dear to them ; these therefore they will not 
 hazard, without the prospect of some advantage 
 to compensate the risk. But with respect to 
 the admission of licentiates into their body, the 
 circumstance which has given birth to the whole 
 of this discussion, what interest have they in 
 acting unjustly? Unless then it shall be clearly 
 established, that they have such an interest, the 
 attempts which have been made by the author 
 of this letter to depreciate their character, must 
 be regarded as the offspring of spleen or disap- 
 pointed ambition, to bestow upon them no 
 harsher appellation." Anticipating, my Lord, 
 these observations, I proceed to reply to them. 
 In doing this, I shall be led to the last purpose 
 of my address, namely, to present to your Lord- 
 ship's view, several proceedings of the college, 
 previous to the decision of the Court of King's 
 
LORD KENYON. 379 
 
 Bench in Dr. Stanger's case, which, if known 
 or minutely considered by you, might have pos- 
 sibly induced an opinion respecting the in- 
 tegrity of their corporate conduct, far different 
 from what you then so warmly expressed. 
 
 In the first place, it will be scarcely denied 
 by any one, in the least acquainted with me- 
 dicine as a practical art in London, that phy- 
 sicians conceive it of much importance to be 
 fellows of the college. This indeed seems suf- 
 ficiently proved, both by the eagerness with 
 which admission into the fellowship has been 
 sought by some of our most celebrated phy- 
 sicians, Hunter, Fothergill, and Fordyce, not 
 to mention other and later names, and by the 
 obstinacy with which their endeavours to gain 
 it have been resisted, by those already in pos- 
 session of the corporation. It will not diminish 
 the force of this argument to assert, that the 
 object in dispute was altogether unworthy of 
 the exertions, to which it gave rise. Men do 
 not always estimate the value of things, either 
 according to the profit they produce, or by the 
 rules which may possibly guide the opinions of 
 superior beings. What more trite, and, at first 
 sight, more just subject of ridicule is there, than 
 the vehement desire which many exhibit, for the 
 possession of a piece of ribbon of a particular 
 
380 LETTER TO 
 
 colour ? Yet this desire exists with persons of the 
 first talents, fortune, and rank in this country : 
 
 te Let school-taught pride dissemble all it can, 
 <f These little things are great to little man." 
 
 Though it be unnecessary, therefore, to pro- 
 ceed further in proving the value of a fellow- 
 ship of the college, I shall, notwithstanding, 
 briefly mention some of the advantages, which 
 accrue to physicians from possessing it. 
 
 There are various offices, lectureships, and 
 appointments in the college, which are attended 
 with profit, and are filled by fellows alone. The 
 emoluments of these, though not considerable, 
 are still of sufficient magnitude to render them 
 objects of desire to physicians in the first years 
 of their residence in London ; and hence, as I 
 have been informed, they are frequently given 
 to the younger fellows, with the view of assist- 
 ing them during that difficult period. 
 
 The chief advantages, however, which a phy- 
 sician enjoys from a fellowship of the college, 
 are in consequence of his being often placed by 
 it, in very conspicuous and honourable situa- 
 tions. Soon after receiving it, he becomes an 
 examiner of the fitness of other physicians to be 
 fellows or licentiates ; a visitor of the shops of 
 apothecaries, for the purpose of inspecting the 
 
LORD KENYON. 381 
 
 quality of their medicines; and a commissioner, 
 under an act of the legislature, for licensing 
 houses for the reception of lunatics. By these 
 means, though he may be a very young phy- 
 sician, he nevertheless appears to the world as 
 a man of rank in his profession. Such a cir- 
 cumstance to the greater part of persons must 
 be highly gratifying, without regard to its con- 
 sequences. But in medicine, the slightest sign 
 of distinction is frequently a source of profit to 
 the possessor ; for as men, in general, have not 
 sufficient knowledge or discernment to choose 
 their physicians on the ground of merit, they 
 commonly take those who exhibit marks of 
 public approbation and confidence. A fellow- 
 ship, therefore, by bestowing such marks, is 
 often greatly conducive to the advancement of 
 the interests of a physician. It is far indeed 
 from always happening, that fellows of the col- 
 lege rise to eminence, as practitioners of me- 
 dicine 5 but the fact is undoubted, that they 
 rise to it more frequently and more quickly, 
 than licentiates in every respect equal to them- 
 selves, except as to the relation in which they 
 stand to the college'. 
 
 But it is evident that these, and all other ad- 
 vantages of a fellowship, will be more or less 
 amply enjoyed by individuals, according as 
 few or many are entitled to partake of them. 
 
382 LETTER TO 
 
 Whether any body of men would be able to resist 
 such a temptation to restrain the increase of 
 their number, I know not. It is certain, at 
 least, that the college have not been so, but have 
 often adopted measures for this purpose, which 
 are declared, by persons of the highest au- 
 thority, to have been contrary to the laws of 
 our country. " Licences," said Lord Mans- 
 field, while delivering a judicial opinion upon 
 the conduct of that corporation, " probably 
 took their rise from that illegal by-law, now at 
 an end, which restrained the number of fellows 
 to twenty. This was arbitrary and unjustifiable ; 
 they were obliged to admit all such as came 
 within the terms of their charter." 
 
 The effect, which was once derived from re- 
 straining by-laws, is now produced by means 
 less odious in appearance, but not less sure in 
 operation. Though a. degree of doctor in me- 
 dicine, from Oxford or Cambridge, has been 
 demanded by the college, almost from its 
 foundation, as a qualification for a fellowship ; 
 yet, for a considerable time, it was occasionally 
 dispensed with, and when it was not, physicians, 
 who had graduated elsewhere, could for a small 
 sum of money, readily procure such a degree 
 from those universities, by incorporation *. But, 
 
 * The degrees, which students of Oxford and Cambridge 
 receive from their own universities, are conferred by creation* 
 
LORD KENYON. 383 
 
 towards the end of the last century, laws were 
 passed by our universities, at the desire, it is 
 said, of the college, to prevent in future the 
 incorporation in them of physicians, who had 
 graduated in any place out of England, except 
 Dublin; and since then, the college have never, 
 I believe, admitted any one to an examination 
 for a fellowship, who did not possess an English 
 degree of doctor in medicine. The consequence 
 has been, that the number of members, which in 
 1677 was sixty-five*, without including twenty 
 honorary fellows, a class no longer existing, is 
 now only forty-eight t, notwithstanding the vast 
 increase, which the capital has in the mean 
 time received, in point both of population and 
 riches. But all surprise at this diminution of 
 the number of members will cease, when it is 
 known, how greatly that of licentiates has during 
 the same interval been augmented. In 1667, 
 
 but when a graduate from a different university is admitted 
 in either of them, ad eundem gradum, this is called incorpora- 
 tion. 
 
 * Fifty-three fellows and twelve candidates, who are both, 
 in the language of the college, named college?. The term 
 candidate is used in a very different sense by the college from 
 what is commonly given to it ; with them it means a person 
 who has passed all the examinations which are required for 
 a fellowship, but who is not actually in possession of it. I 
 have for this reason very seldom employed it. 
 
 f Forty-five fellows and three candidates. 
 
384 LETTER TO 
 
 there were only ten persons in that class; while 
 the present college list contains one hundred 
 and five, the far greater part of whom would 
 have been admitted as fellows, if the English 
 universities had not repealed their former laws 
 for granting degrees by incorporation. 
 
 The system of admission which has produced 
 these effects, is that which the college, after 
 being repeatedly admonished of its narrowness 
 and injustice by Lord Mansfield, professed to 
 amend, by the two by-laws already so often 
 spoken of. That they have an interest, how- 
 ever, directly contrary to the pretended object 
 of the new laws, is clear from the tardiness 
 alone with which these were brought forward. 
 Lord Mansfield began in 1767 to censure the 
 old laws of admission, yet the new were not 
 made before 1778*. The succeeding history 
 of one of the latter demonstrates the existence 
 
 * The college, during the trials of Dr. Stanger's case, 
 seemed to have been much ashamed of the dates of these 
 laws. They were not mentioned in Mr. Roberts's affidavit, 
 and when asked for by the judges, the counsel of the college 
 appeared ignorant of them. If the omission had not been 
 by design, they would surely have been inserted in Dr. Gis- 
 borne's affidavit in answer to Dr. Stanger's second applica- 
 tion ; but upon this subject he was equally silent with Mr. 
 Roberts. At length, after repeated questions from the judges 
 during the second trial also, it was extracted from Mr. Dam- 
 pier, that the new laws were made in 1778. 
 
LORD KENYON. 385 
 
 of the same interest still more strongly. This 
 at first authorized the introduction, by favour, 
 of two licentiates every year into the college. 
 But it was quickly after enacted, that only one 
 should be annually proposed for introduction ; 
 and again, that no proposition of this kind 
 should be made oftener than once in two years. 
 Such are the changes which the letter of the 
 law has undergone. If we look to its execu- 
 tion, it may MOW be regarded as abrogated; 
 since no licentiate has been proposed under it 
 for six years past. 
 
