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 LIGHTNING,THUNDER 
 
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 LIGHTNING CONDUCTORS. 
 
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LIGHTNING, THUNDER 
 
 AND 
 
 LIGHTNING CONDUCTORS. 
 
 WITH AN APPENDIX ON THE RECENT CONTROVERSY 
 
 ON LIGHTNING CONDUCTORS. 
 
 * \ 
 
 BY 
 
 GERALD IMOLLOY, D. D., D.SC 
 
 ILLUSTRATED. 
 
 NEW YORK : 
 THE HUMBOLDT PUBLISHING CO., 
 
 28 LAFAYETTE PLACE. 
 
LIGHTNING, THUNDER, AND LIGHTNING 
 CONDUCTORS. 
 
 CONTENTS. 
 
 LECTURE I. 
 
 LIGHTNING AND THUNDER. 
 
 Identity of Lightning and Electricity Franklin's Experiment Fatal Experiment of 
 Richman Immediate Cause of Lightning Illustration from Electric Spark 
 What a Flash of Lightning Is Duration of a Flash of Lightning Expe- 
 riments of Professor Rood Wheatstone's Experiments Experiment with 
 Rotating Disc Brightness of a Flash of Lightning Various Forms of 
 Lightning Forked Lightning, Sheet Lightning, Globe Lightning St. 
 Elmo's Fire Experimental Illustration Origin of Lightning Length of a 
 Flash of Lightning Physical Cause of Thunder Rolling of Thunder 
 Succession of Peals Variation of Intensity Distance of a Flash of Light- 
 ning, ........... Pages 5-26 
 
 LECTURE II. 
 
 LIGHTNING CONDUCTORS. 
 
 Destructive Effects of Lightning Destruction of Buildings Destruction of 
 Ships at Sea Destruction of Powder Magazines Experimental Illustra- 
 tions Destruction of Life by Lightning The Return Shock Franklin's 
 Lightning Rods Introduction of Lightning Rods into England The Bat- 
 tle of Balls and Points Functions of a Lightning Conductor Conditions 
 of a Lightning Conductor Mischief Done by Bad Conductors Evil Effects 
 of a Bad Earth Contact Danger from Rival Conductors Insulation of 
 Lightning Conductors Personal Safety in a Thunder Storm Practical 
 Rules Security Afforded by Lightning Rods, , . . Pages 26-53 
 
 APPENDIX. 
 
 RECENT CONTROVERSY ON LIGHTNING CONDUCTORS. 
 
 Tfteory of Lightning Conductors Challenged Lectures of Professor Lodge 
 Short Account of his Views and Arguments Effect of Self-induction on a 
 Lightning Rod Experiment on the Discharge of a Leyden Jar Outer Shell 
 only of a Lightning Rod Acts as a Conductor Discussion at the Meeting of 
 
CONTENTS. 
 
 the British Association, September, 1888 Statement by Mr. Preece Lord 
 Rayleigh and Sir William Thomson Professor Rowland and Professor 
 Forbes M. de Fonvielle, Sir James Douglass, and Mr. Symons Reply of 
 Professor Lodge Concluding Remarks of Professor Fitzgerald, President 
 of the Section Summary Showing the Present State of the Ques- 
 tion, , Pages 55-62 
 
 LIST OF ILLUSTRATIONS. 
 
 PAGE 
 
 THE ELECTRIC SPARK : A TYPE OF A FLASH OF LIGHTNING, , . . 8 
 
 CARDBOARD Disc WITH BLACK AND WHITE SECTORS ; AS SEEN WHEN AT REST, 12 
 
 SAME Disc ; AS SEEN WHEN IN RAPID ROTATION, 12 
 
 THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO'S FIRE, . . . .17 
 
 ORIGIN OF SUCCESSIVE PEALS OF THUNDER 22 
 
 VARIATIONS OF INTENSITY IN A PEAL OF THUNDER, 24 
 
 DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES, . . . .27 
 GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY, . . .32 
 
 GUN COTTON SET ON FIRE BY ELECTRIC SPARK, 33 
 
 VOLTA'S PISTOL ; EXPLOSION CAUSED BY ELECTRIC SPARK, . . . .34 
 
 THE RETURN SHOCK ILLUSTRATED, 35 
 
 PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR, 48 
 
 INDUCTION EFFECT OF LEYDEN JAR DISCHARGE, 56 
 
LECTURE I. 
 
 LIGHTNING AND THUNDER. 
 
 THE electricity produced by an ordinary electric machine exhibits, 
 under certain conditions, phenomena which bear a striking re- 
 semblance to the phenomena attendant on lightning. In both cases 
 there is a flash of light ; in both there is a report, which, in the 
 case of lightning, we call thunder ; and, in both cases, intense heat is 
 developed, which is capable of setting fire to combustible bodies. 
 Further, the spark from an electric machine travels through space 
 with extraordinary rapidity, and so does a flash of lightning ; the 
 spark follows a zig-zag course, and so does a flash of lightning; the 
 spark moves silently and harmlessly through metal rods and stout 
 wires, while it forces its way, with destructive effect, through bad 
 conductors, and it is so, too, with a flash of lightning. Lastly, the 
 electricity of a machine is capable of giving a severe shock to the 
 human body; and we know that lightning gives a shock so severe as 
 usually to cause immediate death. For these reasons it was long 
 conjectured by scientific men that lightning is, in its nature, identical 
 with electricity; and that it differs from the electricity of our machines 
 only in this, that it exists in a more powerful and destructive form. 
 
 Identity of Lightning and Electricity. But it was reserved 
 for the celebrated Benjamin Franklin to demonstrate the truth of this 
 conjecture by direct experiment. He first conceived the idea of 
 drawing electricity from a thundercloud in the same way as it is 
 drawn from the conductor of an electric machine. For this purpose 
 he proposed to place a kind of sentry-box on the summit of a lofty 
 tower, and to erect, on the sentry-box, a metal rod, projecting twenty 
 or thirty feet upward into the air, pointed at the end, and having no 
 electrical communication with the earth. He predicted that when a 
 thundercloud would pass over the tower, the metal rod would become 
 charged with electricity, and that an observer, stationed in the sentry- 
 box, might draw from it, at pleasure, a succession of electric sparks. 
 
 With the magnanimity of a really great man, Franklin published 
 this project to the world ; being more solicitous to extend the domain 
 of science by new discoveries, than to secure for himself the glory of 
 
6 LIGHTNING AND THUNDER. 
 
 having made them. The project was set forth in a letter to Mr. 
 Collinson, of London, which bears date July 29, 1750, and which, in 
 the course of a year or two, was translated into the principal lan- 
 guages of Europe. Two years later the experiment suggested by 
 Franklin was made by Monsieur Dalibard, a wealthy man of science, 
 at his villa near Marly-la- Ville, a few miles from Paris. In the mid- 
 dle of an elevated plain Monsieur Dalibard erected an iron rod, forty 
 feet in length, one inch in diameter, and ending above in a sharp 
 steel point. The iron rod rested on an, insulating support, and was 
 kept in position by means of silk cords. 
 
 In the absence of Monsieur Dalibard, who was called by business 
 to Paris, this apparatus was watched by an old dragoon, named 
 Coiffier; and on the afternoon of the tenth of May, 1752, he drew 
 sparks from the lower end of the rod at the time that a thundercloud 
 was passing over the neighborhood. Conscious of the importance 
 that would be attached to this phenomenon, the old dragoon sum- 
 moned, in all haste, the prior of Marly to come and witness it. The 
 prior came without delay, and he was followed by some of the prin- 
 cipal inhabitants of the village. In the presence of the little group, 
 thus gathered together, the experiment was repeated electric sparks 
 were again drawn, in rapid succession, from the iron rod; the prediction 
 of Franklin was fulfilled to the letter; and the identity of lightning and 
 electricity was, for the first time, demonstrated to the world. 
 
 Franklin's Experiment. Meanwhile Franklin had been wait- 
 ing, with impatience, for the completion of the tower of Christchurch, 
 in Philadelphia, on which he intended to make the experiment him- 
 self. He even collected money, it is said, to hasten on the building. 
 But, notwithstanding his exertions, the progress of the tower was 
 slow; and his active mind, which could ill brook delay, hit upon 
 another expedient, remarkable alike for its simplicity and for its com- 
 plete success. He constructed a boy's kite, using, however, a silk 
 pockethandkerchief, instead of paper, that it might not be damaged 
 by rain. To the top of the kite he attached a pointed iron wire about 
 a foot long, and he provided a roll of hempen twine, which he knew 
 to be a conductor of electricity, for flying it. This was the apparatus 
 with which he proposed to explore the nature of a thundercloud. 
 
 The thundercloud came late in the afternoon of the fourth of July ? 
 1752, and Franklin sallied out with his kite, accompanied by his son, 
 and taking with him a common door-key and a Leyden jar. The kite 
 was soon high in air, and the philosopher awaited the result of his 
 experiment, standing, with his son, under the lee of a cowshed, partly 
 to protect himself from the rain that was coming, and partly, it is 
 said, to shield himself from the ridicule of passers-by, who, having no 
 sympathy with his philosophical speculations, might be inclined to 
 regard him as a lunatic. To guard against the danger of receiving a, 
 
LIGHTNING AND THUNDER. 7 
 
 flash of lightning through his body, he held the kite by means of a 
 silk ribbon, which was tied to the door-key, the door-key being itself 
 attached to the lower end of the hempen string. 
 
 A flash of lightning soon came from the cloud, and a second, and a 
 third; but no sign of electricity could be observed in the kite, or the 
 hempen cord, or the key. Franklin was almost beginning to despair 
 of success, when suddenly he noticed that the little fibres of the cord 
 began to bristle up, just as they would if it were placed near an elec- 
 tric machine in action. He presented the door-key to the knob of the 
 Leyden jar, and a spark passed between them. Presently a shower 
 began to fall; the cord, wetted by the rain, became a better con- 
 ductor than it had been before, and sparks came more freely. With 
 these sparks he now charged the Leyden jar, and found, to his intense 
 delight, that he could exhibit all the phenomena of electricity by 
 means of the lightning he had drawn from the clouds. 
 
 In the following year a similar experiment, with even more striking 
 results, was carried out, in France, by de Romas. Though it is said 
 he had no knowledge of what Franklin had done in America, he, too, 
 used a kite; and, with a view of making the string a better conductor, 
 he interlaced with it a thin copper wire. Then, flying his kite in the 
 ordinary way, when it had risen to a height of about 550 feet, he drew 
 sparks from it which, we are told, were upwards of nine feet long, 
 and emitted a sound like the report of a pistol. 
 
 Fatal Experiment of Richman. There can be no doubt that 
 experiments of this kind, made with the electricity of a thunder- 
 cloud, were extremely dangerous ; and this was soon proved by a fatal 
 accident. Professor Richman, of St. Petersburgh, had erected on the 
 roof of his house a pointed iron rod, the lower end of which passed 
 into -a glass vessel, intended, as we are informed, to measure the 
 strength of the charge which he expected to receive from the clouds. 
 On the sixth of August, 1753, observing the approach of a thunder- 
 storm, he hastened to his apparatus ; and as he stood near it, with his 
 head bent down, to watch the effect, a flash of lightning passed 
 through his body and killed him on the spot. This catastrophe served 
 to fix public attention on the danger of such experiments, and gave oc- 
 casion to the saying of Voltaire : " There are some great lords whom 
 we should always approach with extreme precaution, and lightning is 
 one of them." 1 From this time the practice of making experiments 
 directly with the lightning of the clouds seems to have been, by com- 
 mon consent, abandoned. 
 
 Immediate Cause of Lightning. And now, having set before 
 you some of the most memorable experiments by which the identity 
 of lightning and electricity has been demonstrated, I will try to give 
 
 1 u II y a des grands seigneurs dont il ne faut approcher qu'avec d'extremes precautions. Le ton- 
 nerre est de ce nombre." Diet. Philos. art. Foudre. 
 
8 LIGHTNING AND THUNDER. 
 
 you a clear conception regarding the immediate cause of lightning, so 
 far as the subject is understood at the present day by scientific men. 
 You know that there are two kinds of electricity, which are called posi- 
 tivc and negative j and that each of them repels electricity of the 
 same kind as itself, while it attracts electricity of the opposite kind. 
 Now, every thundercloud is charged with electricity of one kind or the 
 other, positive or negative ; and, as it hovers over the earth, it devel- 
 ops, by what is called induction, or influence, electricity of the opposite 
 kind in that part of the earth which is immediately under it. Thus 
 we have two bodies the cloud and the earth charged with opposite 
 kinds of electricity, and separated by a stratum of the atmosphere. 
 The two opposite electricities powerfully attract each other ; but for 
 a time they are prevented from rushing together by the intervening 
 stratum of air, which is a non-conductor of electricity, and acts as a 
 barrier between them. As the electricity, however, continues to accu- 
 mulate, the attraction becomes stronger and stronger, until at length 
 it is able to overcome the resistance of this barrier ; a violent disrup- 
 
 THE ELECTRIC SPARK ; A TYPE OF A FLASH OF LIGHTNING. 
 
 tive discharge then takes place between the cloud and the earth, and 
 the flash of lightning is the consequence of the discharge. , 
 
 The whole phenomenon may be illustrated, on a small scale, by 
 means of this electric machine of Carry's which you see before you. 
 When my assistant turns the handle of the machine negative elec- 
 tricity is developed in that large brass cylinder, which in our experi- 
 ment will represent the thundercloud. At a distance of five or six 
 inches from the cylinder I hold a brass ball, which is in electrical com- 
 munication with the earth through my body. The electrified brass 
 cylinder acts by induction, or influence on the brass ball, and develops 
 in it, as well as in my body, a charge of positive electricity. Now, the 
 positive electricity of the ball and the negative electricity of the 
 cylinder are mutually attracting each other, but the intervening 
 stratum of air offers a resistance which prevents a discharge from 
 taking place. My assistant, however, continues to work the machine ; 
 the two opposite electricities rapidly accumulate on the cylinder and 
 the ball ; at length their mutual attraction is strong enough to over- 
 
LIGHTNING AND THUNDER. 9 
 
 come the resistance interposed between them ; a disruptive discharge 
 follows, and at the same moment a spark is seen to pass, accompanied 
 by a sharp snapping report. 
 
 This spark is a miniature flash of lightning ; and the snapping re- 
 port is a diminutive peal of thunder. Furthermore, at the moment 
 the spark passes you may observe a slight convulsive movement in my 
 hand and wrist. This convulsive movement represents, on a small 
 scale, the violent shock, generally fatal to life, which is produced by 
 a flash of lightning when it passes through the body. 
 
 I can continue to take sparks from the conductor as long as the 
 machine is worked ; and it is interesting to observe that these sparks 
 follow an irregular zig-zag course, just as lightning does. The reason 
 is the same in both cases : a discharge between two electrified bodies 
 takes place along the line of least resistance ; and, owing to the vary- 
 ing condition of the atmosphere, as well as of the minute particles of 
 matter floating in it, the line of least resistance is almost always a 
 zig-zag line. >^\ 
 
 jL-What a Flash of Lightning is. Lightning, then, may be VJ/ 
 j^aqpceived as an electrical discharge, sudden and violent in its charac- 
 ter, which takes place, through the atmosphere, between two bodies 
 highly charged with opposite kinds of electricity. Sometimes this 
 electrical discharge passes, as I have said, between a cloud and the 
 earth ; sometimes it passes between one cloud and another ; some- 
 times, on a smaller scale, it takes place between the great mass of a 
 cloud and its outlying fragments. 
 
 But, if you ask me in what the discharge itself consists, I am 
 utterly unable to tell you. It is usual to speak and write on this sub- 
 ject as if electricity were a material substance, a very subtle fluid, and 
 as if, at the moment the discharge takes place, this fluid passes like a 
 rapid stream, from the body that is positively electrified to the body 
 that is negatively electrified. But we must always remember that this 
 is only a conventional mode of expression, intended chiefly to assist 
 our conceptions, and to help us to talk about the phenomena. It does 
 not even profess to represent the objective truth. All that we know 
 for certain is this : that immediately before the discharge the two 
 bodies are highly electrified with opposite kinds of electricity ; and, 
 that immediately after the discharge, they are found to have returned 
 to their ordinary condition, or, at least, to have become less highly 
 electrified than they were before. 
 