 But, though the college have thus shown, 
 that they possess a strong interest in preventing 
 the increase of their number, from the introduc- 
 tion of licentiates by favour, it is yet easy to 
 prove, that they must have a much more power- 
 ful one, in resisting the entrance of persons of 
 that class, through the means of examination. 
 Licentiates made fellows in the former way will 
 naturally adopt the maxims of their patrons, 
 with respect to the management of the corpora- 
 tion ; and even if they should not, they can 
 never be sufficiently numerous to form in it a 
 party of any consequence. On the other hand, 
 licentiates admitted to be fellows of the college, 
 after an examination of their fitness, would be 
 free to act in all its concerns, according to their 
 own views of what was right. They might 
 
 c c 
 
386 LETTER TO 
 
 consequently dispute both the justice and ex- 
 pediency of acknowledging in the graduates of 
 Oxford and Cambridge, any title to be received 
 into the corporation, which does not depend 
 upon their learning and good character ; and 
 their own number might in a few years become 
 so great, as to exceed that of all the other re- 
 sident fellows. Can we now even imagine, 
 that the present fellows of the college, all of 
 them, except five persons who have been ad- 
 mitted through favour, physicians from Oxford 
 and Cambridge, are not generally hostile to a 
 measure, which, if executed, must immediately 
 diminish some of their own advantages, and 
 may hereafter deprive the members of the En- 
 glish universities of the chief rule in a corpora- 
 tion, which has long been regarded by them as 
 their own ? 
 
 I have thus, my Lord, replied, and I hope 
 satisfactorily, to the question concerning the 
 interest, which the college have in acting un- 
 justly towards those licentiates, who may apply 
 to them to be examined for fellowships ; and, 
 while doing this, I have proved by indubitable 
 testimony, that even before the decision of Dr. 
 Stanger's case, they had not always shaped 
 their conduct by the rules of honour and good 
 faith. It may therefore be thought, that my 
 address to your Lordship ought now to close, 
 
LORD KENYON. 387 
 
 since its various objects have been attained. 
 But^ as in my opinion, it deserves to be still 
 further considered, whether an accurate know- 
 ledge or estimation of some preceding acts of 
 the college might not possibly have produced a 
 doubt in your Lordship's mind, on the pro- 
 priety of surrendering to them the sole deter- 
 mination of claims, which they have various 
 and manifest temptations to determine unjustly, 
 I shall venture to trespass a little longer upon 
 your Lordship's patience, by offering a few 
 additional observations upon this part of my 
 subject. 
 
 The first I shall make is derived from a cir- 
 cumstance in the general conduct of the col- 
 lege, of which your Lordship took notice, when 
 you delivered your opinion upon Dr. Stanger's 
 second application. On that occasion your 
 Lordship said : "By what fatality it is, that 
 almost since this charter has been granted, this 
 learned body has somehow or other lived in a 
 course of litigation, I know not ; one is rather 
 surprised, when one considers, that the several 
 members of this body, including the licentiates, 
 the commonalty of this corporation, are very 
 learned men : and as much as it is not generally 
 the fruits of learning, at least not the best fruits 
 of learning, to get into litigation, one cannot 
 tell how those learned gentlemen have fallen 
 
 c c 2 
 
LETTER TO 
 
 into so much litigation." The fact here men- 
 tioned, though highly important, may not to 
 many, however, appear so surprising as it did 
 to your Lordship. Learned occupations, by 
 withholding their followers, for the most part, 
 from the busy paths of life, necessarily exempt 
 them from many occasions of dispute, to which 
 other persons are exposed; but few are more 
 ready, than literary men, to embrace such oc- 
 casions of dispute as are presented to them. 
 In whatever regards the fruits of their mental 
 labours, this is universally acknowledged to be 
 true ; the title of genus irritabik, though more 
 especially given to poets, is found to be ap- 
 plicable, in a greater or less degree, to every 
 description of authors. Some of the malevolent 
 passions, indeed, frequently become in learned 
 men more than ordinarily strong, from want 
 of that restraint upon their excitement which 
 society imposes. Perhaps too, from a well- 
 known law of human nature, their moral feel- 
 ings may be less correct than those of many other 
 men, in consequence of the great and frequent 
 exercise, which is given to the powers of their 
 understandings. Physicians, therefore, as men 
 of learning, have their causes of dissension with 
 each other ; as men seeking wealth by their 
 learning, or affectation of learning, they have 
 many more. The great bulk of mankind being 
 
LORD KENYON. 389 
 
 unable to judge of the truth of their dogmas, 
 or the propriety of their practices, it is very 
 natural, that a number o them should jointly 
 endeavour to persuade their sovereign, that 
 they are the only fit persons to take care of 
 the health of his subjects ; while in truth, the 
 great object of their combination is to establish 
 a monopoly of medical employment in their own 
 favour. This I believe to be the real origin 
 of our college of Physicians, notwithstanding 
 the praises which have been lavished upon its 
 founders. Its charter was granted in the age 
 of monopolies, when men of much higher rank, 
 and greater private respectability than phy- 
 sicians, were eager to obtain them. Some sur- 
 geons procured, about the same time, a mono- 
 poly of their profession in London ; but being 
 less wary than the physicians, or the operations 
 of their art being more subject to the examina- 
 tion of the external senses, they were shortly 
 after declared by an act of Parliament to have 
 abused their trust most grossly. Though the 
 college have not experienced a similar disgrace, 
 the defence of their monopoly has yet involved 
 them in that constant course of litigation, which 
 has so much excited your Lordship's surprise. 
 But had your Lordship advanced a single step 
 further in this subject, it would certainly, I 
 think, have occurred to you, that the members 
 
390 LETTER TO 
 
 of a body, which for nearly three hundred years 
 had been almost constantly engaged in law-suits, 
 were not very fit persons to be entrusted with 
 the power of deciding on the claims of those, 
 whom it was their interest to depress. The 
 frequent appearance of men in our courts of 
 law, whether as plaintiffs or defendants, is not, 
 I believe, generally held such a proof of their 
 virtue, that they are hence thought capable of 
 exertions of self-denial, to which others of a 
 more retired life are acknowledged to be un- 
 equal. 
 
 Possibly another source of doubt, respecting 
 the fitness of the college to execute with fidelity 
 so difficult a trust, without the inspection or 
 controul of some superior power, would have 
 been furnished to your Lordship, by a com- 
 parison of the circumstances, which precede and 
 attend the admission among them of the two 
 descriptions of men, who are entitled to apply 
 for it. A physician of Oxford or Cambridge, 
 who possesses a desire to enter the corporation, 
 has no obstacle to fear to its completion, from 
 any general prejudice against him in the minds 
 of those who are already members. He has, 
 on the contrary, reason to expect, that he will 
 be received by the body at large with pleasure, 
 both because he comes from one of their own 
 universities, and has completed there the course 
 
LORD KENYON. 
 
 391 
 
 of study, which they regard as by far the most 
 proper to form a physician, and because his 
 admission will tend to prevent the necessity of 
 their adopting persons of a different education, 
 to render their number sufficient for the cus- 
 tomary rotation of corporate offices. Nor can 
 any of the members well entertain a personal 
 dislike to him, as he has scarcely yet begun 
 to contend with them for employment. Under 
 these circumstances he applies to the college, 
 at any time he finds convenient, for an examina- 
 tion of his qualifications, which is immediately 
 granted as a matter of course. The examina- 
 tion is delegated to the president and the four 
 censors, who are all chosen to their offices for 
 only a year, and, to use the language of the 
 college, " are strictly sworn to do justice." It 
 is divided by them into three parts, each of 
 which is .held at one of their separate meet- 
 ings*, and their decision upon his fitness is 
 seldom or never formed, until he has been sub- 
 jected to all the parts. Should the decision be 
 in his favour, at the next general meeting of 
 
 * I know that the president and censors may hold the 
 examination, if they please, at the general meetings of the 
 college ; but no instance of their doing so has, I believe, oc- 
 curred for many years^ and if they were to hold it at those 
 meetings, none^ except themselves would have a title to de- 
 termine on the fitness of .the person examined. 
 
392 LETTER TO 
 
 the college he is proposed for admission. A 
 ballot is then taken, and if a majority of the 
 votes be in support of the proposal, he becomes 
 a member of the corporation, with the title of 
 candidate. The whole of these proceedings, in- 
 cluding the original application, are sometimes 
 finished in a week or two, and always in less 
 than three months. After he has been a can- 
 didate for twelve months, without further ex- 
 amination, and almost without further cere- 
 mony, he is received into the order of fellows. 
 If he has come to London shortly after obtain- 
 ing a doctor's degree, his admission into the 
 fellowship almost always takes place, either be- 
 fore or about the thirtieth year of his age. 
 
 I turn now, my Lord, to the licentiate who is 
 engaged in a similar attempt. Though the col- 
 lege, from deference to the authority of Lord 
 Mansfield, have apparently ceased to view an 
 English degree, as an indispensable part of the 
 title of a physician to be examined for a fel- 
 lowship, the prejudices* and interests, which 
 
 * Some notion may be formed of the extent of these pre- 
 judices, from the undermentioned circumstances in the con- 
 duct of Sir Lucas Pepys, as physician general to the army. 
 I possess indeed a still more flagrant example of their in- 
 fluence ; but I prefer the present, as being of a public nature. 
 
 Suspicions having arisen in the beginning of the present 
 war, that the dreadful mortality of our troops in the West 
 
LORD KENYON. 393 
 
 dictated their former laws of admission, still 
 exist with undiminished force. Whenever, 
 
 Indies, had, in part at least, been owing to their want of 
 proper medical aid, it necessarily became an object of great 
 national concern, that the immense armament, which was 
 preparing, in 1795, to be sent to those countries under the 
 command of Sir Ralph Abercrombie, should be provided 
 with able physicians. In this state of things, Dr. William 
 Wright of Edinburgh was mentioned to a person in power, 
 as being well acquainted with the diseases of the West In- 
 dies j in consequence of which, a gentleman, connected with 
 administration, authorised a common friend to make him the 
 offer of being a physician to the armament. Having signified 
 his willingness to accept this appointment, he was desired 
 to remain in Edinburgh, until his services should be required. 
 It is proper to say somewhat here concerning the fitness of 
 Dr. Wright, for the situation to which he was designed. He 
 was a fellow of the college of Physicians of Edinburgh j and had 
 formerly served his Majesty seventeen years, chiefly in the 
 West Indies. He had, besides, practised medicine in Ja- 
 maica, while unconnected with the army, for thirteen years, 
 during great part of which time he was Physician General to 
 the militia of the island. His talents had not, in the mean 
 while, been confined to the cultivation of the practical part 
 of his profession. Having included natural history among 
 the objects of his study, he had, during his residence in 
 Jamaica, explored almost the whole of it, in his attempts to 
 extend the limits of that science, and had in consequence 
 made many important discoveries of plants, some of which 
 had been published in the Philosophical Transactions of 
 London and Edinburgh, and various other works. By these 
 means, he had become well known to many of the learned 
 in different parts of the world, and had been admitted a 
 member of the Royal Societies of London and Edinburgh, 
 
394 LETTER TO 
 
 therefore, a licentiate applies for an examina- 
 tion, a contest in reality arises between the 
 
 and several other bodies of literary men. In short, if private 
 worth, patient industry, diversified knowledge, great general 
 skill in medicine, and long experience of those diseases in 
 particular, which attack Europeans in the West Indies, were 
 qualities to be desired in a physician to his Majesty's forces 
 there, the fitness of Dr. Wright to be one was most eminent. 
 To return to my narrative ; in September Dr. Wright 
 came to London, expecting to receive the promised appoint- 
 ment immediately upon his arrival ; but he was told at the 
 Army Medical Board, that, by a rule of Sir Lucas Pepys, it 
 could not be given to him, unless he hud a licence to practise 
 medicine from the college of Physicians of London. He declared 
 his readiness to submit to the forms necessary for obtaining 
 one ; but these could not be completed before the end of 
 /December ; and the armament it was intended he should 
 accompany was almost upon the point of sailing. Sir Lucas 
 Pepys was therefore strongly urged by several persons to 
 suspend his rule j among others, by two of his own friends, 
 who told him, that Dr. Wright would certainly be appointed, 
 whether he recommended him or not. His answer was, he 
 would never recommend Dr. Wright, and was sure the King 
 would not sign his commission. But it was quickly seen, that 
 he had grossly overrated his consequence. It was indeed 
 not to be supposed, that a rule of a court physician, whose 
 connexion with the army had commenced only a year or 
 two before, by his being placed at once at the head of its 
 medical department, would long prevent the execution of u 
 measure, deemed by the ablest judges highly beneficial to 
 the military service of our country. In October, by the in- 
 fluence chiefly of Sir Ralph Abercrombie, Dr. Wright was 
 appointed a physician to the armament, and shortly after 
 went with it to the West Indies. 
 
LORD KENYON. 395 
 
 graduates of Oxford and Cambridge, and those 
 of the Scotch and foreign universities. But 
 
 The only possible ground, upon which Sir Lucas Pepys 
 could consistently with his duty to the public have formed 
 his rule, appears to be, that he regarded an examination of 
 medical ability by men whom he knew, and upon whose 
 report he could therefore implicitly rely, as a necessary test 
 of the fitness of those, who were to be entrusted with the 
 important charge of watching over the health of his Ma- 
 jesty's troops. But if this be supposed the principle. of his 
 rule, what must be said of his recommending, notwithstand- 
 ing, several persons to be physicians to the army, who had 
 never undergone such an examination ? Perhaps they were 
 evidently so superior in ability to Dr. Wright, as to justify 
 even a breach of principle in their favour : No j they were 
 young men, who had not yet completed their academical, 
 education, and who probably had never had the entire ma- 
 nagement of a dangerous disease committed to their care. 
 They were, however, Bachelors of Physic from Cambridge. 
 
 The degree of Bachelor of Physic is now given at Oxford, 
 the eighth year after matriculation j about thirty years ago 
 it was not given till the tenth, but even then, so little know- 
 ledge of medicine was thought requisite for it, that he who 
 received it was only said to be admitted, to read the aphorisms 
 of Hippocrates. At Cambridge, the same degree may be 
 obtained as soon as the fifth year after entrance is com- 
 pleted. The candidate first keeps an act ; which consists in 
 defending two questions, one chosen by himself, the other 
 by the professor of medicine; but the latter is given when 
 asked for, however long this may be before the defence is to 
 be made. The statutes of the university require also, that 
 the candidate should oppose another candidate for a degree in 
 Physic; but this is now dispensed with for twenty shillings. 
 
396 LETTER TO 
 
 who are appointed to decide it? graduates of 
 Oxford and Cambridge. The members of the 
 
 These ceremonies then have not the least resemblance to an 
 examination', and no person, I believe., is ever rejected at 
 them for want of medical learning. It is on the contrary, 
 well known, that students at Cambridge, to save time, often 
 take the degree of Bachelor of Medicine, when they have 
 scarcely entered upon the study of their intended profession, 
 meaning no doubt to apply to it with great diligence, during 
 ihcjive years which must afterwards pass away, before they 
 can receive a doctor's degree. Yet, in the sight of Sir Lucas 
 Pepys, a Cambridge bachelor of Physic appears fit, without 
 further trial, to be a physician to his Majesty's forces in the 
 West Indies, while a man, so gifted and adorned as Dr. 
 Wright, appears unfit, and is therefore sent by him to be 
 examined by the college of Physicians of London ! Such are 
 the grounds upon which the physicians of Scotch and foreign 
 universities must build their expectations of justice from the 
 college, when they apply for admission into the fellow- 
 ship. If it be said, that no conclusion from the conduct of 
 an individual ought to be applied to the whole body; my 
 answer is, that the conduct of that individual must, in its 
 principle at least, be approved by the body at large, since he 
 is marked by their opinion to succeed Dr. Gisborne, in the 
 presidency of the corporation. 
 
 It may be gratifying to many to know, that by his Ma- 
 jesty's command, orders were last year issued from the War- 
 Office, to regulate, in future, the appointment of physicians 
 to the army j and that, in consequence, it is now no longer 
 necessary that*- they have licences from the London college, 
 or degrees from the English universities. Those, who 
 formerly nominated physicians to the land forces, were 
 allowed to form their own rules, and a like indulgence was 
 
LORD KENYON. 397 
 
 college being thus both parties and judges in 
 the cause, it will doubtless be thought, that 
 from respect to their own characters, they have 
 attempted by every means in their power to 
 lessen the invidiousness, and even danger of 
 their situation. Have they truly done so ? No, 
 no, my Lord. They have, on the contrary, 
 invented a mode of trial, which places their 
 adversaries in the most difficult and humiliating 
 circumstances, and lays themselves open to the 
 influence of some of the basest passions of the 
 human mind. 
 
 In the first place, before a licentiate is ad- 
 missible to the examination he desires, it is 
 demanded by the college that he be of seven 
 years standing, and upwards of thirty-six years 
 of age. But a rivalship for seven years with 
 his judges, for employment, may have excited 
 considerable animosity against him in the minds 
 
 for some years enjoyed by Sir Lucas Pepys. When this was 
 taken away; some persons thought, that after such a dis- 
 grace, as they termed it, he would feel himself obliged as a 
 man of spirit, to resign his office, as he could in no other 
 way demonstrate the purity, if not the wisdom, of his inten- 
 tions in framing the rules which had been annulled. Fortu- 
 nately, however, he has been influenced by no such extrava- 
 gant notions of personal dignity ; but from unbounded zeal 
 for his sovereign's glory, and a most tender regard for the 
 welfare of our gallant soldiers, in eoery part of the world, 
 still remains Physician General to the army. 
 
398 LETTER TO 
 
 of some of them ; and the disgrace of being 
 rejected at an examination must prove highly 
 injurious, not only to the reputation, but to the 
 fortune also of a physician, who has passed his 
 thirty-sixth year. Such a disgrace may even 
 more readily befal him than a younger man. 
 For many things which he formerly learned, 
 and the knowledge of which is required at the 
 college examinations, are now unknown to him, 
 from never having experienced their use in the 
 exercise of his profession ; and his present oc- 
 cupations may afford little leisure for regaining 
 them. 
 
 But secondly, the application for his examina- 
 tion can be made upon only one day in the year, 
 and it must not even then come directly from 
 himself; he must find some fellow of the col- 
 lege to make it for him. As the number of 
 resident fellows, however, is under thirty, it 
 may surely happen, that they shall all agree to 
 regard it as a point of honour not to propose 
 a licentiate for examination. 
 
 Let it now be granted, that a fellow has pro- 
 posed him ; in this case your Lordship, during 
 the trial of Dr. Stanger's cause, seemed to think, 
 from your acquaintance with the pure and ho- 
 nqurable conduct of the benchers of the inns of 
 court in similar situations, that admission into 
 the college must follow of course. But, in 
 
LORD KENYON. 399 
 
 truth, he has only gained a title to have a vote 
 taken by the secret method of ballot at the 
 present meeting of the corporation, whether 
 his qualifications for a fellowship shall hereafter 
 be examined. If a bare majority be against his 
 being examined, the proceedings are stopped, 
 and cannot be begun again for a twelvemonth. 
 I need not, however, point out to your Lord- 
 ship, how much more likely it is, that a ma- 
 jority of votes, secretly taken, should appear 
 against a licentiate before an examination, than 
 that an English graduate should be rejected by 
 a similar mode of voting, after he has been exa- 
 mined and approved by the president and cen- 
 sors, this being the only time at which tiie latter 
 is subjected to a general ballot, before admission 
 into the college. 
 