 The flash of light that accompanies an electric discharge is often * 
 supposed to be the electricity itself, passing from one body to the 
 other. But it is not ; it is simply an effect produced by the discharge. 
 Heat is generated by the expenditure of electrical energy, in over- 
 coming the resistance offered by the atmosphere ; and this heat is so 
 intense, that it produces a brilliant incandescence along the path of 
 
io LIGHTNING AND THUNDER. 
 
 the discharge. When a spark appears, for example, between the con- 
 ductor of the machine and this brass ball, it can be shown, by very 
 satisfactory evidence, that minute particles of these solid bodies are 
 first converted into vapor, and then made to glow with intense heat. 
 The gases, too, of which the air is composed, and the solid particles 
 floating in the air, are likewise raised to incandescence. So, too, with 
 lightning ; the flash of light is due to the intense heat generated by 
 the electrical discharge, and owes its character to the composition 
 and the density of the atmosphere through which the discharge passes. 
 
 Duration of a Flash of Lightning. How long does a flash 
 of lightning last ? You are aware, I dare say, that when an impres- 
 sion of light is made on the eye, the impression remains for a sensible 
 interval of time, not less than the tenth of a second, after the source 
 of light has been extinguished or removed. Hence we continue, in 
 fact, to see the light, for at least the tenth of a second, after the light 
 has ceased. Now, if you reflect how brief is the moment for which a 
 flash of lightning is visible, and if you deduct the tenth of a second 
 from that brief moment, you will see, at once, that the period of its 
 actual duration must be very short indeed. 
 
 The exact duration of a flash of lightning is a question on which no 
 settled opinion has yet been accepted generally by scientific men. 
 Indeed, the most widely different statements have been made on the 
 subject, quite recently, by the highest authorities, each speaking ap- 
 parently with unhesitating confidence. Thus, for example, Professor 
 Mascart describes an experiment, which he says was made by Wheat- 
 stone, and which showed that a flash of lightning lasts for less than 
 0//-thousandth of a second ;' Professor Everett describes the same 
 experiment, without saying by whom it was made, and gives, as the 
 result, that "the duration of the illumination produced by lightning 
 is certainly less than the /^//-thousandth of a second;" 3 Professor 
 Tyndall, in his own picturesque way, tells us that "a flash of light- 
 ning cleaves a cloud, appearing and disappearing in less than the 
 ////W/r</-thousandth of a second ; " ' and according to Professor Tait, 
 of Edinburgh, "Wheatstone has shown that lightning certainty lasts 
 less than the millionth of a second." 4 
 
 Experiments of Professor Rood. I cannot say which of 
 these statements is best supported by actual observation ; for none of 
 the writers I have quoted gives any reference to the original memoir 
 from which his statement is derived. As far as my own reading 
 goes, I have only come across one original record of experiments, 
 made directly on the flash of lightning itself, with a view to determine 
 the period of its duration. These experiments were carried out by 
 
 1 Electricite Statique, ii., 561. 
 
 a Deschanel's Natural Philosophy, Sixth Edition, p. 641, 
 
 3 Fragments of Science, Fifth Edition, p. 311. 
 
 4 Lecture on Thunderstorms, Nature, vol. xxii., p. 341. 
 
Ll&HTNfNG AND THUNDER. ii 
 
 Professor Ogden Rood, of Columbia College, New York, between the 
 years 1870 and 1873, and are recorded in the American Journal of 
 Science and Arts. 1 
 
 For the description of his apparatus, and for the details of his ob- 
 servations, I must refer you to the memoir itself ; but I may tell you 
 briefly that the results at which he arrived, if they be accepted, must 
 lead to a considerable modification of the views previously entertained 
 on the subject. In the first place, he satisfied himself that what ap- 
 pears to the eye a single flash of lightning is usually, if not always, 
 multiple in its character ; consisting, in fact, of a succession of dis- 
 tinct flashes, which follow one another with such rapidity as to make 
 a continuous inpression on the retina. Next, he proceeded to meas- 
 ure approximately the duration of these several component flashes ; 
 and he found that it varied over a wide range, amounting sometimes 
 to fully the twentieth of a second, and being sometimes less than the 
 sixteen-hundredth of a second. 
 
 Wheatstone's Experiments. These results are extremely in- 
 teresting ; but we can hardly regard them as finally established, until 
 they have been confirmed by other observers. I may remark, how- 
 ever, that they fit in very well with the experiments made by Pro- 
 fessor Wheatstone, many years ago, on the duration of the electric 
 spark, which, as I told you, is a miniature flash of lightning. In these 
 classical experiments, which leave nothing to be desired in point of 
 accuracy, Professor Wheatstone showed that a spark taken directly 
 from a Leyden jar, or a spark taken from the conductor of a power- 
 ful electric machine, that is, just such a spark as you have seen here 
 to-day, lasts for less than the millionth of a second. 
 
 But he also showed that the duration of the spark is greatly in- 
 creased, when a resisting wire is introduced into the path of the dis- 
 charge. Thus, for example, when the discharge from a Leyden jar was 
 made to pass through half a mile of copper wire, with breaks at inter- 
 vals, t|*e sparks that appeared at these breaks were found to last for 
 2To~o -Q f a second. 2 Hence we should naturally expect that the period 
 of illumination would be still further increased, in the case of a flash 
 of lightning, where the resistance interposed is enormously greater 
 than in either of the experiments made by Wheatstone. 3 
 
 Experiment of the Rotating Disk. It would be tedious, on an 
 occasion like the present, to enter into an account of Wheatstone's 
 beautiful and ingenious method of investigation, by which the above 
 facts have been established ; but I will show you a much more 
 simple experiment which brings home very forcibly to the mind how 
 
 i Third Series, vol. v., p. 161. 
 
 3 Phil. Trans. Royal Society, 1834, vol. cxxv., pp. 583-591. 
 
 3 In experiments with a Leyden jar, Feddersen has shown that the duration of the discharge is in- 
 creased, not only by increasing the striking distance, but also by increasing the size of the jar. 
 Now, a flash of lightning may be regarded as the discharge of a Leyden jar of immense size, with an 
 enormous striking distance ; and therefore we should expe'ct that the duration of the discharge should 
 be greatly prolonged. See American Journal of Science and Arts, Third Series, vol. i., p. 15. 
 
LIGHTNING AND ^HUNDER. 
 
 exceedingly short must be the duration of the electric spark. Here is 
 a circular disk of cardboard, the outer part of which, as you see, is di- 
 vided into sectors, black and white alternately, while the space about 
 the centre is entirely white. The disk is mounted on a stand, by means 
 of which I can make it rotate with great velocity. When it is put 
 in rotation, the effect on the eye is very striking the central space 
 remains white as before, but in the outer rim the distinction of black 
 and white absolutely disappears and gives place to a uniform gray. 
 This color is due to the blending together of black and white in equal 
 proportions ; the blending being effected, not on the cardboard disk, 
 but on the retina of the eye. 
 
 I mentioned just now that an impression made on the retina lasts 
 for the tenth of a second after the cause of it has been removed. 
 
 CARDBOARD DISK AS SEEN WHEN AT REST. 
 
 SAME DISK AS SEEN WHEN IN RAPID ROTATION. 
 
 Now, when this disk is in rotation, the sectors follow one another so 
 rapidly that the particular part of space occupied at any moment by a 
 white sector will be occupied by a black sector within a time much 
 less than the tenth of a second. It follows that the impression made 
 by each white sector remains on the retina until the following black 
 sector comes into the same position ; and, in like manner, the impres- 
 sion made by each black sector remains until the following white sec- 
 tor takes up the position of the black. Therefore, the impression 
 made -by the whole outer rim is the impression of black and white 
 combined that is, the impression of gray. 
 
 So far, I dare say, the phenomenon is already familiar to you all. 
 But I propose now to show you the revolving disk illuminated by the 
 
LIGHTNING AND THUNDER. 13 
 
 electric spark ; and you will observe that, at the moment of illumina- 
 tion, the black and white sectors come out as clearly and distinctly as 
 if the disk were standing still. 
 
 For the success of this experiment it is desirable, not only to have a 
 brilliant spark in order to secure a good illumination of the disk, but 
 also to have a succession of such sparks, that you may see the phe- 
 nomenon frequently repeated, and thus be able to observe it at your 
 leisure. To attain these two objects, I have made the arrangement 
 which is here before you. 
 
 In front of the disk is a large and very powerful Leyden jar. The 
 rod connected with the inner coating rises well above the mouth of 
 the jar, and ends in a brass ball nearly opposite the centre of the disk. 
 Connected with the outer coating of the jar is another rod which like- 
 wise ends in a brass ball, and which is so adjusted that the distance 
 between the two balls is about an inch. The two rods are connected 
 respectively with the two conductors of a Holtz machine, so that, 
 when the machine is worked, the jar is first quickly charged, and then 
 it discharges itself, with a brilliant spark, between the two brass balls. 
 Thus, by continuing to work the machine, we can get, as long as we 
 choose, a succession of sparks following one another at short and 
 regular intervals right in front of the disk. 
 
 Everything being now ready, and the room partially darkened, the 
 disk is put in rapid rotation ; and you can see, by the twilight that re- 
 mains, the outer rim a uniform gray, and the central space white. 
 But when my assistant begins to turn the Holtz machine, and brilliant 
 sparks leap out at intervals, the revolving disk, illuminated for a mo- 
 ment at each discharge, seems to be standing still, 'and shows the 
 black and white sectors distinctly visible. 
 
 The reason of this is clear : So brief is the moment for which the 
 spark endures, that the disk, though in rapid motion, makes no sensi- 
 ble advance during that small fraction of time ; therefore, in the im- 
 age on the retina, the impression made by the white sectors remains 
 distinct from the impression made by the black, and the eye sees the 
 disk as it really is. 
 
 I may notice, in passing, a very interesting consideration, suggested 
 by this experiment. A cannon ball is now commonly discharged with 
 a velocity of about 1,600 feet a second. Moving with this velocity it 
 is, as you know, under ordinary circumstances, altogether invisible to 
 the eye. But suppose it were illuminated, in the darkness of night, 
 by this electric spark, which lasts, we will say, for the millionth of a 
 second. During the moment of illumination, the cannon ball moves 
 through the millionth part of 1,600 feet, which is a little less than the 
 fiftieth of an inch. Practically, we may say that the cannon ball does 
 not sensibly change its place while the spark lasts. Further, the im- 
 pression it makes on the eye, from the position it occupies at the 
 
14 LIGHTNING AND THUNDER. 
 
 moment of illumination, remains on the retina for at least the tenth 
 of a second. Therefore, if we are looking toward that particular part 
 of space where the cannon ball happens to be at the moment the spark 
 passes, we must see the cannon ball hanging motionless in the air, 
 though we know it is traveling at the rate of 1,600 feet a second, or 
 about 1,000 miles an hour. 
 
 Brightness of a Flash of Lightning. I should like to say 
 one word about the brightness of a flash of lightning. Somewhat 
 more than thirty years ago, Professor Swan, of Edinburgh, showed 
 that the eye requires a sensible time about the tenth of a second 
 to perceive the full brightness of a luminous object. Further, he 
 proved, by a series of interesting experiments, that when a flash of 
 light lasts for less than the tenth of a second, its apparent brilliancy 
 to the eye is proportional to the time of its duration. 1 Now consider 
 the consequence of these facts in reference to the brightness of our 
 electric spark. If the spark lasted for the tenth of a second, we 
 should perceive its full brightness ; if it lasted for the tenth part of 
 that time, we should see only the tenth part of its brightness ; if it 
 lasted for the hundredth part, we should see only the hundredth part 
 of its brightness ; and so on. But we know, in point of fact, that it 
 lasts for less than the millionth of a second, that is, less than the 
 hundred-thousandth part of the tenth of a second. Therefore we see 
 only the hundred-thousandth part of its real brightness. 
 
 Here is a startling conclusion, and one, I may say, fully justified by 
 scientific evidence. That electric spark, brilliant as it appears to us, 
 is really a hundred thousand times as bright as it seems to be. We 
 cannot speak with the same precision of a flash of lightning ; because 
 its duration has not yet been so exactly determined. But if we sup- 
 pose that a flash of lightning, in a particular case, lasts for the thou- 
 sandth of a second, it would follow, from the above experiments, that 
 the flash is a hundred times as bright, in fact, as it appears to the eye. 
 Various Forms of Lightning. The lightning of which I have 
 spoken hitherto is commonly called forked lightning ; a name which 
 seems to have been derived from the zig-zag line of light it presents 
 to the eye. But there are other forms under which the electricity of 
 the clouds often makes itself manifest ; and to these I would now in- 
 vite your attention for a few moments. The most common of them 
 all, at least in this country, is that which is familiarly known by the 
 name of sheet^ lightning. This is, probabJv, nothing else than the 
 lighting up of the atmosphere, or of the clouds, by forked lightning, 
 which is not itself directly visible. 
 
 Generally speaking, after a flash of sheet lightning, we hear the 
 rolling of distant thunder. But it sometimes happens, especially in 
 
 1 See original paper by Swan, Trans. Royal Society. Edinburgh. 1849. vol. xvi., pp. 581-603 ; also, a 
 second paper, ib. 1861, vol. xxii., pp. 33-39. 
 
LIGHTNING AND THUNDER. 15 
 
 summer time, that the atmosphere is again and again lit up by a sud- 
 den glow of light, and yet no thunder is heard. This phenomenon is 
 commonly called suimner lightning, or heat lightning. It is probably 
 due, in many cases, to electrical discharges in the higher regions of 
 the atmosphere, where the air is greatly rarified ; and, in these cases, it 
 would seem to resemble the discharges obtained by means of an induc- 
 tion coil in glass tubes containing rarified gases. But there is little 
 doubt that in many cases, too, summer lightning, like ordinary sheet 
 lightning, is due to forked lightning, which is so remote that we can 
 neither see the flash itself directly, nor hear the rolling of the thunder. 
 
 Perhaps the most distinct and satisfactory evidence on this subject, 
 derived from actual observation, is contained in the following letter 
 of Professor Tyndall, written in May, 1883 : " Looking to the south 
 and south-east from the Bel Alp, the play of silent lightning among 
 the clouds and mountains is sometimes very wonderful. It may be 
 seen palpitating for hours, with a barely appreciable interval between 
 the thrills. Most of those who see it regard it as lightning without 
 thunder Blitz ohne Donner, Wetterleuchten, I have heard it named 
 by German visitors. The Monte Generoso, overlooking the Lake of 
 Lugano, is about fifty miles from the Bel Alp, as the crow flies. The 
 two points are connected by telegraph ; and frequently when the 
 Wetterleuchten, as seen from the Bel Alp, was in full play, I have 
 telegraphed to the proprietor of the Monte Generoso Hotel and 
 learned, in every instance, that our silent lightning co-existed in time 
 with a thunderstorm more or less terrific in upper Italy." 1 
 
 Another form of lightning, described by many writers, is called 
 globg^ lightning. It is said to appear as a ball of fire, about the size 
 of a child's head, or even larger, which moves for a time slowly about, 
 and then, after the lapse of several seconds, explodes with a terrific 
 noise, sending forth flashes of fire in all directions, which burn what- 
 ever they strike. Many accounts are on record of such phenomena ; 
 but they are derived, for the most part, from the evidence of persons 
 who were not specially competent to observe, and to describe with 
 precision, the facts that fell under their observation. Hence these 
 accounts, while they are accepted by some, are rejected by others ; 
 and it seems to me, in the present state of the question, that the ex- 
 istence of globe lightning can hardly be regarded as a demonstrated 
 fact. ''At all events, if phenomena of this kind have really occurred, 
 I can only say that nothing we know about electricity, at present, will 
 enable us to account for them. 2 
 
 St. Elmo's Fire. A much more authentic and, at the same time, 
 
 1 Nature, vol. xxviii., p. 54. 
 
 2 See, however, an attempt to account for this phenomenon in De Larive's Treatise on Electricity, 
 London, 1853-8, vol. iii., pp. 199, 200 ; and another, quite recently, by Mr. Spottiswoode, in a Lecture 
 on the Electrical Discharge, delivered before the British Association at York, in September, 1881, and 
 published by Longmans, London, p. 42. See also, for recent evidence regarding the phenomenon itself, 
 Scott s Elementary Meteorology, pp. 175-8. 
 