 The examination, which may have been al- 
 lowed to the licentiate in consequence of the 
 ballot, is of the same kind as that which an 
 English graduate undergoes; but the first part 
 of it is not held till three months after the 
 grant, and the same space of time is interposed 
 between its first and second parts, and between 
 the second and third. In this way, if he is not 
 in the mean time rejected, he is to be tortured 
 for nine months with doubt and anxiety respect- 
 ing its event. All its parts too are held, not at 
 the private meetings of the president and cen- 
 sors, as in the case of an English graduate, but 
 
400 LETTER TO 
 
 at the public meetings of the corporation ; and 
 should he, from natural timidity, or from that 
 embarrassment which every man must feel, 
 upon personally submitting his talents to the 
 scrutiny of those, whom he believes to be un- 
 friendly to his views, appear ignorant of any of 
 the subjects proposed, no opportunity is allowed 
 to him, as to an English graduate, of compen- 
 sating such a seeming deficiency by any after- 
 exhibition of knowledge. For the majority of 
 a general meeting must declare their approba- 
 tion of the first part of his examination, before 
 he can be admitted to the second ; and of the 
 second, before he can be admitted to the third. 
 If every part of his examination has been ap- 
 proved, and he has thus obtained four majorities 
 of general meetings of the corporation in his 
 favour, all of them declared by ballot, three 
 months afterwards, that is, twelve months after 
 being proposed for examination, he may be 
 proposed at another general meeting for admis- 
 sion, and if the majority is found by a fifth, 
 ballot to consent, he is then to be received into 
 the college as a fellow. 
 
 These conditions of a licentiate's entry into 
 the college are contained, I confess, in a by- 
 law, which your Lordship pronounced to be, 
 not only free from blemish, but possessed of 
 such virtue, as to render sound an older by- 
 law, emphatically declared by you to have had 
 
LORD KENYON. 401 
 
 in it the seed of death, before it received this new 
 infusion of health. I am much inclined, how- 
 ever, by what has been already mentioned, to 
 suppose, that your Lordship's opinion was de- 
 rived from a very cursory view of the subject 
 to which it relates, and I embrace this conclu- 
 sion more strqngly, when I consider a further 
 point of difference between the by-law in ques- 
 tion, and that for the admission of physicians 
 from Oxford and Cambridge, the simplest notice 
 of which must excite disgust and indignation in 
 every bosom, the least animated by a love of 
 justice. 
 
 The persons, who decide on the examination 
 of an English graduate, are those to whom it 
 is committed, the president and censors. The 
 examination of a licentiate is also committed to 
 the president and censors, but not its decision. 
 When this is given, they vote as individuals 
 only, in a meeting consisting frequently, I be- 
 lieve commonly, of more than twenty members, 
 none of whom, except themselves, are under 
 any other than the ordinary obligations of men 
 to good conduct, or are even required to be 
 present at the examination, whose event they 
 are to determine. But if these obligations have 
 been esteemed insufficient to ensure justice from 
 English graduates to one of their own class, 
 and it has therefore been thought necessary to 
 
 D D 
 
402 LETTER TO 
 
 delegate the decision upon his merits to five 
 persons, who are solemnly sworn to the faithful 
 discharge of their duty, what notion are we to 
 entertain of the design of the college in com- 
 mitting the decision upon the merits of a licen- 
 tiate to the discretion of a general meeting? We 
 are taught, my Lord, by the slightest experience 
 in the affairs of the world, to seek for the mo- 
 tives of men in their actions, when these are at 
 variance with their words. No credit was ever 
 given by the Romans to the declarations of 
 clemency, with which Domitian used to preface 
 his cruelties, or by ourselves to the robbers and 
 murderers of France, when they pretended, 
 that their conduct towards foreign nations arose 
 from a disinterested desire to give liberty and 
 happiness to mankind. When, therefore, I ob- 
 serve, that the college of Physicians have per- 
 mitted themselves to decide upon the examina- 
 tions of licentiates, without the restraint of an 
 oath, at the same time that they strictly swear 
 those to do justice, who are to decide upon the 
 examinations of the graduates of Oxford and 
 Cambridge, I hold myself fully authorised to 
 infer, notwithstanding any protestation to the 
 contrary, that their design in establishing this 
 difference was, to allow room in the former set 
 pf examinations, if any such should ever take 
 place, for the operation of principles, the most 
 
LORD KENYON. 403 
 
 remote that can be conceived from honour and 
 good faith. 
 
 It will perhaps be expected, that I should 
 illustrate what I have said upon the theory of 
 this by-law, by an appeal to the facts which 
 have relation to it. But scarcely any such 
 exist. During the nineteen years which inter- 
 vened between the framing of the law, and the 
 decision of the Court of King's Bench in the 
 case of Dr. Stanger, the licentiates had been so 
 intimidated both by its intrinsic difficulties, and 
 by the threats of fellows of the college, that no 
 person who applied under it should ever obtain 
 what he desired, that only one of them, Dr. 
 James Sims, had endeavoured to profit by it. 
 He was regularly proposed for examination by 
 Dr. Burges, whose motion, however, the college 
 refused even to consider, on the ground that no 
 one had seconded it. With what justice or de- 
 cency this was done, I learn from your Lord- 
 ship. " He is not to wait to be seconded, 5 * 
 your Lordship said, in Dr. Stanger's case, while 
 speaking of a licentiate in the situation of Dr. 
 Sims, " the by-law does not require that." 
 These circumstances respecting Dr. Sims were 
 mentioned to the court by Mr. Christian, one 
 of Dr. Stanger's counsel, but, I suppose, in a 
 manner too unimpressive to fix them in your 
 Lordship's mind. For had they been present 
 
 D D 2 
 
404 LETTER TO 
 
 to it, when your decision was given, you would 
 necessarily have entertained some suspicion, 
 that they, who had openly violated one part of 
 a law, were not to be restrained by honour and 
 good faith from violating any other part of it, 
 when their conduct should be screened by a 
 ballot. 
 
 The last act of the college, to which I shall 
 solicit your Lordship's attention, seems alone 
 sufficient to have demonstrated their total un- 
 fitness to decide between themselves and other 
 men, when the only guard against their doing 
 wrong should consist in their feelings o'f what 
 is right, Some of the circumstances, indeed, 
 which I am going to relate, occurred in your 
 Lordship's presence, in the course of Dr. 
 Stanger's cause ; and I am not ignorant, that 
 you then considered them as unconnected with 
 any serious intention, on the part of the college. 
 Admitting, however, for a moment, this to have 
 been the case, surely the system of morality, 
 which permits its followers to accuse a gentle- 
 man, by way of joke, of a most disgraceful crime 
 before the Lord Chief Justice of England, ought 
 to have no place in the sanctuary of honour and 
 good faith. But not to dwell longer upon this 
 argument, I shall, I think, soon convince your 
 Lordship, that the charge to which I have 
 alluded was deliberately formed, and seriously 
 
LORD KENYON. 405 
 
 urged by the college, with the horrible design 
 of destroying the character of an innocent per- 
 son, because he was bold enough to oppose their 
 injustice. 
 
 When a physician is admitted by the college 
 into the class of licentiates, he gives his promise 
 or faith, that he will observe their statutes, or 
 readily pay the fines which shall be imposed 
 upon him for disobedience*. Sir William 
 Blackstone, who, I believe, is not generally 
 reckoned a loose moralist, holds it established, 
 that, when a penalty is annexed to the non- 
 compliance with laws, " which enjoin only 
 positive duties, and forbid only such things as 
 are not mala in se, but mala prohibita merely, 
 without any intermixture of moral guilt the 
 alternative is offered to every man, * either 
 abstain from this, or submit to such a penalty ;' 
 and [that] his conscience will be clear, which- 
 ever side of the alternative he thinks proper to 
 embrace." Possibly some doubt may be enter- 
 tained of the justness of this doctrine when 
 applied to laws, which affect all persons equally, 
 and are made by those who are to be controlled 
 
 * The president says to him dabis fidem, te observaturum 
 statuta collegii, aut mXiltas tibi contri facienti irrogandas 
 prompte persoluturum, omniaque in arte medica pro viribus 
 facturum in honorem collegii, et reipublicse utilitatem to 
 which he assents. 
 
406 LETTER TO 
 
 by them. But, however this maybe, it is at 
 least certain, that no doubt can exist, whether 
 a licentiate is entitled to take either side he 
 pleases of the alternative, which is offered to 
 him by the college themselves, not by implica- 
 tion, but by the most direct and explicit expres- 
 sion, with respect to the observance of statutes, 
 made always without his consent, and sometimes 
 tvith the avowed design of placing him beneath 
 men, whom the laws of their common country 
 declare to be no more than his equals. He will 
 even merit no blame from them, as lawgivers, 
 by disobeying such of their statutes as forbid 
 what is evil in itself, provided he immediately 
 pays the fines which are demanded from him. 
 iThe blame, which he here incurs, depends upon 
 his having broken the laws of some far higher 
 power, those of God or his country. But I fear 
 I render this subject confused, by holding it up 
 too long to view. Luminous objects are best 
 discerned by a single glance of the eye ; if we 
 suffer our sight to dwell upon them, their very 
 brightness soon causes them to appear indistinct. 
 The degree of obedience, which is due by a 
 licentiate to the laws of the college, being then 
 so evident, no one can imagine, that it was ever 
 unknown to the many learned and well-informed 
 men, who are members of that body. The in- 
 tention, therefore, of those men, in acting even 
 
LORD KEN YON. 407 
 
 for the shortest time, as if it were unknown to 
 them, could not have been honourable ; but as 
 they persisted in this conduct for nearly three 
 years, they must necessarily have been serious. 
 Shortness of duration is essential to every kind 
 of joke, whether verbal or practical. 
 