16 LIGHTNING AND THUNDER. 
 
 very interesting form, under which the electricity of the clouds some- 
 times manifests its presence, is known by the name of St. Elmo's fire. 
 This phenomenon at one time presents the appearance of a star, shining 
 at the points of the lances or bayonets of a company of soldiers; at an- 
 other, it takes the form of a tuft of bluish light, which seems to stream 
 away from the masts and spars of a ship at sea, or from the pointed 
 spire of a church. It was well known to the ancients. Caesar, in his 
 Commentaries, tells us that, after a stormy night, the iron points of 
 the javelins of the fifth legion seemed to be on fire ; and Pliny says 
 that he saw lights, like stars, shining on the lances of the soldiers, 
 keeping watch by night upon the ramparts. When two such lights 
 appeared at once, on the masts of a ship, they were called Castor and 
 Pollux, and were regarded by sailors as a sign of a prosperous voyage 
 When only one appeared, it was called Helen, and was taken as an 
 unfavorable omen. 
 
 In modern times St. Elmo's fire has been witnessed by a host of 
 observers, and all its various phases have been repeatedly described. 
 In the memoirs of Forbin we read that, when he was sailing once, in 
 1696, among the Balearic Islands, a sudden storm came on during the 
 night, accompanied by lightning and thunder. "We saw on the ves- 
 sel," he says, " more than thirty St. Elmo's fires. Among the rest 
 there was one on the vane of the mainmast more than a foot and a 
 half high. I sent a man up to fetch it down. When he was aloft he 
 cried out that it made a noise like wetted gunpowder set on fire. 1 
 told him to take off the vane and come down ; but, scarcely had he 
 removed it from its place, when the fire left it and reappeared at the 
 end of the mast, so that it was impossible to take it away. It remained 
 for a long time, and gradually went out." 
 
 On the i4th of January, 1824, Monsieur Maxadorf happened to look 
 at a load of straw in the middle of a field just under a dense black 
 cloud. The straw seemed literally on fire a streak of light went 
 forth from every blade ; even the driver's whip shone with a pale-blue 
 flame. As the black cloud passed away, the light gradually disap- 
 peared, after having lasted about ten minutes. Again, it is related 
 that on the 8th of May, 1831, in Algiers, as the French artillery offi- 
 cers were walking out after sunset without their caps, each one saw a 
 tuft of blue light on his neighbor's head ; and, when they stretched 
 out their hands, a tuft of light was seen at the end of every finger. 
 Not infrequently a traveler in the Alps sees the same luminous tuft 
 on the point of his alpenstock. And quite recently, during a thunder- 
 storm, a whole forest was observed to become luminous just before 
 each flash of lightning, and to become dark again at the moment of 
 the discharge. 1 
 
 See Jamin, " Cours de Physique," i., 480-1; Tomlinson, " The Thunderstorm," Third Edition, pp. 
 95-103 ; " Thunderstorms," a Lecture by Professor Tail, Nature, vol. xxii., p. 356. 
 
LIGHTNING AND THUNDER. 17 
 
 This phenomenon may be easily explained. It consists in a gradual 
 and comparatively silent electrical discharge between the earth and 
 the cloud ; and generally, but not always, it has the effect of prevent- 
 ing such an accumulation of electricity as would be necessary to pro- 
 duce a flash of lightning. I can illustrate this kind of discharge with 
 the aid of our machine. If I hold a pointed metal rod toward the 
 large conductor, you can see, when the machine is worked and the 
 room darkened, how the point of the rod becomes luminous and shines 
 like a faint blue star. I substitute for the pointed rod the blunt han- 
 dles of a pair of pliers, and a tuft of blue light is at once developed at 
 the end of each handle, and seems to stream away with a hissing 
 noise. I now put aside the pliers, and open out my hand under the 
 conductor and observe how I can set up, at pleasure, a luminous 
 tuft at the tips of my fingers. Now and tjien a spark passes, giving 
 me a smart shock, and showing how the electricity may sometimes 
 accumulate so fast that it cannot be sufficiently discharged by the lu- 
 
 THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO S FIRE. 
 
 minous tuft. Lastly, I present a small bushy branch of a tree to the 
 conductor, and all its leaves and twigs are aglow with bluish light, 
 which ceases for a moment when a spark escapes, to be again renewed 
 when electricity is again developed by the working of the machine. 
 
 Now, if you put a thundercloud in the place of that conductor, you 
 can easily realize how, through its influence, the lance and bayonet of 
 the soldier, the alpenstock of the traveler, the pointed spire of a church, 
 the masts of a ship at sea, the trees of a forest, can all be made to 
 glow with a silent electrical discharge which may or may not, accord- 
 ing to circumstances, culminate at intervals in a genuine flash of 
 lightning. 
 
 Origin of Lightning. When we seek to account for the origin 
 of lightning, we are confronted at once with two questions of great 
 interest and importance first, What are the sources from which the 
 electricity of the thundercloud is derived ? and, secondly, How does 
 this electricity come to be developed in a form which so far transcends 
 in power the electricity of our machines ? These questions have long 
 
1 8 LIGHTNING AND THUNDER. 
 
 engaged the attention of scientific men, but I cannot say that they 
 have yet received a perfectly satisfactory solution. Nevertheless, 
 some facts of great scientific value have been established, and some 
 speculations have been put forward, which are well deserving of con- 
 sideration. 
 
 In the first place, it is quite certain that the atmosphere which sur- 
 rounds our globe is almost always in a state of electrification. Fur- 
 ther, the electrical condition of the atmosphere would seem to be as 
 variable as the wind. It changes with the change of season ; it 
 changes from day to day ; it changes from hour to hour. The charge 
 of electricity is sometimes positive, sometimes negative ; sometimes it 
 is strong, sometimes feeble ; and the transition from one condition to 
 another is sometimes slow and gradual, sometimes sudden and 
 violent. 
 
 As a general rule, in fine, clear weather, the electricity of the at- 
 mosphere is positive, and not very strongly developed. In wet 
 weather the charge may be either positive or negative, and is gen- 
 erally strong, especially when there are sudden heavy showers. In 
 fog it is also strong, and almost always positive. In a snowstorm it is 
 very strong, and most frequently positive. Finally, in a thunderstorm 
 it is extremely strong, and generally negative ; but it is subject to a 
 sudden change of sign, when a flash of lightning passes or when rain 
 begins to fall. 
 
 So far I have simply stated facts, which have been ascertained by 
 careful observations, made at different stations by competent observ- 
 ers, and extending over a period of many years. But as regards the 
 process by which the electricity of the atmosphere is developed, we 
 have, up to the present time, no certain knowledge. It has been said 
 that electricity may be generated in the atmosphere by the friction of 
 the air itself, and of the minute particles floating in it, against the 
 surface of the earth, against trees and buildings, against rocks, cliffs, 
 and mountains. But this opinion, however probable it may be, has 
 not yet been confirmed by any direct experimental investigation. 
 
 The second theory is that the electricity of the atmosphere is due, 
 in great part at least, to the evaporation of salt water. Many years 
 ago, Pouillet, a French philosopher, made a series of experiments in the 
 laboratory, which seemed to show that evaporation is generally 
 attended with the development of electricity ; and, in particular, he 
 satisfied himself that the vapor which passes off from the surface of 
 salt water is always positively electrified. Now, the atmosphere is 
 everywhere charged, more or less, with vapor which comes, almost 
 entirely, from the salt water of the ocean. Hence Pouillet inferred 
 that the chief source of atmospheric electricity is the evaporation of 
 sea water. This explanation would certainly go far to account for 
 the presence of electricity in the atmosphere, if the fact on which it 
 
LIGHTNING AND THUNDER. 19 
 
 i 
 rests were established beyond dispute. But there is some reason to 
 
 doubt whether the development of electricity, in the experiments of 
 Pouillet, was due simply to the process of evaporation, and not rather 
 to other causes, the influence of which he did not sufficiently take into 
 account. 
 
 A conjecture has recently been started that electricity may be gen- 
 erated by the mere impact of minute particles of water vapor against 
 minute particles of air. 1 If this conjecture could be established as a 
 fact, it would be amply sufficient to account for all the electricity of 
 the atmosphere. From the very nature of a gas, the molecules of 
 which it is composed are forever flying about with incredible velocity; 
 and therefore the particles of water vapor and the particles of air, 
 which exist together in the atmosphere, must be incessantly coming 
 into collision. Hence, however small may be the charge of electricity 
 developed at each individual impact, the total amount generated over 
 any considerable area, in a single day, must be very great indeed. It 
 is evident, however, that this method of explaining the origin of at- 
 mospheric electricity can only be regarded as, at best, a probable 
 hypothesis, until the assumption on which it rests is supported by the 
 evidence of observation or experiment. 
 
 Length of a Flash of Lightning. It would seem, then, that 
 we are not yet in a position to indicate with certainty the sources from 
 which the electricity of the atmosphere is derived. But whatever 
 these sources may be, there can be little doubt that the electricity of 
 the atmosphere is intimately associated with the minute particles of 
 water vapor of which the thundercloud is eventually built up. This 
 consideration is of great importance when we come to consider the 
 special properties of lightning, as compared with other forms of elec- 
 tricity. The most striking characteristic of lightning is the wonder- 
 ful power it possesses of forcing its way through the resisting me- 
 dium of the air. In this respect it incomparably surpasses all forms 
 of electricity that have hitherto been produced by artificial means. 
 The spark of an ordinary electric machine can leap across a space of 
 three or four inches ; the machine we have employed in our experi- 
 ments to-day can give, under favorable circumstances, a spark of nine 
 or ten inches ; the longest electric spark ever yet produced artificially 
 is probably the spark of Mr. Spottiswoode's gigantic induction coil ; 
 and it does not exceed three feet six inches. But the length of a flash 
 of lightning is not to be measured in inches, or in feet or in yards ; 
 it varies from one or two miles, for ordinary flashes, to eight or ten 
 miles in exceptional cases. 
 
 This power of discharging itself violently through a resisting me- 
 dium, in which the thundercloud so far transcends the conductor of 
 an electric machine, is due to the property commonly known among 
 
 1 Professor Tait, On Thunderstorms, Nature, vol. xxii., pp. 436-7. 
 
26 LIGHTNING AND THUNDER. 
 
 scientific men as electrical potential. The greater the distance to 
 which an electrified body can shoot its flashes through the air, the 
 higher must be its potential. Hence the potential of a thundercloud 
 must be exceedingly high, since its flashes can pierce the air to a dis- 
 tance of several miles. And what I want to point out is, that we are 
 able to account for this exceedingly high potential, if we may only 
 assume that the minute particles of water vapor in the atmosphere 
 have, from any cause, received ever so small a charge of electricity. 
 The number of such particles that go to make up an ordinary drop of 
 rain are to be counted by millions of millions ; and it is capable of 
 scientific proof that, as each new particle is added, in the building up 
 of the drop, a rise of potential is necessarily produced. It is clear, 
 therefore, that there is practically no limit to the potential that may 
 be developed by the simple agglomeration of very small cloud parti- 
 cles, each carrying a very small charge of electricity.' 
 
 This explanation, which traces the exceedingly high potential of 
 lightning to the building up of rain drops in the thundercloud, sug- 
 gests a reason why it so often happens that immediately after a flash 
 of lightning "the big rain comes dancing to the earth." The poten- 
 tial has been steadily rising as the drops have been getting larger and 
 larger, until at length the potential has become so high that the thun- 
 dercloud is able to discharge itself, and almost at the same moment 
 the drops have become so large that they can no longer be held aloft 
 against the attracting force of gravity. 
 
 Physical Cause of Thunder. Let us now proceed to consider 
 the phenomenon of thunder, which is so intimately associated with 
 lightning, and which, though perfectly harmless in itself, and though 
 never heard until the real danger is past, often excites more terror in 
 the mind than the lightning flash itself. The sound of thunder, like 
 that of the electric spark, is due to a disturbance caused in the air by 
 the electric discharge. The air is first expanded by the intense heat 
 that is developed along the line of discharge, and then it rushes back 
 again to fill up the partial vacuum which its expansion has produced. 
 This sudden movement gives rise to a series of sound waves, which 
 reach the ear in the form of thunder. But there are certain peculiar 
 characteristics of thunder which are deserving of special consid- 
 eration. 
 
 Rolling of Thunder. They maybe classified, I think, under two 
 heads. First, the sound of thunder is not an instantaneous report 
 like the sound of the electric spark it is a prolonged peal lasting, 
 sometimes, for several seconds. Secondly, each flash of lightning 
 gives rise, not to one peal only, but to a succession ( of peals following 
 one another at irregular intervals. These two phenomena, taken 
 together, produce that peculiar effect on the ear which is commonly 
 
 1 See note at the end of this Lecture, p. 26. 
 
LIGHTNING AND THUNDER. *i 
 
 described as the rolling of thunder ; and both of them, I think, may 
 be sufficiently accounted for in accordance with the well-established 
 properties of sound. 
 
 To understand why the sound of thunder reaches the ear as a pro- 
 longed peal, we have only to remember that sound takes time to 
 travel. Since a flash of lightning is practically instantaneous, we 
 may assume that the sound is produced at the same moment all along 
 the line of discharge. But the sound waves, setting out at the same 
 moment from all points along the line of discharge, must reach the 
 ear in successive instants of time, arriving first from that point which 
 is nearest to the observer, and last from that point which is most dis- 
 tant. Suppose, for example, that the nearest point of. the flash is a 
 mile distant from the observer, and the farthest point two miles the 
 sound will take about five seconds to come from the nearest point, 
 and about ten seconds to come from the farthest point ; and more- 
 over, in each successive instant from the time the first sound reaches 
 the ear, sound will continue to arrive from the successive points be- 
 tween. Therefore the thunder, though instantaneous in its origin, 
 will reach the ear as a prolonged peal extending over a period of five 
 seconds. 
 
 Succession of Peals. The succession of peals produced by a 
 single flash of lightning is due to several causes, each one of which 
 may contribute more or less, according to circumstances, toward the 
 general effect. First, if we accept the results arrived at by Professor 
 Ogden Rood, of Columbia College, what appears to the eye as a sin- 
 gle flash of lightning, consists, in fact, as a general rule, of a succes- 
 sion of flashes, each one of which must naturally produce its own peal 
 of thunder ; and although the several flashes, if they follow one an- 
 other at intervals of the tenth of a second, will make one continuous 
 impression on the eye, the several peals of thunder, under the same 
 conditions, will impress the ear as so many distinct peals. 
 
 The next cause that I would mention is the zigzag path of the 
 lightning discharge. To make clear to you the influence of this cir- 
 cumstance, I must ask your attention for a moment to the diagram 
 on next page. Let the broken line represent the path of a flash of 
 lightning, and let o represent the position of an observer. The sound 
 will reach him first from the point A, which is nearest to him, and then 
 it will continue to arrive in successive instants from the successive 
 points along the line A N and along the line A M, thus producing the 
 effect of a continuous peal. Meanwhile the sound waves have been 
 traveling from the point B, and in due time will reach the observer at 
 o. Coming as they do in a different direction from the former, they 
 will strike the ear as the beginning of a new peal which, in its turn, 
 will be prolonged by the sound waves arriving, in successive instants, 
 from the successive points along the line B M and B H. A little later, 
 
22 LIGHTNING AND THUNDER. 
 
 / 
 
 the sound will arrive from the more distant point c, and a third peal 
 will begin. And so there will be several distinct peals proceeding, so 
 to speak, from several distinct points in the path of the lightning 
 flash, 
 
 A third cause to which the succession of peals may be referred is to 
 be found in the minor electrical discharges that must often take place 
 within the thundercloud itself. A thundercloud is not a continuous 
 mass like the metal cylinder of this electric machine it has many 
 outlying fragments, more or less imperfectly connected with the prin- 
 cipal body. Moreover, the material of which the cloud is composed 
 is only a very imperfect conductor as compared with our brass cylin- 
 der. For these two reasons it must often happen, about the time a 
 flash of lightning passes, that different parts of the cloud will be in 
 such different electrical conditions as to give rise to electrical dis- 
 charges within the cloud itself. Each of these discharges produces 
 
 ORIGIN OF SUCCESSIVE PEALS OF THUNDER. 
 
 its own peal of thunder ; and thus we may have a number of minor 
 peals, sometimes preceding and sometimes following the great crash 
 which is due to the principal discharge. 
 
 Lastly, the influence of echo has often a considerable share in multi- 
 plying the number of peals of thunder. The waves of sound, going 
 forth in all directions, are reflected from the surfaces of mountains, 
 forests, clouds, and buildings, gnd coming back from different quar- 
 ters, and with varying intensity, reach the ear like the roar of distant 
 artillery. The striking effect of these reverberations in a mountain 
 district has been described by a great poet in words which, I daresay, 
 
 are familiar to most of you : 
 
 " Far along, 
 From peak to peak, the rattling crags among, 
 
 Leaps the live thunder ! Not from one lone cloud, 
 But every mountain now has found a tongue, 
 And Jura answers from her misty shroud 
 Back to the joyous Alps, that call to her aloud ! " 
 
LIGHTNING AND THUNDER. 23 
 
 Variations of Intensity in Thunder, From what has been said, 
 it is easy to understand how the general roar of thunder is subject to 
 great changes of intensity, during the time it lasts, according to the 
 number of peals that may be arriving at the ear of an observer in 
 each particular moment. But every one must have observed that 
 even an individual peal of thunder often undergoes similar changes, 
 swelling out at one moment with great power, and the next moment 
 rapidly dying away. To account for this phenomenon, I would ob- 
 serve, first, that there is no reason to suppose that the disturbance 
 caused by lightning is of exactly the same magnitude at every point 
 of its path. On the contrary, it would seem very probable that the 
 amount of this disturbance is, in some way, dependent on the resist- 
 ance which the discharge encounters. Hence the intensity of the 
 sound waves sent forth by a flash of lightning is probably very differ- 
 ent at different parts of its course ; and each individual peal will 
 swell out on the ear or die away, according to the greater or less in- 
 tensity of the sound waves that reach the ear in each successive 
 moment of time. 
 
 But there is another influence at work which must produce varia- 
 tions in the loudness of a peal of thunder, even though the sound 
 waves, set in motion by the lightning, were everywhere of equal in- 
 tensity. This influence depends on the position of the observer in 
 relation to the path of the lightning flash. At one part of its course 
 the lightning may follow a path which remains for a certain length 
 at nearly the same distance from the observer ; then all the sound 
 produced along this length will reach the observer nearly at the same 
 moment, and will burst upon the ear with great intensity. At another 
 part, the lightning may for an equal length go right away from the 
 observer ; and it is evident that the sound produced along this length 
 will reach the observer in successive instants, and consequently pro- 
 duce an effect comparatively feeble. 
 
 With a view to investigate this interesting question a little more 
 closely, let me suppose the position of the observer taken as a centre, 
 and a number of concentric circles drawn, cutting the path of the 
 lightning flash, and separated from one another by a distance of no 
 feet, measured along the direction of the radius. It is evident that 
 all the sound produced between any two consecutive circles will reach 
 the ear within a period which must be measured by the time that 
 sound takes to travel no feet, that is, within the tenth of a second. 
 Hence, in order to determine the quantity of sound that reaches the 
 ear in successive periods of one-tenth of a second, we have only to 
 observe how much is produced between each two consecutive cir- 
 cles. But on the supposition that the sound waves, set in motion by 
 the flash of lightning, are of equal intensity at every point of its 
 path, it is clear that the quantity of sound developed between each 
 
24 LIGHTNING AND THUNDER. 
 
 two consecutive circles will be simply proportional to the length of 
 the path enclosed between them. 
 
 With these principles established, let us now follow the course of a 
 peal of thunder, in the diagram before us. This broken line, drawn 
 almost at random, represents the path of a flash of lightning ; the 
 observer is supposed to be placed at o, which is the centre of the con- 
 centric circles ; these circles are separated from one another by a dis- 
 tance of no feet, measured in the direction of the radius; and we 
 want to consider how any one peal of thunder may vary in loudness 
 in the successive periods of one-tenth of a second. 
 
 Let us take, for example, the peal which begins when the sound 
 waves reach the ear from the point A. In the first unit of time the 
 sound that reaches the ear is the sound produced along the lines A B 
 and A c ; in the second unit, the sound produced along the lines B D 
 and c E ; in the third unit, the sound produced along D F and E G. 
 So far the peal has been fairly uniform in its intensity ; though there 
 has been a slight falling off in the second and third units of time, as 
 
 VARIATIONS OF INTENSITY IN A PEAL OF THUNDER. 
 
 compared with the first. But in the fourth unit there is a consider- 
 able falling away of the sound ; for the line F K is only about one- 
 third as long as D F and E G taken together ; therefore the quantity 
 of sound that reaches the ear in the fourth unit of time is only one- 
 third of that which reaches it in each of the three preceding units ; 
 and consequently the sound is only one-third as loud. In the fifth 
 unit, however, the peal must rise to a sudden crash ; for the portion 
 of the lightning path inclosed between the fifth and sixth circles is 
 about six times as great as that between the fourth and fifth ; there- 
 fore the intensity of the sound will be suddenly increased about six- 
 fold. After this sudden crash, the sound as suddenly dies away in 
 the sixth unit of time ; it continues feeble as the path of the light- 
 ning goes nearly straight away from the observer ; it swells again 
 slightly in the ninth unit of time ; and then continues without much 
 variation to the end. This is only a single illustration, but it seems 
 quite sufficient to show that the changes of intensity in a peal of 
 
LIGHTNING AND THUNDER. 25 
 
 thunder must be largely due to the position of the spectator in rela- 
 tion to the several parts of the lightning flash. 
 
 Distance of a Flash of Lightning. I need hardly remind 
 you that, by observing the interval that elapses between the flash of 
 lightning and the peal of thunder that follows it, we may estimate 
 approximately the distance of the nearest point of the discharge. 
 Light travels with such amazing velocity that we may assume, with- 
 out any sensible error, that we see the flash of lightning at the very 
 moment in which the discharge takes place. But sound, as we have 
 seen, takes a sensible time to travel even short distances ; and there- 
 fore a measurable interval almost always elapses between the moment 
 in which the flash is seen and the moment in which the peal of thunder 
 first reaches the ear. And the distance through which sound travels 
 in this interval will be the distance of the nearest point through which 
 the discharge has passed. Now, the velocity of sound in air varies 
 slightly with the temperature ; but, at the ordinary temperature of 
 our climate, we shall not be far astray if we allow 1,100 feet for every 
 second, or about one mile for every five seconds. 
 
 You will observe also that, by repeating this observation, we can 
 determine whether the thundercloud is coming toward us, or going 
 away from us. So long as the interval between each successive flash 
 and the corresponding peal of thunder, continues to get shorter and 
 shorter, the thundercloud is approaching ; when the interval begins 
 to increase, the thundercloud is receding from us, and the danger is 
 passed. 
 
 The crash of thunder is terrific when the lightning is close at hand ; 
 but it is a curious fact, that the sound does not seem to travel as far 
 as the report of an ordinary cannon. We have no authentic record of 
 thunder having been heard at a greater distance than from twelve to 
 fifteen miles, whereas the report of a single cannon has been heard at 
 five times that distance ; and the roar of artillery, in battle, at a 
 greater distance still. On the occasion of the Queen's visit to Cher- 
 bourg, in August, 1858, the salute fired in honor of her arrival was 
 heard at Bonchurch, in the Isle of Wight, a distance of sixty miles. 
 It was also heard at Lyme Regis, in Dorsetshire, which is eighty-five 
 miles from Cherbourg, as the crow flies ; and we are told that, not 
 only was it audible in its general effect, but the report of individual 
 guns was distinctly recognized. The artillery of Waterloo is said to 
 have been heard at the town of Creil, in France, 115 miles from the 
 field of battle ; and the cannonading at the siege of Valenciennes, in 
 1793, was heard, from day to day, at Deal, on the coast of England, 
 a distance of 120 miles. 1 
 
 So far, I have endeavored to set forth some general ideas on the 
 nature and origin of lightning, and of the thunder that accompanies 
 
 1 See Tomlinson, The Thunderstprm, pp. 87-9. 
 
26 LIGHTNING AND THUNDER. 
 
 it. In my next Lecture I propose to give a short account of the de- 
 structive effects of lightning, and to consider how these effects may 
 best be averted by means of lightning conductors. 
 
 NOTE TO PAGE 20. 
 
 ON THE HIGH POTENTIAL OF A FLASH OF LIGHTNING. 
 
 The potential of an electrified sphere is equal to the quantity of electricity with 
 which the sphere is charged, divided by the radius of the sphere. Now the minute 
 cloud particles, which go to make up a drop of rain, may be taken to be very small 
 spheres ; and if v represent the potential of each one, q the quantity of electricity with 
 which it is charged, and r the radius of the sphere, we have v = Suppose 1,000 
 of these cloud particles to unite into one ; the quantity of electricity in the drop, thus 
 formed, will be i.ooo q ; and the radius, which increases in the ratio of the cube root 
 of the volume, will be lor. Therefore the potential of the new sphere will be 
 1000 ?, or 100 ^ ; that is to say, it will be 100 times as great as the potential of each 
 of the cloud particles which compose it. When a million of cloud particles are blended 
 into a single drop, the same process will show that the potential has been increased 
 ten thousandfold ; and when a drop is produced by the agglomeration of a million of 
 millions of cloud particles, the potential of the drop will be a hundred million times 
 as great as that of the individual particles. 1 
 
 LECTURE II. 
 
 LIGHTNING CONDUCTORS. 
 
 THE effects of lightning, on the bodies that it strikes, are analo- 
 gous to those which may be produced by the discharge of our 
 electric machines and Leyden jar batteries. When the discharge of 
 a battery traverses a metal conductor of sufficient dimensions to allow 
 it an easy passage, it makes its way along silently and harmlessly. 
 But if the conductor be so thin as to offer considerable resistance, 
 then the conductor itself is raised to intense heat, and may be melted, 
 or even converted into vapor, by the discharge. 
 
 On opposite page is shown a board on which a number of very thin 
 wires have been stretched, over white paper, between brass balls. The 
 wires are so thin that the full charge of the battery before you, which 
 consists of nine large Leyden jars, is quite sufficient to convert them in 
 an instant into vapor. I have already, on former occasions, sent the 
 charge through two of these wires, and nothing remains of them now 
 
 1 See Tait on Thunderstorms, Nature, vol. xxii., p. 436. 
 
LIGHTNING AND THUNDER. 
 
 27 
 
 but the traces of their vapor, which mark the path of the electric dis- 
 charge from ball to ball. At the present moment the battery stands 
 ready charged, and I am going to discharge it through a third wire, by 
 means of this insulated rod which I hold in my hand. The discharge 
 
 DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES. 
 
 has passed ; you saw a flash, and a little smoke ; and now, if you look 
 at the paper, you will find that the wire is gone, but that it has left 
 behind the track of its incandescent vapor, marking the path of the 
 discharge. 
 
 Destruction of Buildings by Lightning. We learn from this 
 experiment that the electricity stored up in our battery passes, with- 
 out visible effect, through the stout wire of a discharging rod, but 
 that it instantly converts into vapor the thin wire stretched across the 
 spark board. And so it is with a flash of lightning. It passes harm- 
 lessly, as every one knows, through a stout metal rod, but when it 
 comes across bell wires or telegraph wires, it melts them, or converts 
 them into vapor. On the sixteenth of July, 1759, a flash of lightning 
 struck a house in Southwark, on the south side of London, and fol- 
 lowed the line of the bell wire. After the lightning had passed, the 
 wire was no longer to be found ; but the path of the lightning was 
 clearly marked by patches of vapor which were left, here and there, 
 adhering to the surface of the wall: In the year 1754, the lightning 
 fell on a bell tower at Newbury, in the United States of America, 
 and having dashed the roof to pieces, and scattered the fragments 
 about, it reached the bell. From this point it followed an iron wire, 
 about as thick as a knitting needle, melting it as it passed along, leav- 
 ing behind a black streak of vapor on the surface of the walls. 
 
 Again, the electric discharge, passing through a bad conductor, 
 produces mechanical disturbance, and, if the substance be combus- 
 tible, often sets it on fire. So, too, as you know, the lightning flash, 
 falling on a church spire, dashes it to pieces, knocking the stones 
 about in all directions, while it sets fire to ships and wooden buildings; 
 and more than once it has caused great devastation by exploding 
 powder magazines, 
 
28 LIGHTNING AND THUNDER. 
 
 Let me give you one or two examples : In January, 1762, the light- 
 ning fell on a church tower in Cornwall, and a stone three hundred- 
 weight was torn from its place and hurled to a distance of 180 feet, 
 while a smaller stone was projected as far as 1,200 feet from the 
 building. Again, in 1809, the lightning struck a house not far from 
 Manchester, and literally moved a massive wall twelve feet high and 
 three thick to a distance of several feet. You may form some con- 
 ception of the enormous force here brought into action, when I tell 
 you that the total weight of mason-work moved on this occasion was 
 not less than twenty-three tons. 
 
 The church of St. George, at Leicester, was severely damaged by 
 lightning on the ist of August, 1846. About 8 o'clock in the evening 
 the rector of the parish saw a vivid streak of light darting with incred- 
 ible velocity against the upper part of the spire. " For the distance of 
 forty feet on the eastern side, and nearly seventy on the west, the 
 massive stonework of the spire was instantly rent asunder and laid in 
 ruins. Large blocks of stone were hurled in all directions, broken 
 into small fragments, and in some cases, there is reason to believe, 
 reduced to powder. One fragment of considerable size was hurled 
 against the window of a house three hundred feet distant, shattering 
 to pieces the woodwork, and strewing the room within with fine dust 
 and fragments of glass. It has been computed that a hundred tons 
 of stone were, on this occasion, blown to a distance of thirty feet in 
 three seconds. In addition to the shivering of the spire, the pinnacles 
 at the angles of the tower were all more or less damaged, the flying 
 buttresses cracked through and violently shaken, many of the open 
 battlements at the base of the spire knocked away, the roof of the 
 church completely riddled, the roofs of the side entrances destroyed, 
 and the stone staircases of the gallery shattered." 1 
 
 Lightning has been at all times the cause of great damage to prop- 
 erty by its power of setting fire to whatever is combustible. Fuller 
 says, in his Church History, that "scarcely a great abbey exists in 
 England which once, at least, has not been burned by lightning from 
 heaven." He mentions, as examples, the Abbey of Croylarid twice 
 burned, the Monastery of Canterbury twice, the Abbey of Peterbor- 
 ough twice ; also the Abbey of St. Mary's, in Yorkshire, the Abbey of 
 Norwich, and several others. Sir William Snow Harris, writing about 
 twenty years ago, tells us that " the number of churches and church 
 spires wholly or partially destroyed by lightning is beyond all belief, 
 and would be too tedious a detail to enter upon. Within a compara- 
 tively few years, in 1822 for instance, we find the magnificent Cathedral 
 of Rouen burned, and, so lately as 1850, the beautiful Cathedral of 
 Saragossa, in Spain, struck by lightning during divine service and set 
 on fire. In March of last year a dispatch from our Minister at Brussels, 
 
 > The Thunderstorm, by Charles Tomlinson, F. R. S., Third Edition, pp. 153-4. 
 
LIGHTNING AND THUNDER. 29 
 
 Lord Howard de Walden, dated the 24th of February, was forwarded 
 by Lord Russell to the Royal Society, stating that, on the preceding 
 Sunday, a violent thunderstorm had spread over Belgium ; that twelve 
 churches had been struck by lightning ; and that three of these fine 
 old buildings had been totally destroyed." 1 
 
 Even in our own day the destruction caused by fires produced 
 through the agency of lightning is very great far greater than is 
 commonly supposed. No general record of such fires is kept, and 
 consequently our information on the subject is very incomplete and 
 inexact. I may tell you, however, one small fact which, so far as it 
 goes, is precise enough and very significant. In the little province of 
 Schleswig-Holstein, which occupies an area less than one-fourth of 
 the area of Ireland, the Provincial Fire Assurance Association has 
 paid in sixteen years, for damage caused by lightning, somewhat over 
 ^100,000, or at the rate of more than ^"6,000 a year. The total loss 
 of property every year in this province, due to fires caused by light- 
 ning, is estimated at not less than ^"i2,5oo. 2 
 
 Destruction of Ships at Sea. The destructive effects of light- 
 ning on ships at sea, before the general adoption of lightning con- 
 ductors, seems almost incredible at the present day. From official 
 records it appears that the damage done to the Royal Navy of Eng- 
 land alone involved an expenditure of from ^"6,000 to ,10,000 a 
 year. We are told by Sir William Snow Harris, who devoted himself 
 for many years to this subject with extraordinary zeal and complete 
 success, that between the year 1810 and the year 1815 that is, within 
 a period of five years " no less than forty sail of the line, twenty 
 frigates, and twelve sloops and corvettes were placed hors de combat 
 by lightning. In the merchant navy, within a comparatively small 
 number of years, no less than thirty-four ships, most of them large 
 vessels with rich cargoes, have been totally destroyed been either 
 burned or sunk to say nothing of a host of vessels partially destroyed 
 or severely damaged." 3 . 
 