 About the middle of 1794, a rumour became 
 prevalent among medical men in London, that 
 the college viewed, as a breach of faith to them, 
 the attempt of certain licentiates to render the 
 corporate distinctions of their profession ac- 
 cessible to every physician of sound morals and 
 learning ; but it was thought by those licentiates 
 too absurd to be credited. " We know," said 
 they, " of no statute of the college, by which 
 we are forbidden to endeavour to gain admission 
 into it. If there be any such, let it be pointed 
 out, and let the fine be demanded, which is an- 
 nexed to our disobedience. Were indeed such 
 a statute to exist, it would be not only tyran- 
 nical, but contrary to the laws of our country, 
 and therefore without force. At all events, to 
 desire the removal of a grievance can never be 
 justly held a breach of our promise to the col- 
 lege. For to what purpose has the Court of 
 King's Bench been charged with the inspection 
 and controul of corporations, if applications to 
 it against the oppression of by-laws can, by 
 other by-laws, be legally declared violations of 
 
408 LETTER TO 
 
 faith in those who seek for relief ?" But they 
 soon discovered their mistake in supposing that 
 the rumour must be false, because it seemed to 
 them absurd ; for in October of the same year, 
 the accusation which it contained was publicly 
 brought against them by Dr. John Latham, one 
 of the fellows of the college. " We are at- 
 tacked*/ 5 said Dr. Latham in his Harvei'an 
 oration, " by ferocious, daring, and obstinate 
 enemies, regardless of the faith which they have 
 
 pledged for the observance of our statutes. 
 
 I might complain at greater length of the injury 
 which they have rashly done us, but liberality 
 forbids me to say more/' 
 
 Flagitious conduct, my Lord, ought, in my 
 poor opinion, never to pass uncalled by its 
 proper name. If vice be not termed vice, if 
 baseness and dishonour be suffered to come 
 forth into the world, without the mark of in- 
 famy, we remove one of the most powerful 
 checks upon the evil inclinations of man, and 
 
 * " Hostis aggreditur, ferox, audax, pertinax, posthabita 
 fide de observandis [collegii] statutis. Verum enimvero 
 tametsi mihi esset occasio querendi prolixius de facta nobis 
 temere injuria, vetat amplius disserere liberalitas." These 
 quotations are from the printed copy. The author of this 
 letter did not hear Dr. Latham deliver his oration, but from 
 the reports of others he has reason to believe, that the whole 
 of the abuse, which was then thrown upon the associated 
 licentiates, has not been printed. 
 
LORD KENYON. 409 
 
 indirectly discourage the practice of virtue. 
 If, therefore, the titles of reproach used by Dr. 
 Latham had been merited, it would have been 
 gallant, it would have been praiseworthy in 
 him to have bestowed them. But to whom 
 were they applied ? To fourteen persons of his 
 own profession, all of whom, except one, were 
 at least equal to himself in every quality and 
 accomplishment, which physicians are required 
 to possess. And upon what occasion ? Because 
 these men had, in a temperate, and even re- 
 spectful address to the college, set forth their 
 claims to admission into the fellowship, and 
 had requested to know, whether they would be 
 allowed to prove their fitness for what they de- 
 sired, by undergoing the examinations which 
 are prescribed for the graduates of Oxford and 
 Cambridge. This was the only measure they 
 had hitherto taken for obtaining their object. 
 Your Lordship will now assuredly conceive, 
 that such expressions were heard with disgust 
 by the other members of the college. I firmly 
 believe, my Lord, that they were heard with 
 great disgust by some of its members. But the 
 body at large hastened to adopt them, by so- 
 liciting their author to print his oration. Happy, 
 however, would it have been for Dr. Latham, 
 if their zeal to injure the moral characters of 
 those, whom they denominated their enemies, had 
 
410 LETTER TO 
 
 not blinded them to the danger, to which they 
 were about to expose the literary reputation of 
 one of their dearest friends ; if they had not by 
 their own praises so fanned his desire for general 
 applause, as to occasion his giving a work to the 
 world, which sets at defiance every principle of 
 taste in composition, and exhibits more than a 
 schoolboy's ignorance of the common language 
 of the learned. 
 
 The next public indication of the plan of the 
 college to defame the associated licentiates, (for 
 I purposely avoid mentioning any private proof 
 of it) was furnished in April 1796, by Sir George 
 Baker's swearing before your Lordship, that 
 Dr. Stanger, upon being made a licentiate, had 
 given his faith, or promise, that he would obey 
 their statutes. It now became clearly evident 
 to those, who had watched the conduct of the 
 college, that they meant to urge this, among 
 other arguments against the claim of that gen- 
 tleman, that he was unworthy of admission into 
 their body, from having, by his present applica- 
 tion to the court, forfeited all title to confidence 
 in his future declarations. No notice, indeed, 
 was taken of this part of Sir George Baker's 
 affidavit, in the pleadings which immediately 
 followed ; but Mr. Erskine was the only one of 
 their counsel, who completed his speech upon 
 that occasion, and there are strong grounds for 
 
LORD KENYON. 411 
 
 concluding, (with which, however, I shall not 
 trouble your Lordship,) that his omitting to 
 bring it forward was highly disagreeable to his 
 employers. 
 
 In January, 1797, the circumstance of Dr. 
 Stanger' s having given his faith to observe the 
 statutes of the college was a second time sworn 
 to by their president, and in the trial which 
 took place in May, Mr. Erskine did not again 
 neglect to touch upon it. But the whole of this 
 part of his speech seemed to denote a struggle 
 between the ingenuous feelings of a gentleman, 
 and the desire of an advocate to gratify his 
 clients. " I do not mean to say any thing 
 offensive to Dr. Stanger ; he will understand 
 that I am using the words of Lord Mansfield. 
 I have done justice to this gentleman, who, 
 I have no doubt, is a learned man, and a person 
 of honour and character in his profession." 
 These were expressions employed by Mr. 
 Erskine, while speaking of the engagement 
 under consideration. But as the only possible 
 view of the college, in producing it to the 
 court, must have been to pretend that it had 
 been violated, to call Dr. Stanger " a person of 
 honour" was directly in opposition to their de- 
 sign, and plainly demonstrated the aversion of 
 their principal advocate to lend his aid towards 
 its completion. 
 
412 LEWER TO 
 
 The two advocates of the college, who spoke 
 next, were silent upon the subject of Dr. Stan- 
 ger's engagement. But their deficiency on this 
 point was fully supplied by the youngest counsel, 
 Mr. Warren. He was the son of one of his em- 
 ployers, and consequently possessed the most 
 ample opportunities of being acquainted with 
 their real motives and views, and as he had 
 evidently been retained in the present cause, 
 for reasons unconnected with his general fame, 
 he must have been strongly disposed to requite 
 the favour he had received, by doing what he 
 knew would be most agreeable to them. He 
 therefore did not inform the court, as Mr. 
 Erskine had done, that he was not instructed 
 to make any insinuation against the character 
 of Dr. Stanger, but boldly and explicitly 
 charged that gentleman, with " a violation of 
 something, less formal, but not less sacred, than 
 an oath." The court now exerted their au- 
 thority, and prevented his proceeding further 
 in this strain*. But, my Lord, had the dagger, 
 
 * My authority for saying, that Mr. Warren was in- 
 terrupted in this part of his speech, is the following con- 
 versation between Lord Kenyon and Mr. Christian, one of 
 Dr. Stanger's counsel, which took place two days after, 
 while the latter was replying to the arguments against the 
 issuing of the mandamus. 
 
 Mr. Christian. " An argument was pressed the other day 
 
LORD KEN YON. 413 
 
 which he drew from beneath a robe, intended 
 to give dignity to the assertor of innocence and 
 right, been even suffered to reach its destined 
 object with all the force that his arm could im- 
 part, it would have still struck harmless upon 
 the armour of honourable reputation, to the 
 
 which I was sorry to hear, because it might wound the feel- 
 ings of a very honourable mind ; it was said that Dr. Stanger 
 had pledged his faith to observe the statutes." 
 
 Lord Kenyan. " That was put an end to immediately as 
 it was mentioned." 
 
 Mr. Christian. " It seemed to be pressed and relied upon 
 as a serious argument." 
 
 Lord Kenyan. " Certainly not." 
 
 I must, however, confess, that I see no mark of any such 
 interruption, in Mr. Gurney's report of Mr. Warren's speech. 
 I presume, therefore, that the Court's disapprobation of the 
 shameful attack upon Dr. Stanger's character must have 
 been expressed by some gesture or look from the Bench, 
 which, though sufficiently intelligible to Mr. Warren, might 
 easily pass unobserved by a writer intent upon his papers. 
 How far his Lordship himself thought the honour of that 
 physician affected by his application to the court, may be 
 known from the following passage in his speech at the close 
 of the trial. " It is fit that I should put the mind of Dr. 
 Stanger, in case it is in an uneasy situation, in a perfect 
 state of repose with regard to one thing. Undoubtedly his 
 moral character is not at all tainted by the application that 
 is now made. I have not the honour of knowing him ; I 
 have heard nothing but to his advantage when I have heard 
 him spoken of, and I dare say all the eulogy, which his 
 warmest friends could bestow upon him, his character both 
 as a moral and professional man deserves." 
 