 And these statements, be it observed, take no account of ships that 
 were simply reported as missing, some of which, we can hardly doubt, 
 were struck by lightning in the open sea, and went down with all 
 hands on board. A famous ship of forty-four guns, the Resistance, 
 was struck by lightning in the Straits of Malacca, and the powder 
 magazine exploding, she went to the bottom. Of her whole crew only 
 three were saved, who happened to be picked up by a passing boat. 
 It has been well observed that, were it not for these three chance sur- 
 vivors, nothing would have been known concerning the fate of the 
 vessel, and she would have been simply recorded as missing in the 
 Admiralty lists. 
 
 1 Two Lectures on Atmospheric Electricity and Protection from Lightning, published at the end of 
 his Treatise on Frictional Electricity, p. 273. 
 
 3 See Report of Lightning Rod Conference, p. 119. 8 Loco citato. 
 
30 LIGHTNING AND THUNDER. 
 
 Nothing is more fearful to contemplate than the scene on board a 
 ship when she is struck by lightning in the open sea, with the winds 
 howling around, the waves rolling mountains high, the rain coming 
 down in torrents, and the vivid flashes lighting up the gloom at inter- 
 vals, and carrying death and destruction in their track. I will read 
 you one or two brief accounts of such a scene, given in the pithy but 
 expressive language of the sailor. In January, 1786, the Thisbe, of 
 thirty-six guns, was struck by lightning off the coast of Scilly, and 
 reduced to the condition of a wreck. Here is an extract from the 
 ship's log : " Four A. M., strong gales ; handed mainsail and main top- 
 sail ; hove to with storm staysails; blowing very heavy, S. E. 4.15, 
 a flash of lightning, with tremendous thunder, disabled some of our 
 people. A second flash set the mainsail, main-top, and mizen stay- 
 sails on fire. Obliged to cut away the mainmast ; this carried away 
 mizen top-mast and fore top-sail yard. Found foremast also shivered 
 by the lightning. Fore top-mast went over the side about 9 A. M. 
 Set the foresail." 1 
 
 A few years later, in March, 1796, the Lowestoffe was struck in the 
 Mediterranean, and we read as follows in the log of the ship : " North 
 end of Minorca ; heavy squalls ; hail, rain, thunder, and lightning. 
 12.15, ship struck by lightning, which knocked three men from the mast- 
 head, one killed. 12.30, ship again struck; main top-mast shivered in 
 pieces ; many men struck senseless on the decks. Ship again struck, 
 and set on fire in the masts and rigging ; mainmast shivered in pieces ; 
 fore top-mast shivered ; men benumbed on the decks, and knocked 
 out of the top ; one man killed on the spot. 1.30, cut away the main- 
 mast ; employed clearing wreck. 4, moderate; set the foresail." 8 
 
 Again, in 1810, the Repulse, a ship of seventy-four guns, was struck, 
 off the coast of Spain. "The wind had been variable in the morning- 
 and at 12.35 there was a heavy squall, with rain, thunder, and light- 
 ning. The ship was struck by two vivid flashes of lightning, which 
 shivered the maintop-gallant mast, and severely damaged the main- 
 mast. Seven men were killed on the spot ; three others only survived 
 a few days ; and ten others were maimed for life. After the second 
 discharge the rain fell in torrents. The ship was more completely 
 crippled than if she had been in action, and the squadron, then en- 
 gaged on a critical service, lost for a time one of its fastest and best 
 ships." 1 
 
 Destruction of Powder Magazines. Not less appalling is 
 the devastation caused by lightning when it falls on a powder maga- 
 zine. Here is a striking example : On the eighteenth of August, 
 1769, the tower of St. Nazaire, at Brescia, was struck by lightning. 
 Underneath the tower about 200,000 pounds of gunpowder, belonging 
 
 1 Sir William Snow Harris, loco citato^ p. 274. 
 
 3 Id., p. 275. 
 
 * The Thunderstorm, by Charles Toralinson, F.R.S., Third Edition, p. 172. 
 
LIGHTNING AND THUNDER. 31 
 
 to the Republic of Venice, were stored in vaults. . The powder ex- 
 ploded, leveling to the ground a great part of the beautiful city of 
 Brescia, and burying thousands of its inhabitants in the ruins. It is 
 said that the tower itself was blown up bodily to a great height in 
 the air, and came down in a shower of stones. This is, perhaps, the 
 most fearful disaster of the kind on record. But we are not without 
 examples in our own times. In the year 1856 the lightning fell on the 
 Church of St. John, in the Island of Rhodes. A large quantity of 
 gunpowder had been deposited in the vaults of the church. This was 
 ignited by the flash ; the building was reduced to a mass of ruins, a 
 large portion of the town was destroyed, and a considerable number 
 of the inhabitants were killed. Again, in the following year, the 
 magazine of Joudpore, in the Bombay Presidency, was struck by 
 lightning. Many thousand pounds of gunpowder were blown up, five 
 hundred houses were destroyed, and nearly a thousand people are 
 said to have been killed. 1 
 
 Experimental Illustrations. And now, before proceeding fur- 
 ther, I will make one or two experiments, with a view of showing that 
 the electricity of our machines is capable of producing effects similar 
 to those produced by lightning, though immeasurably inferior in point 
 of magnitude. Here is a common tumbler, about three-quarters full 
 of water. Into it I introduce two bent rods of brass, which are care- 
 fully insulated below the surface of the water by a covering of india- 
 rubber. The points, however, are exposed, and come to within an inch 
 of one another, near the bottom of. the tumbler. Outside the tum- 
 bler, the brass rods are mounted on a stand, by means of which I can 
 send the full charge of this Leyden jar battery through the water, 
 from point to point. Since water is a bad conductor of electricity, 
 as compared with metals, the charge encounters great resistance in 
 passing through it, and in overcoming this resistance produces con- 
 siderable mechanical commotion, which is usually sufficient to shiver 
 the glass to pieces. 
 
 To charge the battery will take about twenty turns of this large 
 Holtz machine. Observe how the pith ball of the electroscope rises 
 as the machine is worked, showing 'that the charge is going in. And 
 now it remains stationary ; which is a sign that the battery is fully 
 charged, and can receive no more. You will notice that the outside 
 coating of the battery has been already connected with one of the 
 brass rods dipping into the tumbler of water. By means of this dis- 
 charger I will now bring the inside coating into connection with the 
 other rod. And see, before contact is actually made, the spark has 
 leaped across, and our tumbler is violently burst asunder from top to 
 bottom. 
 
 1 See for these facts, Anderson, Lightning Conductors, p. 197 ; 1'omlinson, The Thunderstorm, pp. 
 167-9 ; Harris, loco citato, pp. 273-4. 
 
3* LIGHTNING AND THUNDER. 
 
 This will probably appear to you a very small affair, when com- 
 pared with the tearing asunder of solid masonry, and the hurling 
 about of stones by the ton weight. No doubt it is ; and that is just 
 one of the lessons we have to learn from the experiment we have 
 made. For, not only does it show us that effects of this kind may be 
 caused by electricity artificially produced, but it brings home forcibly 
 to the mind how incomparably more powerful is the lightning of the 
 clouds than the electricity of our machines. 
 
 The property which electricity has of setting fire to combustible 
 substances may be easily illustrated. This india rubber tube is con- 
 nected with the gas pipe under the floor, and to the end of the tube is 
 fitted a brass stop-cock which I hold in my hand. I open the cock, 
 
 GLASS VESSEL BROKEN MY DISCHARGE OF LEYUEN JAK BATTKKY. 
 
 and allow the jet of gas to flow toward the conductor of Carry's ma- 
 chine, while my assistant turns the handle ; a spark passes, and the 
 gas is lit. Again, my assistant stands on this insulating stool, placing 
 his hand on the large conductor of the machine, while I turn the han- 
 dle. His body becomes electrified, and when he presents his knuckle 
 to this vessel of spirits of wine, which is electrically connected with 
 the earth, a spark leaps across, and the spirits of wine are at once in 
 a blaze. Once more ; I tie a little gun-cotton around one knob of 
 the discharging rod, and then use it to discharge a small Leyden jar ; 
 at the moment of the discharge the gun-cotton is set on fire. 
 
 It would be easy to explode gunpowder with the electric spark, 
 but the smoke of the explosion would make the lecture-hall very un- 
 pleasant for the remainder of the lecture. I propose, therefore, to 
 
LIGHTNING AND THUNDER. 33 
 
 substitute for gunpowder an explosive mixture of oxygen and hydro- 
 gen, with which I have filled this little metal flask, commonly known 
 as Volta's pistol. By a very simple contrivance, the electric spark is 
 discharged through the mixture, when I hold the flask toward the 
 conductor of the machine. A cork is fitted tightly into the neck of 
 the flask, and at the moment the spark passes you hear a loud explo- 
 sion, and you see the cork driven violently up to the ceiling. 
 
 Destruction of Life. The last effect of lightning to which 1 
 shall refer, and which, perhaps, more than any other, strikes us with 
 terror, is the sudden and utter extinction of life, when the lightning 
 flash descends on man or on beast. So swift is this effect, in most 
 cases, that death is, in all probability, absolutely painless, and the vic- 
 tim is dead before he can feel that he is struck. I cannot give you, 
 with any degree of exactness, the number of people killed every year 
 by lightning, because the record of such deaths has been hitherto very 
 imperfectly kept, in almost all countries, and is, beyond doubt, very 
 
 GUN-COTTON SET ON FIRE BY ELECTRIC SPARK. 
 
 incomplete. But perhaps you will be surprised to learn that the num- 
 ber of deaths by lightning actually recorded is, on an average, in 
 England about 22 every year, in France 80, in Prussia no, in Austria 
 212, in European Russia 440.' 
 
 So far as can be gathered from the existing sources of information, 
 it would seem that the number of persons killed by lightning is, on 
 the whole, about one in three of those who are struck. The rest are 
 sometimes only stunned, sometimes more or less burned, sometimes 
 made deaf for a time, sometimes partially paralyzed. On particular 
 occasions, however, especially when the lightning falls on a large as- 
 sembly of people, the number of persons struck down and slightly in- 
 jured, in proportion to the number killed, is very much increased. 
 
 An interesting case of this kind is reported by Mr. Tomlinson. 
 " On the twenty-ninth of August, 1847, at the parish church of Welton, 
 
 1 See Anderson, Lightning Conductors, pp. 170-5. 
 
34 LIGHTNING AND THUNDER. 
 
 Lincolnshire, while the congregation were engaged in singing the 
 hymn before the sermon, and the Rev. Mr. Williamson had just as- 
 cended the pulpit, the lightning was seen to enter the church from 
 the belfry, and instantly an explosion occurred in the centre of the 
 edifice. All that could move made for the door, and Mr. Williamson 
 descended from the pulpit, endeavoring to allay the fears of the peo- 
 ple. But attention was now called to the fact that several of the 
 congregation were lying in different parts of the church, apparently 
 dead, some of whom had their clothing on fire. Five women were 
 found injured, and having their faces blackened and burned, and a 
 boy had his clothes almost entirely consumed. A respected old par- 
 ishioner, Mr. Brownlow, aged sixty-eight, was discovered lying at the 
 bottom of his pew, immediately beneath one of the chandeliers, quite 
 dead. There were no marks on the body, but the buttons of his 
 
 VOLTA'S PISTOL ; EXPLOSION CAUSED BY ELECTRIC SPARK. 
 
 waistcoat were melted, the right leg of his trousers torn down, and his 
 coat literally burnt off. His wife in the same pew received no injury." ' 
 Not less striking is the story told by Dr. Plummer, surgeon of the 
 Illinois Volunteers, in the Medical and Surgical Reporter of June 19, 
 1865: "Our regiment was yesterday the scene of one of the most 
 terrible calamities which it has been my lot to witness. About two 
 o'clock a violent thunderstorm visited us. While the old guard was 
 being turned out to receive the new, a blinding flash of lightning was 
 seen, accompanied instantly by a terrific peal of thunder. The whole 
 of the old guard, together with part of the new, were thrown violently 
 to the earth. The shock was so severe and sudden that, in most 
 cases, the rear rank men were thrown across the front rank men. One 
 
 1 The Thunderstorm, pp. 158-9. See also an account of four persons who were struck on the Mat- 
 terhorn, in July, 1869, all of whom were hurt, and none killed : Whymper's Scrambles Among the Alps, 
 PP- 4H, 415- 
 
LIGHTNING AND THUNDER. 
 
 35 
 
 man was instantly killed, and thirty-two men were more or less se- 
 verely burned by the electric fluid. In some instances the men's 
 boots and shoes were rent from their feet and torn to pieces, and, 
 strange as it may appear, the men were injured but little in the feet. 
 In all cases the burns appear as if they had been caused by scalding- 
 hot water, in many instances the skin being shriveled and torn off. 
 The men all seem to be doing well, and a part of them will be able to 
 resume their duties in a few days." 
 
 The Return Shock. It sometimes happens that people are 
 struck down and even killed at the moment a discharge of lightning 
 takes place between a cloud and the earth, though they are very far 
 from the point where the flash is actually seen to pass ; while others, 
 who are situated between them and the lightning, suffer very little, or 
 perhaps not at all. This curious phenomenon was first carefully in- 
 vestigated by Lord Mahon in the year 1779, and was called by him 
 
 THE RETURN SHOCK ILLUSTRATED. 
 
 the "return shock." His theory, which is now commonly accepted, 
 may be easily understood with the aid of the sketch before you. 
 
 Let us suppose ABC to represent the outline of a thundercloud 
 which dips down toward the earth at A and at c. The electricity of 
 the cloud develops by inductive action a charge of the opposite kind 
 in the earth beneath it. But the inductive action is most powerful at 
 E and F, where the cloud comes nearest to the earth. Hence, bodies 
 situated near these points may be very highly electrified as compared 
 with bodies at a point between them, such as D. Now, when a flash 
 of lightning passes at E, the under part of the cloud is at once relieved 
 of its electricity, its inductive action ceases, and, therefore, a person 
 situated at F suddenly ceases to be electrified. This sudden change 
 from a highly electrified to a neutral state involves a shock to his 
 system which may be severe enough to stun or even to kill him. 
 
36 LIGHTNING AND THUNDER. 
 
 Meanwhile, people at r>, having been also electrified to some extent 
 by the influence of the thundercloud, must in like manner undergo a 
 change in their electrical condition when the flash of lightning passes, 
 but this change will be less violent because they were less highly 
 electrified. 
 
 Many experiments have been devised to illustrate this theory of 
 Lord Mahon. But the best illustration I know is furnished by this 
 electric machine of Carre's. If you stand near one end of the large 
 conductor when the machine is in action and sparks are taken from 
 the other end, you will feel a distinct electric shock every time a spark 
 passes. The large conductor here takes the place of the cloud, the 
 spark that passes at one end represents the flash of lightning, and the 
 observer at the other end gets the return shock, though he is at a 
 considerable distance from the point where the flash is seen. 
 
 An experiment of this kind, of course, cannot be made sensible to 
 a large audience like the present. But I can give you a good idea of 
 the effect by means of this tuft of colored papers. While the machine 
 is in action I hold the tuft of papers near that end of the conductor 
 which is farthest from the point where the discharge takes place. 
 You see the paper ribbons are electrified by induction, and, in virtue, 
 of mutual repulsion, stand out from one another "like quills upon the 
 fretful porcupine." But, when a spark passes, the inductive action 
 ceases, the paper ribbons cease to be electrified, and the whole tuft 
 suddenly collapses into its normal state. 
 
 While fully accepting Lord Mahon's theory of the return shock 
 as perfectly good so far as it goes, I would venture to point out an- 
 other influence which must often contribute largely to produce the 
 effect in question, and which is not dependent on the form of the 
 cloud. It may easily happen, from the nature of the surface in the 
 district affected by a thundercloud, that the point of most intense 
 electrification say E in the figure is in good electrical communica- 
 tion with a distant point, such as F, while it is very imperfectly con- 
 nected with a much nearer point, D. In such a case it is evident that 
 bodies at F will share largely in the highly-electrified condition of E, 
 and also share largely in the sudden change of that condition the 
 moment the flash of lightning passes ; whereas bodies at D will be less 
 highly electrified before the discharge, and less violently disturbed 
 when the discharge takes place. 
 
 This principle may be illustrated by a very simple experiment. 
 Here is a brass chain about twenty feet long. One end of it I hand 
 to any one among the audience who will kindly take hold of it ; the 
 other end I hold in my hand. I now stand near the conductor of the 
 machine ; and will ask some one to stand about ten feet away from 
 me, near the middle of the chain, but without touching it. Now ob- 
 serve what happens when the machine is worked and I take a spark 
 
LIGHTNING AND THUNDER. 37 
 
 from the conductor : My friend at the far end of the chain, twenty 
 feet away, gets a shock nearly as severe as. the one I get myself, be- 
 cause he is in good electrical communication with the point where the 
 discharge takes place. But my more fortunate friend, who is ten feet 
 nearer to the flash, is hardly sensible of any effect, because he is con- 
 nected with me only through the floor of the hall, which is, compara- 
 tively speaking, a bad conductor of electricity. 
 
 Summary. Let me now briefly sum up the chief destructive 
 effects of lightning. First, with regard to good conductors : though 
 it passes harmlessly through them if they be large enough to afford it 
 an easy passage, it melts and converts them into vapor if they be of 
 such small dimensions as to offer considerable resistance. Secondly, 
 lightning acts with great mechanical force on bad conductors ; it is 
 capable of tearing asunder large masses of masonry, and of project- 
 ing the fragments to a considerable distance. Thirdly, it sets fire to 
 combustible materials. And lastly, it causes the instantaneous death 
 of men and animals. 
 
 Franklin's Lightning Rods. The object of lightning con- 
 ductors is to protect life and property from these destructive effects. 
 Their use was first suggested by Franklin, in 1749, even before his 
 famous experiment with the kite ; and immediately after that experi- 
 ment, in 1752, he set up, on his own house, in Philadelphia, the first 
 lightning conductor ever made. He even devised an ingenious con- 
 trivance, by means of which he received notice when a thundercloud 
 was approaching. The contrivance consisted of a peal of bells, which 
 he hung on his lightning conductor, and which were set ringing when- 
 ever the lightning conductor became charged with electricity. 
 
 Franklin's lightning rods were soon adopted in America ; and he 
 himself contributed very much to their popularity by the simple and 
 lucid instructions he issued every year, for the benefit of his country- 
 men, in the annual publication known as "Poor Richard's Almanac." 
 It is very interesting at this distance of time to read the homely prac- 
 tical rules laid down by this great philosopher and statesman ; and, 
 though some modifications have been suggested by the experience of 
 a hundred and thirty years, especially as regards the dimensions of 
 the lightning conductor, it is surprising to find how accurately the 
 general principles of its construction, and of its action, are here set 
 forth. 
 
 "It has pleased God," he says, "in His goodness to mankind, at 
 length to discover to them the means of securing their habitations 
 and other buildings from mischief by thunder and lightning. The 
 method is this : Provide a small iron rod, which may be made of the 
 rod-iron used by nailors, but of such a length that one end being 
 three or four feet in the moist ground, the other may be six or eight 
 feet above the highest part of the building. To the upper end of the 
 
38 LIGHTNING AND THUNDER. 
 
 rod fasten about a foot of brass wire, the size of a common knitting 
 needle, sharpened to a fine point ; the rod may be secured on the 
 house by a few small staples. If the house or barn be long, there 
 may be a rod and point at each end, and a middling wire along the 
 ridge from one to the other. A house thus furnished will not be dam- 
 aged by lightning, it being attracted by the points and passing through 
 the metal into the ground, without hurting anything. Vessels also 
 having a sharp-pointed rod fixed on the top of their masts, with a 
 wire from the foot of the rod reaching down round one of the shrouds 
 to the water, will not be hurt by lightning." 
 
 Introduction of Lightning Rods into England. The pro- 
 gress of lightning conductors was more slow in England and on the 
 Continent of Europe, owing to a fear, not unnatural, that they might, 
 in some cases, draw down the lightning where it would not otherwise 
 have fallen. People preferred to take their chance of escaping as 
 they had escaped before, rather than invite, as it were, the lightning 
 to descend on their houses, in the hope that an iron rod would convey 
 it harmless to the earth. But the immense amount of damage done every 
 year by lightning, soon led practical men to entertain a proposal which 
 offered complete immunity from all danger on such easy terms ; and 
 when it was found that buildings protected by lightning conductors 
 were, over and over again, struck by lightning without suffering any 
 harm, a general conviction of their utility was gradually established 
 in the public mind. 
 
 The first public building protected by a lightning rod in England 
 was St. Paul's Cathedral, in London. On the eighteenth of June, 
 1764, the beautiful steeple of Saint Bride's Church, in the city, was 
 struck by lightning and reduced to ruin. This incident awakened the 
 attention of the dean and chapter of St. Paul's to the danger of a 
 similar calamity, which seemed, as it were, impending over their own 
 church. After long deliberation, they referred the matter to the Royal 
 Society, asking for advice and instruction. A committee of scientific 
 men was appointed by the Royal Society to consider the question. 
 Benjamin Franklin himself, who happened to be in London at the 
 time, as the representative of the American States in their dispute 
 with England, was nominated a member of the committee. And the 
 result of its deliberation was that, in the year 1769, a number of 
 lightning conductors were erected on St. Paul's Cathedral. 
 
 It was on this occasion that arose the celebrated controversy about 
 the respective merits of points and balls. Franklin had recommended 
 a pointed conductor ; but some members of the committee were of 
 opinion that the conductor should end in a ball and not in a point. 
 The decision of the committee was in favor of Franklin's opinion, and 
 pointed conductors were accordingly adopted for St. Paul's Cathedral. 
 But the controversy did not end here. The time was one of great 
 
LIGHTNING AND THUNDER. 39 
 
 political excitement, and party spirit infused itself even into the peace- 
 ful discussions of science. The weight of scientific opinion was on 
 the side of Franklin ; but it was hinted, on the other side, that the 
 pointed conductors were tainted with republicanism, and pregnant 
 with danger to the empire. As a rule, the whigs were strongly in 
 favor of points ; while the Tories were enthusiastic in their support of 
 balls. 
 
 For a time the Tories seemed to prevail. The king was on their 
 side. Experiments on a grand scale were conducted in his presence, 
 at the Pantheon, a large building in Oxford street ; he was assured 
 that these experiments proved the great superiority of balls over 
 points ; and to give practical effect to his convictions, his majesty 
 directed that a large cannon ball should be fixed on the end of the 
 lightning conductor attached to the royal palace at Kew. But the 
 committee of the Royal Society remained unconvinced. In course of 
 time the heat of party spirit abated; experience as well as reason 
 was found to be in favor of Franklin's views ; and the battle of the 
 balls and points has long since passed into the domain of history. 1 
 
 Functions of a Lightning Conductor. A lightning conductor 
 fulfills two functions. First, it favors a silent and gradual discharge of 
 electricity between the cloud and the earth, and thus tends to prevent 
 that accumulation which must of necessity take place before a flash 
 of lightning will pass. Secondly, if a flash of lightning come, the 
 lightning conductor offers it a safe channel through which it may pass 
 harmless to the earth. 
 
 These two functions of a lightning conductor may be easily illus- 
 trated by experiment. When our machine is in action, if I present 
 my closed hand to the large brass conductor, a spark passes between 
 them, and I feel, at the same moment, a slight electric shock. Here 
 the conductor of the machine, as usual, holds the place of the electri- 
 fied cloud ; my closed hand represents, as it were, a lofty building 
 that stands out prominently on the surface of the earth ; the spark is 
 the flash of lightning, and the electric shock just suggests the de- 
 structive power of the sudden disruptive discharge. 
 
 Now let me protect this building by a lightning conductor. For 
 this purpose, I take in my hand a brass rod, which I connect with the 
 earth by a brass chain. In the first instance, I will have a metal ball 
 on the end of my lightning conductor. You see the effect ; sparks 
 pass rapidly, but I feel no shock. I can increase the strength of the 
 discharge by hanging this condensing jar on the conductor of the ma- 
 chine. Sparks pass now, much more brilliant and powerful than be- 
 fore, but still I get no shock. It is evident, therefore, that my light- 
 ning rod does not prevent the flash from passing, but it conveys it 
 harmless to the ground. 
 
 1 See Philosophical Transactions of the Royal Society, 1773, p. 42, and 1778, part i., p. 232 ; Ander- 
 son's Lightning Conductors, pp. 40-2 ; Lightning Rod Conference, pp. 76-9. 
 
40 LIGHTNING AND THUNDER. 
 
 . 
 
 I next take a rod which is sharply pointed, and connecting it as 
 fore with the earth by a brass chain, I present the sharp point to the 
 conductor of the machine. Observe how different is the result ; there 
 is no disruptive discharge : no spark passes ; no shock is felt. Elec- 
 tricity still continues to be generated in the machine, and electricity 
 is generated, by induction, in the brass rod, and in my body. But 
 these two opposite electricities discharge themselves silently, by means 
 of this pointed rod, and no sensible effect of any kind is exhibited. 
 
 These experiments are very simple, but they really put before us, 
 in the clearest possible way, the whole theory of lightning conductors. 
 In particular, they give us ocular demonstration that an efficient light- 
 ning rod not only makes the lightning harmless when it comes, but 
 tends very much to prevent its coming. A remarkable example, on a 
 large scale, of this important property, is furnished by the town of 
 Pietermaritzburg, the capital of the colony of Natal, in South Africa. 
 This town is subject to the frequent visitation of thunderstorms, at 
 certain seasons of the year, and much damage was formerly done by 
 lightning, but since the erection of lightning conductors on the prin- 
 cipal buildings, the lightning has never fallen within the town. 
 Thunderclouds come as before, but they pass silently over the city, 
 and only begin to emit their lightning flashes when they reach the 
 open country, and have passed beyond the range of the lightning 
 conductors. 1 
 
 But it will often happen, even in the case of a pointed conductor, 
 that the accumulation of electricity goes on so fast that the silent dis- 
 charge is insufficient to keep it in check. A disruptive discharge will 
 then take place, from time to time, and a flash of lightning will pass. 
 Under these circumstances, the lightning conductor is called upon to 
 fulfill its second function, and to convey the lightning harmless to the 
 earth. 
 
 Conditions of a Lightning Conductor. From the considera- 
 tion of the functions which it has to fulfill, we may now infer what are 
 the conditions necessary for an efficient lightning conductor. The 
 first condition is that the end of the conductor, projecting into the air, 
 should have, at least, one sharp point. Our experiments have shown 
 us that a pointed conductor tends, in a manner, to suppress the flash 
 of lightning altogether ; whereas a blunt conductor, or one ending in 
 a ball, tends only to make it harmless when it comes. It is evident, 
 therefore, that the pointed conductor offers the greater security. 
 
 But a fine point is very liable to be melted when the lightning falls 
 upon it, and thus to be rendered less efficient for future service. To 
 meet this danger, it has recently been suggested, by the Lightning 
 Rod Conference, that the extreme end of the conductor should be a 
 blunt point, destined to receive the full force of the lightning flash, 
 
 1 See A Lecture on Thunderstorms, by Professor Tait of Edinburgh, published in Nature, vol. 
 xxii., p. 365. 
 
LIGHTNING AND THUNDER. 41 
 
 when it comes ; and that, a little lower down, a number of very fine 
 points should be provided, with a view to favor the silent discharge. 
 This suggestion, which appears admirably fitted to provide for the 
 twofold function of a lightning conductor, deserves to be recorded in 
 the exact terms of the official report. 
 
 " It seems best to separate the double functions of the point, pro- 
 longing the upper terminal to the very summit, and merely beveling 
 it off, so that, if a disruptive discharge does take place, the full con- 
 conducting power of the rod may be ready to receive it. At the same 
 time, having regard to the importance of silent discharge from sharp 
 points, we suggest that, at one foot below the extreme top of the 
 upper terminal, there be firmly attached, by screws and solder, a cop- 
 per ring bearing three or four copper needles, each six inches long, 
 and tapering from a quarter of an inch diameter to as fine a point as 
 can be made ; and with the object of rendering the sharpness as per- 
 manent as possible, we advise that they be platinized, gilded, or 
 nickel plated." 1 
 
 The second condition of a lightning conductor is, that it should be 
 made of such material, and of such dimensions, as to offer an easy 
 passage to the greatest flash of lightning likely to fall on it ; other- 
 wise it might be melted by the discharge, and the lightning, seeking 
 for itself another path, might force its way through bad conductors, 
 which it would partly rend asunder, and partly consume by fire. Cop- 
 per is now generally regarded as the best material for lightning con- 
 ductors, and it is almost universally employed in these countries. If 
 it is used in the form of a rope, it should not be less than half an inch 
 in diameter ; if a band of copper is preferred and it is often found 
 more convenient by builders it should be about an inch and a half 
 broad and an eighth of an inch thick. In France it has been hitherto 
 more usual to employ iron rods for lightning conductors, but since 
 iron is much inferior to copper in its conducting power, the iron rod 
 must be of much larger dimensions ; it should be at least one inch in 
 diameter. 2 
 
 The third condition is that the lightning conductor should be 
 continuous throughout its whole length, and should be placed in good 
 electrical contact with the earth. This is a condition of the first im- 
 portance, and experience has shown that it is the one most likely of 
 all to be neglected. In a -large town the best earth connection is fur- 
 nished by the system of water-mains and gas-mains, each of which 
 constitutes a great network of conductors everywhere in contact with 
 
 1 Report of the Lightning Rod Conference, p. 4. 
 
 3 The dimensions here set forth are greater in some respects than those " recommended as a mini- 
 mum" in the report of the Lightning Rod Conference, page 6. But it will be oboeryed by those who 
 consult the report that the minimum recommended is just the size which, in the preceding paragraph of 
 the report, is said to have been actually melted by a flash of lightning ; and, therefore, it seems not to 
 be a very safe minimum. It wiii be also seen that there is some confusion in the figures given, and that 
 they contradict one another. For the dimensions of iron rods, see the instructions adopted by the 
 Academy of Science, Paris, May ao, 1875 ; Lightning Rod Conference, pp. 67-8. 
 
42 LIGHTNING AND THUNDER. 
 
 the earth. Two points, however, must be carefully attended to first, 
 that the electrical contact between the lightning conductor and the 
 metal pipe should be absolutely perfect ; and, secondly, that the pipe 
 selected should be of such large dimensions as to allow the lightning 
 an easy passage through it to the principal main. 
 
 If no such system of water-pipes or gas-pipes is at hand, then the 
 lightning rod should be connected with moist earth by means of a 
 bed of charcoal or a metal plate not less than three feet square. This 
 metal plate should be always of the same material as the conductor, 
 otherwise a galvanic action would be set up between the two metals, 
 which in course of time might seriously damage the contact. Dry 
 earth, sand, rock, and shingle are bad conductors ; and, if such mate- 
 rials exist near the surface of the earth, the lightning rod must pass 
 through them and be carried down until it reaches water or perma- 
 nently damp earth. 
 
 Mischief Done by Bad Conductors. If the earth contact is 
 bad, a lightning conductor does more harm than good. It invites the 
 lightning down upon the building without providing for it, at the same 
 time, a free passage to earth. The consequence is that the lightning 
 forces a way for itself, violently bursting asunder whatever opposes 
 its progress, and setting fire to whatever is combustible. 
 