414 LETTER TO 
 
 confusion of every hope conceived by the cold- 
 blooded, corporate cruelty, which had urged 
 him to the deed. 
 
 I cannot forbear making one observation 
 more upon this atrocious attempt of the college. 
 Though a licentiate is obliged to give his faith, 
 that he will observe their statutes, he is never 
 furnished with any opportunity of learning what 
 they are. The last printed edition of them is 
 dated in 1765, and is now so scarce, that many, 
 I believe I may justly say most, of the licen- 
 tiates have never seen a copy of it. The code 
 too, since 1765, has undergone very consider- 
 able alterations, none of which, as far as I 
 know, have ever been communicated to the 
 licentiates. In 1796, Dr. Stanger swore before 
 the Court of King's Bench, that to the best of 
 his knowledge and belief, no person could be 
 admitted into the order of candidates, who did 
 not enjoy, by birth, all the privileges of a 
 British subject ; and yet it was afterwards de- 
 clared by the counsel of the college, that the 
 statute requiring this condition had been re- 
 pealed upwards of twenty years. Dr. Stanger 
 swore also, that he had shortly before applied 
 to the president and register of the college, for 
 some information respecting their laws, but that 
 both those officers had refused to give it to 
 him. Caligula, among other acts of tyranny, 
 
LORD KENYON. 415 
 
 caused several of his edicts to be written in 
 very small letters, and afterwards fixed in situa- 
 tions of difficult access, in order that those 
 who were to be affected by them might offend 
 through ignorance. His ultimate object, how- 
 ever, was only to procure the pecuniary fines 
 which were imposed upon the want of obe- 
 dience ; when these were obtained, he readily 
 acquitted the transgressors of all further blame* 
 Men calling themselves Britons likewise con- 
 ceal their laws, but, with a refinement in cruelty 
 beyond the conception even of a Roman tyrant, 
 declare persons to be infamous, who do not ob- 
 serve them. 
 
 I have now, my Lord, finished my journey 
 through the dreary waste, which I undertook to 
 explore. In my progress, no spot of verdure 
 has been found, upon which the weaned eye 
 might repose, and scarcely an object of terror 
 has occurred, to break the flat uniformity of 
 the scene, one wide expanse of pitiful fraud, 
 and paltry chicane. My labour has been in- 
 glorious ; but should it furnish your Lordship 
 with a more accurate knowledge of the ground 
 I have passed over, than that which you for- 
 merly possessed, I shall esteem it most amply 
 repaid. 
 
 That the conduct which I have described 
 
416 LETTER TO 
 
 should have been exhibited by men, many, per- 
 haps all, of whom discharge with propriety the 
 duties of their private stations in society, is 
 one of those facts relative to the human cha- 
 racter, which, however difficult to be explained, 
 are still unquestionably true. There is a certain 
 gallantry in doing a wrong thing for the sake 
 of another, which in some degree lessens the 
 deformity of the action. The odiousness of 
 such an action is still further diminished, should 
 it tend to the benefit of many. If it promises 
 to promote the interests or happiness of a whole 
 nation, its name, if not its nature, is often 
 changed ; and what in private life would have 
 been denominated vicious, may now be re- 
 garded not only as pardonable, but even as me- 
 ritorious. Besides-, the members of corpora- 
 tions commonly imagine, that they have a right 
 to do every thing which has been done by their 
 predecessors, notwithstanding the circumstances 
 may have long ceased to exist, under which 
 their ancient rules were established. Again ; 
 the actions of most persons, when they are not 
 under the dread of general laws, seem to be 
 chiefly regulated by the praise and blame of 
 those by whom they are immediately sur- 
 rounded. The peasantry upon our coasts, who 
 in the ordinary situations of life do not appear 
 
LORD KENYON. 417 
 
 to be more depraved than other men, have 
 often been known to commit, in bodies, the 
 most detestable' cruelties upon shipwrecked ma- 
 riners ; and the vilest malefactors often meet 
 death at the. gallows with the greatest firmness, 
 if strengthened by the presence and approba- 
 tion of their former companions. If to such 
 considerations we add, that no one is personally 
 answerable for the acts of a corporation, and 
 that these often proceed from a bare majority, 
 or a number even less than a majority of its 
 members, we may possibly obtain from the 
 whole an explanation, why the public conduct 
 of the college of Physicians is frequently so 
 very different from what any one might expect, 
 who has looked only to the private characters 
 of some of those .who compose it. But what- 
 ever opinion may be formed concerning the 
 grounds of explanation which I have offered, 
 the fact, to which they are meant to apply, 
 still rests upon the basis of testimony, and is 
 laterally supported by innumerable other facts 
 of the same kind. "All men," said an author, 
 whose wisdom and eloquence have produced a 
 change in the state of human affairs scarcely 
 inferior to any, that has ever been effected by 
 the arms of a conqueror, but who most un- 
 fortunately does not live to witness the grati- 
 tude of the world, for his noble, energetic, and 
 
 E E 
 
418 LETTER TO 
 
 invigorating exhortations to resistance against 
 its common and most dangerous enemy, when 
 almost every one was benumbed by despair, 
 and sought only to prolong a miserable exist- 
 ence by base submission ; " all men," said Mr. 
 Burke, " possessed of an uncontrolled discre- 
 tionary power, leading to the aggrandizement 
 and profit of their own body, have always 
 abused it ; and I see no particular sanctity 
 in our own times, that is at all likely, by a 
 miraculous operation, to over-rule the course 
 of nature." I have thought proper to add thus 
 much, to free myself from the suspicion of being 
 actuated, in what I have written, by private re- 
 sentments against individual members of the 
 college. If such feelings had ever been pro- 
 duced in me, it would have become my duty, 
 and I trust I should have had strength to per- 
 form it, either to stifle them as unworthy of 
 life, or to make known their existence, in a 
 more direct way than the present, to those who 
 had given them birth. 
 
 A more difficult task, my Lord, remains for 
 me to perform that of again apologising to 
 you for this letter. When I began it, my only 
 view was to acquaint your Lordship with the 
 event of an application to the college of Phy- 
 sicians, which had been occasioned by your 
 advice. But, after I had proceeded some way 
 
LORD KENYON. 419 
 
 in accomplishing this design, I thought it might 
 be both curious and useful to show, that their 
 rejection of the application was not incon- 
 sistent, either with the principles which it might 
 have been supposed would influence a body of 
 physicians in their situation, or with the actual 
 tenour of their ^conduct, prior to the decision 
 of the Court of King's Bench in the case of Dr. 
 Stanger. I saw, indeed, that such an attempt 
 would be an indirect attack upon the propriety 
 of that decision, not as connected with the in- 
 tentions of those who gave it - 9 the honour, and 
 integrity, and uprightness of English judges, like 
 axioms in science, are always beyond doubt j 
 but as far as it was founded in considerations, 
 the strength, or weakness of which many per- 
 sons had better opportunities of knowing than 
 your Lordship or brethren. I imagined, there- 
 fore, that in making the attempt, I should only 
 act similarly to one, who applies to a court of 
 justice for a new trial of his cause, in 'con- 
 sequence of obtaining new evidence to support 
 it, or who appeals from the jurisdiction of one 
 court to that of another; and hence I con- 
 cluded with some confidence, that the plan of 
 my letter would be regarded by your Lordship 
 as blameless. But now that it is finished, I 
 greatly fear, that the execution will not be 
 
420 LETTER TO 
 
 esteemed altogether so ; that, on the contrary, 
 the liberties of expression in which I have 
 sometimes indulged may appear to your Lord- 
 ship, if indeed you should ever bestow a mo- 
 ment's thought upon the subject, as not a little 
 reprehensible. 
 
 The plainness and freedom of speech, my 
 Lord, which so remarkably distinguish English- 
 men, have always seemed to me, not only to 
 be essentially connected with the existence of 
 their thrice happy and unparalleled form of go- 
 vernment, but even to give rise, in great mea- 
 sure, to some of their characteristic virtues; 
 among others, to their humanity. I mean not 
 the humanity which is dictated by policy, or 
 that which originates in a morbid sensibility in- 
 capable of bearing the sight of distress ; but 
 the humanity which is so firmly ingrafted upon 
 the wild stock of our populace, that the greatest 
 storms cannot tear it away; the humanity which 
 withholds our mobs, in their most guilty ex- 
 cesses, and while maddened by strong liquors, 
 from the spilling of blood. Hatred and re- 
 venge spring up in concealment, and must be 
 nourished by long and painful meditation upon 
 injuries received, before they can attain any 
 vigour. But Englishmen, by loudly and fear- 
 lessly declaring their wrongs as soon as they 
 
LORD KENYON. 421 
 
 feel themselves aggrieved, prevent the very 
 beginnings of those baleful passions, and thus 
 preserve their hearts always in a condition to 
 obey the great command of their Maker, to 
 venerate his image in man. Our climate, my 
 Lord, may be rude and boisterous, but still it 
 is free from the hurricanes, which desolate 
 countries possessing skies, for the most part, 
 calm and serene. Under the influence of these 
 opinions, I have long been accustomed to give 
 free expression to my sentiments upon the con- 
 duct of other men, and experience of the benefit 
 hence derived to the health of my mind has 
 contributed to establish the practice. If, there- 
 fore, I should be regarded by your Lordship 
 as having employed too great liberty of speech 
 in this address, I humbly request that you will 
 ascribe my fault, either to error of principle, 
 or inveteracy of habit, but in no degree to any 
 deficiency of respect for your high station and 
 character. 
 