 I will give you some recent and striking examples. In the month 
 of May, 1879, the church of Laughton-en-le-Morthen, in England, 
 though provided with a conductor, was struck by lightning and sus- 
 tained considerable damage. On examination it was found that the 
 lightning followed the conductor down along the spire as far as the 
 roof ; then, changing its course, it forced its way through a buttress 
 of. massive masonwork, dislodging about two cartloads of stones, and 
 leaped over to the leads of the roof, about six feet distant. It now 
 followed the leads until it came to the cast-iron down-pipes intended 
 to discharge the rain-water, and through these it descended to the 
 earth. When the earth contact of the lightning conductor was ex- 
 amined, it was found exceedingly deficient. The rod was simply bent 
 underground, and buried in dry loose rubbish at a depth not exceed- 
 ing eighteen inches. This is a very instructive example. The light- 
 ning had a choice of two paths one by the conductor prepared for 
 it, the other by the leads of the roof and the down-pipes and, by a 
 kind of instinct which, however we may explain, we must always con- 
 template with wonder, it chose the path of least resistance, though in 
 doing so it had to burst its way at the outset through a massive wall 
 of solid masonry. 1 
 
 On the 5th of June, in the same year, a flash of lightning struck the 
 house of Mr. Osbaldiston, near Sheffield, and, notwithstanding the 
 supposed protection of a lightning conductor, it did damage to the 
 
 1 See letter of Mr. R. S. Newall, F. R. S M in the Times , May 30, 1879. 
 
LIGHTNING AND THUNDER. 43 
 
 amount of about five hundred pounds. The lightning here followed 
 the conductor to a point about nine feet from the ground, then passed 
 through a thick wall to a gas-pipe at the back of the drawing-room mir- 
 ror. It melted the gas-pipe, set fire to the gas, smashed the mirror to 
 atoms, broke the Sevres vases on the chimney-piece, and dashed the 
 furniture about. In this case, as in the former, it was found that the 
 earth contact was bad; and, in addition, the conductor itself was of too 
 small dimensions. Hence, the electric discharge found an easier path 
 to earth through the gas-pipes, though to reach them it had to force 
 for itself a passage through a resisting mass of non-conductors. 1 
 
 Again in the same year, on the 28th of May, the house of Mr. 
 Tomes, of Caterham, was struck by lightning, and some slight dam- 
 age was done. After a careful examination it was found that the 
 greater part of the discharge left the lightning conductor with which 
 the house was provided, and passed over the slope of the roof to an 
 attic room, into which it forced its way through a brick wall, and 
 reached a small iron cistern. This cistern was connected by an iron 
 pipe of considerable dimensions with two pumps in the basement 
 story ; and through them the lightning found an easy passage to the 
 earth, and did but little harm on its way. When the earth contact of 
 the lightning conductor was examined, it was discovered that the end 
 of the rod was simply stuck into a dry chalky soil to a depth of about 
 twelve inches. Thus in this case, as in the two former, it was made 
 quite clear that the lightning conductor failed to fulfill its functions 
 because the earth contact was bad. 2 
 
 Cases are not uncommon in which builders provide underground a 
 carefully constructed reservoir of water, into which the lower end of 
 the lightning rod is introduced. The idea seems to prevail that a 
 reservoir of water constitutes a good earth contact ; and this is quite 
 true of a natural reservoir, such as a lake, where the water is in con- 
 tact with moist earth over a considerable area. But an artificial res- 
 ervoir may have quite an opposite character, and practically insulate 
 the lightning conductor from the earth. One which came under my 
 notice lately, in the neighborhood of this city, consists of a large 
 earthenware pipe set on end in a bed of cement, and kept half full of 
 water. Now, the earthenware pipe is a good insulator, and so is the 
 bed of cement in which it rests ; and the whole arrangement is identi- 
 cal, in all essential features, with the apparatus of Professor Richman, 
 in which he introduced his lightning rod into a glass bottle, and by 
 which he lost his life a hundred and thirty years ago. 
 
 A conductor mounted in this manner will, probably enough, draw 
 down lightning from the clouds ; but it is more likely to discharge it, 
 with destructive effect, into the building it is intended to guard, than 
 to transmit it harmlessly to the earth. An example is at hand in the 
 
 1 See Nature, June 12, 1879, vol. xx., p. 146. 
 
 8 See letter of Mr. Tomes in Nature, vol. xx., p. 145 ; also Lightning Rod Conference, pp. 210-15. 
 
44 LIGHTNING AND THUNDER. 
 
 case of Christ Church, in the town of Clevedon, in Somersetshire. 
 This church was provided with a very efficient system of lightning 
 conductors, five in number, corresponding to the four pinnacles and the 
 flagstaff, on the summit of the principal tower. The five conductors 
 consisted of good copper-wire rope ; all were united together inside 
 the tower, through which they were carried down to earth, and there 
 ended in an earthenware drain. This kind of earth contact might be 
 pretty good as long as water was flowing in the drain ; but whenever 
 the drain was dry the conductor was practically insulated from the 
 earth. On the fifteenth of March, 1876, the church was struck by 
 lightning, which for some distance followed the line of the conductor; 
 then finding its passage barred by the earthenware drain, which was 
 dry at the time, it burst through the walls of the church, displacing 
 several hundredweight of stone, and making its way to earth through 
 the gas-pipe. 1 
 
 Another very instructive example is furnished by the lightning con- 
 ductor attached to the lighthouse of Berehaven, on the south-west 
 coast of Ireland. It consists of a half-inch copper-wire rope, which 
 is carried down the face of the tower " until it reaches the rock at 
 it base, where it terminates in a small hole, three inches by three inches, 
 jumped out of the rock, about six inches under the surface" Here, again, 
 we have a good imitation of Professor Richman's experiment, with 
 only this difference, that a small hole in the rock is substituted for a 
 glass bottle. A lightning conductor of this kind fulfills two functions: 
 it increases the chance of the lightning coming down on the building, 
 and it makes it positively certain that, having come, it cannot get to 
 earth without doing mischief. 
 
 The lightning did come down on the Berehaven Lighthouse, about 
 five years ago. As might have been expected, it made no use of the 
 lightning conductor in finding a path to earth, but forced its way 
 through the building, dealing destruction around as it descended from 
 stage to stage. The Board of Irish Lights furnished a detailed report 
 of this accident to the Lightning Rod Conference, in March, 1880, from 
 which the above particulars have been derived.* 
 
 Precaution Against Rival Conductors. But it is not enough 
 to provide a good lightning conductor, which is itself able to convey 
 the electric discharge harmless to the earth ; we must take care that 
 there are no rival conductors near at hand in the building, to draw off 
 the lightning from the path prepared for it, and conduct it by another 
 route in which its course might be marked with destruction. This 
 precaution is of especial importance at the present day, owing to the 
 great extent to which metal, of various kinds, is employed in the con- 
 struction and fittings of modern buildings. I will take a typical case 
 which will bring home this point clearly to your minds. 
 
 1 See Anderson, Lightning Conductors, pp. 208-10. 
 
 8 See Lightning Rod Conference, pp. 208-10 ; see also the note at the end of this Lecture, p. 3, 
 
LIGHTNING AND THUNDER. 45 
 
 A great part of the roof of many large buildings is covered with 
 lead. The lead, at one or more points may come near the gutters in- 
 tended to collect the rain water ; the gutters are in connection with 
 the cast-iron down-pipes into which the water flows, and these down- 
 pipes often pass into the earth, which, under the circumstances, is 
 generally moist, and, therefore, in good electrical contact with the 
 metal pipes. Here, then, is an irregular line of conductors, which, 
 though it has gaps here and there, may, under certain conditions, 
 offer to the lightning discharge a path not less free than the lightning 
 conductor itself. What is the consequence ? The flash of lightning, or 
 a part of it, will quit the lightning rod, and make its way to earth 
 through the broken series of conductors, doing, perhaps, serious mis- 
 chief, as it leaps across, or bursts asunder, the non-conducting links 
 in the chain. 
 
 Another illustration may be taken from the gas and water-pipes, 
 with which almost all buildings in great cities are now provided, and 
 which constitute a network of conductors, spreading out over the 
 walls and ceilings, and stretching down into the earth, with which they 
 have the best possible electrical contact. Now, it often happens that 
 a lightning conductor, at some point in its course, comes within a short 
 distance of this network of pipes. In such a case, a portion of the elec- 
 trical discharge is apt to leave the lightning conductor, force .its way 
 destructively through masses of masonry, enter the network of pipes, 
 melt the leaden gas-pipe, ignite the gas, and set the building on fire. 
 
 These are not merely the speculations of philosophers. All the 
 various incidents I have just described have occurred, over and over 
 again, during the last few years. You will remember, in some of the 
 examples I have already set before you, when the electric discharge 
 failed to find a sufficient path to earth through the lightning rod, it 
 followed some such broken series of chance conductors as we are now 
 considering. But this broken series of conductors seems to bring 
 with it a special danger of its own, even when the lightning conductor 
 is otherwise in efficient working order. I will give you just one case 
 in point. 
 
 On the fifth of June, 1879, the Church of Saint Marie, Rugby, was 
 struck by lightning and set on fire, and narrowly escaped being burned 
 to the ground. A number of workmen were engaged on that day in 
 repairing the spire of the church. About three o'clock they saw a 
 dense black cloud approaching, and they came down to take shelter 
 within the building. In a few minutes they heard a terrific crash just 
 overhead ; at the same moment the gas was lighted under the organ 
 loft and the woodwork was set in a blaze. The men soon succeeded 
 in putting out the fire, and the church escaped with very little 
 damage. 
 
 Now, in this case there was no reason to suppose that the lightning 
 
46 LIGHTNING AND THUNDER. 
 
 conductor was in any way defective. But about half-way up the spire 
 there was, a peal of eight bells. Attached to these bells were iron 
 wires, about the eighth of an inch in diameter, leading from the clap- 
 pers down to the organ-loft, where they came within a short distance 
 of a gas-pipe fixed in the wall. It would seem that a great part of 
 .the discharge was carried safely to earth by the lightning conductor. 
 But a part branched off at the bells in the spire, descended by the 
 iron wires, and forced its way into the organ loft, to reach the net- 
 work of gas-pipes, through which it passed down to the earth, melting 
 the soft leaden gas-pipe in its course and lighting the gas. 
 
 The remedy foi this danger is obvious. All large masses of metal 
 used in the structure of a building the leads and gutters of the roof, 
 the cast-iron down-pipes, the iron gas and water mains should be 
 put in good metallic connection with the lightning conductor, and, as 
 far as may be, with one another. Connected in this way they furnish 
 a continuous and effective line of conductors leading safely down to 
 earth ; and, instead of being a dangerous rival, they become a useful 
 auxiliary to the lightning rod. 
 
 I would observe, however, that the lightning conductor ought not 
 to be connected directly with the soft leaden pipes which are com- 
 monly employed to convey gas and water to the several parts of a 
 building. Such pipes, as we have seen, are liable to be melted when 
 any considerable part of the lightning discharge passes through them ; 
 and thus much harm might be done, and the building might even be 
 set on fire by the lighting of the gas. Every good end will be at- 
 tained if the conductor is put in metallic connection with the iron gas 
 and water mains either inside or outside the building. 
 
 Insulation of Lightning Conductors. It is a question often 
 asked whether a lightning rod should be insulated from the building 
 it is intended to protect. I believe that this practice was formerly rec- 
 ommended by some writers, and I have observed that glass insulators 
 are still employed not infrequently by builders in the erection of light- 
 ning conductors ; but, from the principles I have set before you to- 
 day, it seems clear that any insulation of this kind is, to say the least, 
 altogether useless. The building to be protected is itself in electrical 
 communication with the earth, and the lightning conductor, if effi- 
 cient, is also in electrical communication with the earth therefore, 
 the lightning conductor and the building are in electrical communica- 
 tion with each other through the earth, and any attempt at insulating 
 them from one another above the earth is only labor thrown away. 
 
 Further, I have just shown you that the masses of metal employed 
 in the structure or decoration of a building ought to be electrically 
 connected with each other and with the lightning conductor. Now, 
 if this be done, the lightning conductor is, by the fact, in direct com- 
 munication with the building, and the glass insulators are utterly 
 
LIGHTNING AND THUNDER. 47 
 
 futile. Again, the building itself, during a thunderstorm, becomes 
 highly electrified by the inductive action of the cloud, and needs to be 
 discharged through the conductor just as the surrounding earth needs 
 to be discharged ; therefore, the more thoroughly it is connected with 
 the conductor, the more effectively will the conductor fulfill its 
 functions. 
 
 Personal Safety in a Thunderstorm. I suppose there is 
 hardly any one to whom tjie question has not occurred, at some time 
 or another, what he had best do to secure his personal safety during 
 a thunderstorm. This question is of so much practical interest that I 
 think I shall be excused if I say a few words about it, though perhaps, 
 strictly speaking, it is somewhat beside the subject of lightning con- 
 ductors. 
 
 At the outset, perhaps, I shall surprise you when I say that you 
 would enjoy the most perfect security if you were in a chamber en- 
 tirely composed of metal plates, or in a cage constructed of metal 
 bars, or if you were incased, like the knights of old, in a complete suit 
 of metal armor. This kind of defense is looked upon as so perfect, 
 among scientific men, that Professor Tait does not hesitate to recom- 
 mend his adventurous young friends devoted to the cause of science 
 to provide themselves with a light suit of copper, and, thus protected, 
 take the first opportunity of plunging into a thundercloud, there to 
 investigate, at its source, the process by which lightning is manu- 
 factured. 1 t 
 
 The reason why a metal covering affords complete protection is 
 that, when a conductor is electrified, the whole charge of electricity 
 exists on the outside surface of the conductor ; and therefore, when a 
 discharge takes place, it is only the outside surface that is affected. 
 Thus, if you were completely incased in a metal covering, and then 
 charged with electricity by the inductive action of a thundercloud, it 
 is only the metal covering that would undergo any change of electri- 
 cal condition ; and when the lightning flash would pass, it is only the 
 metal covering that would be discharged. 
 
 Let me show you a very pretty and interesting experiment to illus- 
 trate this principle : Here is a hollow brass cylinder, open at the ends, 
 mounted on an insulating stand. On the outside is erected a light 
 brass rod with two pith balls suspended from it by linen threads. Two. 
 pith balls are also suspended by linen threads from the inner surface 
 of the cylinder. You know that these pith balls will indicate to us the 
 electrical condition of the surfaces to which they are attached. If the 
 surface be electrified, the pith balls attached to it will share in its elec- 
 trical condition, and will repel each other ; if the surface be neutral, 
 the pith balls attached to it will be neutral, and will remain at rest. 
 
 1 Lecture on Thunderstorms, Nature, vol. xxii., pp. 365, 437. See, also, a very interesting paper 
 by the late Professor J. Clerk Maxwell, read before the British Association at Glasgow in 1876, and 
 reprinted in the report of the Lightning Rod Conference, pp. 109, no. 
 
4 8 
 
 LIGHTNING AND THUNDER. 
 
 I now put this apparatus under the influence of our thundercloud, 
 that is, the large brass conductor of our machine. The moment my 
 assistant turns the handle, the electricity begins to be developed on 
 the conductor, and you see, at once, the effect on the brass cylinder. 
 The pith balls attached to the outer surface fly asunder ; those at- 
 tached to the inner surface remain at rest. And now a spark passes ; 
 our thundercloud is discharged ; the inductive action ceases ; the pith 
 balls on the outside suddenly collapse, while those on the inside are 
 in no way affected. 
 
 It is not necessary that the brass cylinder should be insulated. To 
 
 PROTECTION FROM LIGHTNING FURNISHED BV A CLOSED CONDUCTOR. 
 
 vary the experiment, I will now connect it with the earth by a chain ; 
 you will observe that the effect is precisely the same as before. Flash 
 after flash passes while the machine continues in action ; the outside 
 pith balls fly about violently, being charged and discharged alter- 
 nately ; the inside pith balls remain all the time at rest. Thus you 
 see clearly that, if you were sitting inside such a metal chamber as 
 this, or covered with a complete suit of metal armor, you would be 
 perfectly secure during a thunderstorm, whether the chamber were 
 electrically connected with the earth or insulated from it. 
 
LIGHTNING AND THUNDER. 49 
 
 Practical Rules. But it rarely happens, when a thunderstorm 
 comes, that an iron hut or a complete suit of armor is at hand, and 
 you will naturally ask me what you ought to do under ordinary cir- 
 cumstances. First, let rne tell you what you ought not to do. You 
 ought not to take shelter under a tree, or under a haystack, or under 
 the lee of a house ; you ought not to stand on the bank of a river, or 
 close to a large sheet of water. If indoors, you ought not to stay 
 near the fireplace, or near any of the flues or chimneys ; you ought 
 not to stand under a gasalier hanging from the ceiling ; you ought 
 not to remain close to the gas pipe's or water-pipes, or any large 
 masses of metal, whether used in the construction of the building, or 
 lying loosely about. 
 