 I retire at length, my Lord, from your pre- 
 sence, and at the same time relinquish my 
 struggle with the college of Physicians. I con- 
 sider myself now as a veteran in the contest, 
 and therefore as entitled to repose ; 
 
 Spectatum satis, et jam donatum rude. 
 
 To those, however, who still combat on the 
 
ACCOUNT 
 
 OF A FEMALE 
 
 OF THE WHITE RACE OF MANKIND, 
 
 PART OF WHOSE SKIN 
 
 RESEMBLES THAT OF A NEGRO, &c. 
 
 INSTANCES of the absence of the black co- 
 lour, in the whole or part of the skin in persons 
 of the negro race, are not very uncommon ; bui 
 there is, I believe, no one upon record of an 
 individual of the white race having any part of 
 the body, covered with a skin similar to that of 
 a negro. The following account, therefore, of 
 such an instance, will, perhaps, be acceptable 
 to the philosophical public. I have been ena- 
 bled to form it by the permission of Dr. Turner, 
 one of my colleagues at St. Thomas's Hospital, 
 into which the person, whose case I am to de- 
 scribe, was lately admitted by him, on account 
 of some bodily ailment. 
 
426 AN ACCOUNT OF, &c. 
 
 Hannah West, the subject of this account, 
 was born in a village in Sussex, about three 
 miles distant from the sea, and is now in the 
 twenty-thirfl year of her age. Both of her 
 parents were natives of the same county. Her 
 father was a footman in a gentleman's family, 
 and died while she was very young. She can- 
 not, therefore, remember his appearance ; but 
 she has never heard, that it was in any way ex- 
 traordinary. Her mother is still alive, and has 
 black hair and hazel eyes, but a fair skin, with- 
 out any stain or mark upon it. West was the 
 only child of her father ; but her mother, having 
 married a second time, has had eleven other 
 children. Nine of these are living, all of whom 
 are without any blackness of the skin. Her 
 mother, she says, received a fright, while preg- 
 nant with her, by accidentally treading on a live 
 lobster ; and to this was attributed the blackness 
 of part of her skin, which was observed at her 
 birth. 
 
 West is somewhat above the middle size, is 
 rather of a full habit, and till she came to Lon- 
 don from Sussex, which was about four months 
 ago, always enjoyed very good health. The 
 hair of her head is of a light brown colour, and 
 is very soft ; her eyes of a faint blue ; her nose 
 prominent and a little aquiline ; her lips thin ; 
 the skin of the greater portion of the uncovered 
 
AN ACCOUNT OF, &c. 427 
 
 parts of her body very white ; in short, her ap- 
 pearance is in every respect, except the one 
 which has been mentioned, that of a very fair 
 female of the white race of mankind. 
 
 The parts covered by the black skin are, the 
 left shoulder, arm, fore-arm, and hand. All 
 these parts, however, are not universally black ; 
 for on the outside of the fore-arm, a little below 
 the elbow, a stripe of white skin commences, 
 about two inches in breadth, and differing in 
 no circumstance from the skin of the other arm, 
 which, proceeding upwards, gradually bends 
 under the arm, and at the arm-pit joins with the 
 white skin of the trunk of the body. The black 
 skin, wherever it is contiguous to the white, 
 terminates rather abruptly, so that its boundary 
 may always be distinctly traced. 
 
 The colour of the black skin is not every 
 where uniformly dark. Thus, the skin of the 
 back of the hand, and of the wrist, is marked 
 by fine lines of a reddish black, which cross 
 one another at right angles, while the small 
 rectangular spaces bounded by these lines are 
 entirely black. Part of the cuticle of the hand 
 having been removed by exciting a blister, the 
 reddish lines were found to be the summits of 
 very thin folds of the true skin, which were raised 
 above its general level, and were less thickly 
 covered with the black rete mucosum than the 
 
428 AN ACCOUNT OF, &c. 
 
 more depressed parts. Their reddish colour 
 was, no doubt, occasioned by the external air, as 
 the skin of the other hand was red from that 
 cause. All the other parts of the black skin 
 are fully as dark, as I found on making the 
 comparison, as the corresponding parts of a 
 dark negro, and are much darker than those of 
 many negroes. One part, indeed, of her skin 
 is considerably darker than the corresponding 
 part in any negro whom I have seen ; for the 
 palm of her hand and inside of her fingers are 
 black, whereas these parts in a negro are only 
 of a tawny hue. 
 
 A considerable part of the black skin is as 
 smooth to the touch, as the skin of the white 
 arm ; but the cuticular lines in the black arm, 
 appeared everywhere stronger to the sight, than 
 similar lines in the arm of a black man, whose 
 skin I examined at the same time. In the 
 greater part, however, of West's black skin, 
 those lines sink deeper beneath its general sur- 
 face, than the lines of any other human skin 
 that I have seen, which was not evidently dis- 
 eased. These depressions are extremely narrow, 
 and proceed chiefly in one direction, obliquely 
 upwards and inwards from the outer part of the 
 arm. On removing a small portion of the cuti- 
 cle, they were found to be occasioned by the 
 sinking down of that membrane between very 
 
AN ACCOUNT OF, &c. 429 
 
 narrow and slightly elevated folds of the true 
 skin, nearly contiguous to one another, which 
 held the direction mentioned. 
 
 A great part of the black shoulder exhibits 
 a singular appearance; for, near to the back 
 bone, the skin, over an extent of six inches in 
 length and two in breadth, resembles a thick 
 coat of pitch, or black paint, which by drying 
 had split into a great number of small square 
 portions. The fissures in the skin are about a 
 line in depth. Mr. James Wilson, teacher of 
 anatomy, and fellow of the Royal Society, who 
 saw this person once along with me, pulled 
 away a little of this black matter, upon which 
 several narrow processes of the skin, perpendi- 
 cular to the plane of the part, became visible. 
 
 Winslow says, that the cuticle of a negro is 
 black, and that the contrary supposition arose 
 from its tenuity and transparency, in like man- 
 ner as a thin film of black horn appears almost 
 colourless. I have found by my own observa- 
 tions, that this opinion of Winslow is just; 
 and I found also, that the cuticle of West's 
 black skin is likewise dark. I may add, that 
 the nails of her black fingers are darker than 
 those of the white, and darker also than those 
 of a negro's hand. 
 
 Sir Everard Home, who likewise saw this 
 person once along with me, thought that the 
 
430 AN ACCOUNT OF, &c. 
 
 black arm smelt more strongly than the white. 
 I made the experiment immediately after him, 
 and thought so too, But on repeating it several 
 times with more attention, I could perceive no 
 difference. It seems to me, indeed, from a 
 similar experiment made on the arm of a dark 
 negro, whose appearance did not lead me to 
 suppose, that he had been very careful with 
 respect to the cleanliness of his person, either 
 that all negroes do not possess a strong smell, 
 or that this does not proceed from all parts of 
 their skin, since I could perceive no difference 
 between the odour of his arm, and that of the 
 white arm of West. 
 
 . On the black fore-arm are about a dozen small 
 hard substances, the largest of which are of the 
 size of a common pea. Some of them are very 
 black ; others are less black, and one or two 
 are of a reddish black colour. I thought, at 
 first, that they consisted of thickened cuticle, 
 but I found afterwards, that they readily bled 
 upon being punctured with a needle. 
 
 The upper and outer part of the black arm 
 has a number of very black hairs growing from 
 it, some of which are three quarters of an inch 
 long. The inner part of the arm, which is 
 equally black, is free from hairs. 
 
 The black arm is as firm to the touch, and as 
 fleshy as the white j and according to the young 
 
AN ACCOUNT OF, &c. 431 
 
 woman's own report, there is no difference in 
 their strength or feelings of any kind. 
 
 The last circumstance which I shall mention 
 concerning her is, that no change has taken 
 place within her remembrance, either in the 
 degree or extent of the blackness of her skin. 
 
 Two inferences may, I think, be made from 
 what has been related respecting Hannah West. 
 
 The first is, that the blackness of the skin in 
 negroes is no proof of their forming a different 
 species of men from the white race. 
 
 When a white man is much exposed to the 
 action of the sun, his skin becomes more or less 
 brown, and as the intensity of this colour, after 
 equal degrees of exposure, is generally propor- 
 tional to the heat of the climate, it has hence 
 been supposed, that the colour of negroes is 
 derived from a very great degree of the same 
 cause. But this conclusion seems to me very 
 faulty. For, setting aside that a white man, 
 rendered brown by the sun's rays, begets as 
 white children as those of another of the same 
 race, the colour of whose skin had never been 
 altered, it appears to me probable, from ob- 
 servations lately made on two negroes, that the 
 action of the sun tends rather to diminish than 
 augment the colour of their race. Both of 
 those persons were born in European settle- 
 ments, and had been accustomed to have their 
 
432 AN ACCOUNT OF, &c. 
 
 bodies clothed, yet, in both, the trunk, arms, 
 and lower extremities, were considerably darker 
 than the face, and in one, were somewhat darker 
 than the hands. But admitting this observa- 
 tion to be of no force, still it must be granted, 
 in consequence of what has been said upon the 
 state of part of West's skin, that great heat is 
 not indispensably necessary to render the human 
 colour black ; which is the second conclusion to 
 be drawn from the account which has been given 
 of her. 
 
 On considering the difference of colour be- 
 tween Europeans and Africans, a view has oc- 
 curred to me of this subject, which has not been 
 given by any author, whose works have fallen 
 into my hands. I shall, therefore, venture to 
 mention it here, though at the hazard of its 
 being thought rather fanciful than just. 
 
 There is no circumstance, perhaps, in which 
 these two races differ so much, as in their capa- 
 city to bear, with impunity, the action of the 
 causes of many diseases. The fatality to Eu- 
 ropeans of the climate of the middle parts of 
 Africa, which are, however, inhabited by ne- 
 groes without injury to their health, is well 
 known. Let it then be supposed, that any 
 number of Europeans were to be sent to that 
 
AN ACCOUNT OF, &c. 433 
 
 country, and that they were to subsist them- 
 selves by their bodily labour ; it seems certain, 
 that the whole colony would soon become 
 extinct. On the other hand, the greater liabi- 
 lity of negroes in Europe to be attacked with 
 fatal diseases is equally well established. If, 
 therefore, a colony of the former race were 
 brought to Europe, and forced to labour in the 
 open air for their subsistence, many of them 
 would quickly die, and the remainder, from 
 their inability to make great bodily exertions 
 in cold weather, and their being frequently 
 diseased, would be prevented from working an 
 equal number of days in the year with the 
 whites. The consequence would be, that with- 
 out taking farther into account the unfriend- 
 liness of the climate to them, their gains would 
 be inadequate to the maintenance of themselves 
 and their families. They would thence become 
 feeble, and be rendered still more incapable of 
 supporting life by their labour. In the mean 
 time, their children would die from want, or 
 diseases induced by deficient or improper nou- 
 rishment, and in this way, a colony of the negro 
 race in a cold country would quickly cease to 
 exist. 
 
 This difference in the capacity of the two 
 races to resist the operation of the causes of 
 many diseases, I assume as a fact, though I am 
 
 F F 
 
434 AN ACCOUNT OF, &c. 
 
 utterly unable to explain it. I do not, however, 
 suppose, that their different susceptibility of 
 diseases depends, properly, on their difference 
 of colour. On the contrary, I think it proba- 
 ble, that this is only a sign of some difference 
 in them, which, though strongly manifested by 
 its effects in life, is yet too subtle to be disco- 
 vered by an anatomist after death 5 in like 
 manner as a human body, which is incapable of 
 receiving the small-pox, differs in no observable 
 thing from another, which is still liable to be 
 affected with that disease. 
 
 Regarding then as certain, that the negro 
 race are better fitted to resist the attacks of the 
 diseases of hot climates than the white, it is 
 reasonable to infer, that those, who only ap- 
 proach the black race, will be likewise better 
 fitted to do so, than others who are entirely 
 white. This is, in fact, found to be true, with 
 regard to the mixture of the two races ; since 
 mulattoes are much more healthy in hot cli- 
 mates than whites. But amongst men, as well 
 as among other animals, varieties of a greater 
 or less magnitude are constantly occurring. In 
 a civilized country, which has been long peo- 
 pled, those varieties, for the most part, quickly 
 disappear, from the intermarriages of different 
 families. Thus, if a very tall man be produced, 
 he very commonly marries a woman much less 
 
AN ACCOUNT OF, &c. 435 
 
 than himself, and their progeny scarcely differs 
 in size from their countrymen. In districts, 
 however, of very small extent, and having little 
 intercourse with other countries, an accidental 
 difference in the appearance of the inhabitants 
 will often descend to their late posterity. The 
 clan of the Macras, for instance, possess both 
 sides of Loch-Duich in Scotland ; but those who 
 inhabit one side of the loch are called the black 
 Macras, and the others the white, from a dif- 
 ference which has always been observed in their 
 complexions. Again, those who attend to the 
 improvement of domestic animals, when they 
 find individuals possessing, in a greater degree 
 than common, the qualities they desire, couple 
 a male and female of these together, then take 
 the best of their offspring as a new stock, and 
 in this way proceed, till they approach as near 
 the point in view, as the nature of things will 
 permit. But, what is here done by art, seems to 
 be done, with equal efficacy, though more slowly, 
 by nature, in the formation of varieties of man- 
 kind, fitted for the country which they inhabit. 
 Of the accidental varieties of man, which would 
 occur among the first few and scattered inha- 
 bitants of the middle regions of Africa, some 
 one would be better fitted than the others to 
 bear the diseases of the country. This race 
 would consequently multiply, while the others 
 
436 AN ACCOUNT OF, &c, 
 
 would decrease, not only from their inability to 
 sustain the attacks of disease, but from their in- 
 capacity of contending with their more vigorous 
 neighbours. The colour of this vigorous race 
 I take for granted, from what has been already 
 said, would be dark. But the same disposition 
 to form varieties still existing, a darker and a 
 darker race would in the course of time occur, 
 and as the darkest would be the best fitted for 
 the climate, this would at length become the 
 most prevalent, if not the only race, in the par- 
 ticular country in which it had originated. 
 
 In like manner, that part of the original stock 
 of the human race, which proceeded to the colder 
 regions of the earth, would in process of time 
 become white, if they were not originally so, 
 from persons of this colour being better fitted 
 to resist the diseases of such climates, than 
 others of a dark skin. 
 
 The cause which I have stated, as likely to 
 have influence on the colour of the human race, 
 would necessarily operate chiefly during its in- 
 fancy, when a few wandering savages, from 
 ignorance and improvidence, must have found 
 it difficult to subsist throughout the various 
 seasons of the year, even in countries the most 
 favourable to their health. But, when men 
 have acquired the knowledge of agriculture, 
 and other arts, and in consequence adopt a 
 
AN ACCOUNT OF, &c. 437 
 
 more refined mode of life, it has been found, 
 that an adherence to their ancient customs and 
 practices will preserve them long as a distinct 
 race from the original inhabitants of the coun- 
 try to which they had emigrated. Examples 
 of this kind are frequent in the islands in the 
 eastern seas in the torrid zone, where the in- 
 habitants of the sea-coast, evidently strangers, 
 are in some degree polished, and of a brown 
 colour, while the ancient natives, who live in 
 the interior parts, are savage and black. Simi- 
 lar facts occur in respect to other species of 
 animals. It seems certain, for instance, that 
 fine woolled sheep, like the Spanish, never both 
 arose and sustained their breed in the northern 
 parts of Europe ; yet, by care, this feeble race, 
 after being formed in Spain, has been propa- 
 gated and preserved in very cold countries. 
 Thus the late Mr. Dryander, the learned libra- 
 rian of the Royal Society, informed me, that 
 the breed of fine woolled Spanish sheep had 
 been kept perfect in Sweden during a very long 
 term of years, I think he said a century. I 
 then, my memory be accurate upon this point, 
 we have here an example of a variety of animals, 
 much more liable to be affected by external 
 circumstances than the human race, being pre- 
 served without change, in a country very dif- 
 ferent from their own, by assimilating their new 
 
438 AN ACCOUNT OF, &c. 
 
 state as much as possible to their old, during at 
 least fifty generations, that is, during a period 
 equivalent to 1500 years in the history of man. 
 
 Hitherto, while speaking of the external ap- 
 pearance of negroes, I have taken notice only 
 of their colour. I shall now say a few words 
 upon their woolly hair, and, according to our 
 notions of beauty, the deformity of their features. 
 
 There are several facts which seem to show, 
 that these circumstances are somehow connected 
 with their low state of civilization. 
 
 First j the black inhabitants of the Indian Pe- 
 ninsula within the Ganges, who, compared with 
 the African negroes, may be regarded as a po- 
 lished people, have hair and features much less 
 dissimilar to the European. 
 
 Secondly ; Woolly heads, and deformed fea- 
 tures, appear again, as we proceed further to 
 the east, among the savage inhabitants of New 
 Guinea, and the adjacent islands, at the distance 
 nearly of half of the circumference of our globe 
 from Africa, and consequently without the 
 smallest probability of any communication 
 having ever existed between the two countries. 
 
 Lastly ; it appears probable from the reliques 
 of ancient art, that the early inhabitants of Egypt 
 were of the negro race. If, then, the negroes 
 of Africa were ever to be civilized, their woolly 
 hair and deformed features would, perhaps, in 
 
AN ACCOUNT OF, &c. 439 
 
 a long series of years, like those of the Egyp- 
 tians, be changed. On the other hand, their 
 present external appearance may possibly be 
 regarded not only as a sign, but as a cause of 
 their degraded condition, by preventing, in 
 some unknown way, the proper developement 
 of their mental faculties ; for the African ne- 
 groes have in all ages been slaves ; and the 
 negroes in the eastern seas are in no instance, 
 I believe, masters of their handsomer neigh- 
 bours, but are in many places in entire subjec- 
 tion to them, though the latter be frequently 
 less numerous. 
 
 It will no doubt be objected, to what I have 
 advanced respecting the difference of colour 
 between Europeans and Africans, that the 
 Indian inhabitants of the greater part of the 
 immense continent of America have skins 
 nearly of one hue. Plausible reasons may, I 
 think, be given for this fact, consistently with 
 what has been said* upon the colour of the two 
 former races; but I forbear trespassing any 
 longer upon the time of the reader, in discuss- 
 ing a subject which admits only of conjectural 
 reasoning. 
 
 THE END. 
 
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