 The necessity for these precautions is sufficiently evident from the 
 principles I have already put before you. You want to prevent your 
 body from becoming a link in that broken chain of conductors which, 
 as we have seen, the electric discharge between earth and cloud is 
 likely to follow. Now a tree is a better conductor than the air ; and 
 your body is a betfer conductor than a tree. Hence, the lightning, in 
 choosing the path of least resistance, would leave the air to pass 
 through the tree, and would leave the tree to pass through you. A 
 like danger would await you if you stood under the lee of a haystack 
 or of a house. 
 
 The number of people who lose their lives by taking refuge under 
 trees in thunderstorms is very remarkable. As one instance out of 
 many, I may cite the following case which was reported in the Times, 
 July 14, 1887 : "Yesterday the funeral of a negress was being con- 
 ducted in a graveyard at Mount Pleasant, sixty miles north of Nash- 
 ville, Tennessee, when a storm came on, and the crowd ran for shelter 
 under the trees. Nine persons stood under a large oak, which the 
 lightning struck, killing everyone, including three clergymen, and the 
 mother and two sisters of the girl who had been buried." 
 
 Again, every -large sheet of water constitutes practically a great 
 conductor, which offers a very perfect medium of discharge between 
 the earth round about and the cloud. Therefore, when a thunder- 
 cloud is overhead, the sheet of water is likely to become one end of 
 the line of the lightning discharge ; and if you be standing near it, 
 the line of discharge may pass through your body. 
 
 When lightning strikes a building, it is very apt to use the stack of 
 chimneys in making its way to earth, partly because the stack of 
 chimneys is generally the most prominent part of the building, and 
 partly because, on account of the heated air and the soot within the 
 chimney, it is usually a moderately good conductor. Therefore, if 
 you be indoors, you must keep well away from the chimneys ; and for 
 a similar reason, you must keep as far as you can from large masses 
 of metal of every kind. 
 
50 LIGHTNING AND THUNDER. 
 
 Having pointed out the sources of danger which you must try to 
 avoid in a thunderstorm, I have nearly exhausted all the practical ad- 
 vice that I have at my command. But there are some occasions on 
 which it may be possible, not only to avoid evident sources of dan- 
 ger, but to make special provision for your own security. Thus, for 
 example, in the open country, if you stand a short distance from a 
 wood, you may consider yourself as practically protected by a light- 
 ning conductor. For a wood, by its numerous branches and leaves, 
 favors very much a quiet discharge of electricity, thus tending to sup- 
 press altogether the flash of lightning ; and if the flash of lightning 
 does come, it is much more likely to strike the wood than to strike you, 
 because the wood is a far more prominent body, and offers, on the whole, 
 an easier path to earth. In like manner, if you place yourself near 
 a tall solitary tree, some twenty or thirty yards outside its longest 
 branches, you will be in a position of comparative safety. If the storm 
 overtake you in the open plain, far away from trees and buildings, 
 you will be safer lying flat on the ground than standing erect. 
 
 In an ordinary dwelling house, the best situation is probably the 
 middle story, and the best position in the room is in the middle of the 
 floor ; provided, of course, that there is no gasalier hanging from the 
 ceiling above or below you. Strictly speaking, the middle of the room 
 would be a still safer position than the middle of the floor ; and noth- 
 ing could be more perfect than the plan suggested by Franklin, to 
 get into "a hammock, or swinging bed, suspended by silk cords, and 
 equally distant from the walls on every side, as well as from the ceil- 
 ing and floor, above and below." An interesting case has been re- 
 cently recorded, by a resident of Venezuela, which illustrates in a 
 remarkable way the excellence of this advice. "The lightning," he 
 says, " struck a rancho a small country house, built of wood and 
 mud, and thatched with straw or large leaves where one man slept 
 in a hammock, another lay under the hammock on the ground, and 
 three women were busy about the floor ; there were also several 
 hens and a pig. The man in the hammock did not receive any in- 
 jury whatever, while the other four persons and the animals were 
 killed." 1 
 
 But, as I can hardly hope that many of you when the thunderstorm 
 actually comes will find yourselves provided with a hammock, I would 
 recommend, as more generally useful, another plan of Franklin's, which 
 is simply to sit on one chair in the middle of the floor and put your 
 feet up on another. This arrangement will approach very nearly to 
 absolute security if you take the further precaution, also mentioned 
 by Franklin, of putting a feather bed or a couple of hair mattresses 
 under the chairs. 8 
 
 1 Nature, vol. xxxi., p. 459. 
 
 9 See further information on this interesting subject in the Report of the Lightning Rod Confer- 
 ence, pp. 233-5. 
 
LIGHTNING AND THUNDER. 51 
 
 Security Afforded by Lightning Rods. You might, perhaps, 
 be inclined to infer hastily, from the examples I have set before you, in 
 the course of this lecture, of buildings which were struck and severely 
 injured by lightning though provided with lightning conductors, that 
 a lightning rod affords a very imperfect protection to life and prop- 
 erty. But such an idea would be entirely at variance with the evi- 
 dence at hand on the subject. In all the cases to which I have re- 
 ferred, and in many others which might easily have been cited, the 
 damage was done simply because the lightning rods were deficient in 
 one or more of the conditions on which I have so much insisted. 
 Where these conditions are fulfilled, the lightning flash will either not 
 come down at all upon the building, or, if it do come, it will be carried 
 harmless to the earth. 
 
 Perhaps there is no one fact that so forcibly brings home to the 
 mind the complete protection afforded by lightning conductors as the 
 change which followed their introduction into the Royal Navy. I 
 have already told you that in former times the damage done by light- 
 ning to ships of the Royal Navy was a regular source of expenditure, 
 amounting every year to several thousand pounds sterling. But, after 
 the general adoption of lightning conductors about forty, years ago, 
 through the indefatigable exertions of Sir William Snow Harris, this 
 source of expenditure absolutely disappeared, and injury to life and 
 property has long been practically unknown in Her Majesty's Fleet. 
 
 I should say, however, that the trial of lightning conductors in the 
 Navy, though it lasted long enough to prove their perfect efficiency, 
 has almost come to an end in our own days. The great iron monsters 
 which in recent times have taken the place of the wooden ships of 
 Old England are quite independent of lightning rods in the common 
 sense of the word. Their ponderous masts are virtually lightning 
 rods of colossal dimensions, and their unsightly hulls are, so to speak, 
 earth-plates of enormous size in perfect electrical contact with the 
 ocean. To add to such structures lightning conductors of the com- 
 mon kind would be nothing better than " wasteful and ridiculous 
 excess." 
 
 As regards buildings on land, I may refer to the little province of 
 Schleswig-Holstein, of which I have already spoken to you. From 
 some cause or other this small peninsula is singularly exposed to 
 thunderstorms, and of late years it has been more abundantly pro- 
 vided with lightning conductors than, perhaps, any other district of 
 equal extent in Europe. Now, as a simple illustration of the protec- 
 tion afforded by these lightning conductors, I may mention that, on 
 the 26th of May, 1878, a violent thunderstorm burst over the little 
 town of Utersen. Five several flashes of lightning fell in different 
 parts of the town, but not the slightest harm was done, each flash 
 being safely carried to earth by a lightning conductor. Further, it 
 
52 LIGHTNING AND THUNDER. 
 
 appears from the records of the fire insurance company that, out of 
 552 buildings injured by lightning during a period of eight years from 
 j8yo to 1878 only four had lightning conductors ; and in these four 
 cases it was found, on examination, that the lightning conductors 
 were defective. 1 
 
 It would be easy to multiply evidence on this subject. But as I 
 have already trespassed, I fear, too far on your patience, I will con- 
 tent myself with saying, in conclusion, that according to all the high- 
 est authorities, both practical and theoretical, any structure provided 
 with a lightning conductor properly fitted up in conformity with the 
 principles I have set before you to-day is perfectly secure against light- 
 ning. The lightning, indeed, may fall upon it, but it will pass harm- 
 less to the earth ; and the experience of more than a hundred years 
 has fully justified the simple and modest words of the great inventor 
 of lightning conductors : " It has pleased God, in His goodness to 
 mankind, at length to discover to them the means of securing their 
 habitations and other buildings from mischief by thunder and 
 lightning." 
 
 NOTE I. 
 
 ON THE LIGHTNING CONDUCTOR AT BEREHAVEN. 9 
 
 It is satisfactory to know that the lightning conductor referred to in my lecture as 
 attached to the lighthouse at Berehaven has been put in good order under the best 
 scientific guidance. The following interesting letter from Professor Tyndall, which 
 appeared in the Times, August 31, 1887, gives-the history of the matter very clearly, 
 and fully bears out the views put forward in my lecture : 
 
 "Your recent remarks on thunderstorms and their effects induce me to submit to 
 you the following facts and considerations. Some years ago a rock lighthouse on the 
 coast of Ireland was struck and damaged by lightning. An engineer was sent down 
 to report on the occurrence ; and, as I then held the honorable and responsible 
 post of scientific adviser to the Trinity House and Board of Trade, the report was 
 submitted to me. The lightning conductor had been carried down the lighthouse 
 tower, its lower extremity being carefully embedded in a stone perforated to receive it. 
 If the object had been to invite the lightning to strike the tower, a better arrangement 
 could hardly have been adopted. 
 
 " I gave directions to have the conductor immediately prolonged, and to have added 
 to it a large terminal plate of copper, which was to be completely submerged in the 
 sea The obvious convenience of a chain as a prolongation of the conductor caused 
 the authorities in Ireland to propose it ; but I was obliged to veto the adoption of the 
 chain. The contact of link with link is never perfect. I had, moreover, beside me a 
 portion of a chain cable through which a lightning discharge had passed, the electricity 
 in passing from link to link encountering a resistance sufficient to enable it to partially 
 fuse the chain. The abolition of resistance is absolutely necessary in connecting a 
 lightning conductor with the earth, and this is done by closely embedding in the earth 
 a plate of good conducting material and of large area. The largeness of area makes 
 atonement for the imperfect conductivity of earth. The plate, in fact, constitutes 
 
 1 See " Die Theorie, die Anlage, und die Prufung der Blitzableiter," von Doctor W. Holtz, Griefs- 
 wald, 1878. 
 
 See page 44 
 
LIGHTNING AND THUNDER. 53 
 
 a wide door through which the electricity passes freely into the earth, its disruptive 
 and damaging effects being thereby avoided. 
 
 " These truths are elementary, but they are often neglected. I watched with inter- 
 est some time ago the operation of setting up a lightning conductor on the house of a 
 neighbor of mine in the country. The wire rope which formed part of the conductor 
 was carried down the wall and comfortably laid in the earth below without any ter- 
 minal plate whatever. I expostulated with the man who did the work, but he obvi- 
 ously thought he knew more about the matter than I did. I am credibly informed 
 that this is a common way of dealing with lightning conductors by ignorant practi- 
 tioners, and the Bishop of Winchester's palace at Farnham has been mentioned to me 
 as an edifice ' protected ' in this fashion. If my informant be correct, the ' protection ' 
 is a mockery, a delusion, and a snare." 
 
 NOTE II. 
 
 BOOKS OF REFERENCE. 
 
 As some of my readers may wish to pursue the study of lightning and light- 
 ning conductors beyond the limits to which a popular lecture must, of neces- 
 sity, be confined, I subjoin a list of the books which I think they would be likely to 
 find most useful for the purpose. Among ordinary text-books on physics Jamin, 
 Cours de Physique, vol. i., pp. 470-494; Mascart, Traite d'Electricite Statique, vol. 
 ii., pp. 555-579 ; De Larive, A Treatise on Electricity, in three volumes, London, 
 1853-8, vol. Hi., pp. 90-201; Daguin, Traite de Physique, vol. iii., pp. 209-280; 
 Riess, Die Lehre von der Reibungs-Elektricitat, vol. ii., pp. 494-564 ; Miiller-Pouillet, 
 Lehrbuch der Physik, Braunschweig, 1881, vol. iii., pp. 210-225 ; Scott, Elementary 
 Meteorology, chap. x. Of the numerous special treatises and detached papers on the 
 subject, I would recommend Instruction sur les Paratonnerres adopte par 1' Academic 
 des Sciences, Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago, Sur le 
 Tonnerre, Paris, 1837 ; also his Meteorological Essays, translated by Sabine, London, 
 1855 ; Sir William Snow Harris, On the Nature of Thunderstorms, London, 1843 ; 
 also by the same writer, A Treatise on Frictional Electricity, London, 1867 ; and 
 various papers on lightning conductors, from 1822 to 1859 ; Tomlinson, The Thun- 
 derstorm, London, 1877 ; Anderson, Lightning Conductors, London, 1880 ; Holtz, 
 Ueber die Theorie, die Anlage, und die Priifung der Blitzableiter, Greifswald, 1878 ; 
 Weber, Berichte ttber Blitzschlage in der Provinz Schleswig-Holstein, Kiel, 1880-1 ; 
 Tait, A Lecture on Thunderstorms, delivered in the City Hall, Glasgow, in 1880, 
 Nature, vol. xxii. ; Report of. the Lightning Rod Conference, London, 1882. This 
 last-mentioned volume comes to us with very high authority, representing, as it does, 
 the joint labors of several eminent scientific men selected from the following societies : 
 The Meteorological Society, the Royal Institute of British Architects, the Society of 
 Telegraph Engineers and Electricians, the Physical Society. 
 
 Since the above was in print, two lectures given before the Society of Arts by Pro- 
 fessor Oliver Lodge, F. R. S., have appeared in the Electrician, June and July, 1888. 
 in which some new views are put forward respecting lightning conductors, that seem 
 deserving of careful consideration. 
 
APPENDIX. 
 
 RECENT CONTROVERSY ON LIGHTNING CONDUCTORS. 
 
 THE lecture on lightning conductors contained in this volume 
 fairly represents, I think, the theory hitherto received on the 
 subject. It is, moreover, entirely in accord with the report of the 
 Lightning Rod Conference, brought out in 1883, by a committee of 
 most eminent men, representing several branches of science, who were 
 specially chosen to consider this question some ten years ago. 
 
 Lectures of Professor Lodge. But, in the month of March, 
 1888, two lectures were given before the Society of Arts, in London, 
 by Professor Oliver Lodge, in which this theory was directly chal- 
 lenged, and attacked with cogent arguments, supported by striking 
 and original experiments. These lectures gave rise to an animated 
 controversy, which culminated in a formal discussion at the recent 
 meeting of the British Association in Bath. The discussion wascar- 
 ried on with great spirit, and most of the leading representatives of 
 physical and mechanical science took an active part in it. The 
 greater portion of this volume was printed off before the meeting of 
 the British Association took place. But the discussion on the theory of 
 lightning conductors seemed to me so interesting and important that 
 I thought it right, in the form of an Appendix, to give some 'account 
 of the questions at issue, and of the opinions expressed upon them. 
 
 Professor Lodge maintains 1 that the received theory of lightning 
 rods is open to two objections. First, it takes account only of the 
 conducting power of the lightning rod, and takes no account of the 
 phenomenon known as self-induction, or electrical inertia. Secondly, 
 it assumes that the whole substance of a lightning rod acts as a con- 
 ductor, in all cases of lightning discharge ; whereas there is reason to 
 believe that, in many cases, it is only a thin outer shell that really 
 comes into action. I will deal with these two points separately. 
 
 The Effect of Self-induction. When an electric discharge be- 
 gins to pass through a conductor, a momentary back electro-motive 
 force is developed in the conductor, which obstructs its passage. 
 This phenomenon is called by some self-induction, by others electrical 
 inertia ; but its existence is admitted by all. Now, when a flash of 
 lightning, so to say, falls on a lightning rod, the back electro-motive 
 
 1 See his Lectures, published in the Electrician, June 22, June 29, July 6, and July 13, 1888. 
 
56 APPENDIX. 
 
 force developed is very considerable ; and it may offer so great an 
 obstruction that the discharge will find an easier passage by some other 
 route, such as the stone walls and woodwork, and furniture of the 
 building. 
 
 According to this view, the obstruction which a flash of lightning 
 encounters in a conductor consists partly of the resistance of the con- 
 ductor, in the ordinary sense of the word resistance, and partly of the 
 back electro-motive force due to self-induction. The sum of these 
 two Professor Lodge calls the impedance of the lightning rod ; and 
 he considers that the impedance may be enormously great, even when 
 the resistance, in the ordinary sense, is comparatively small. 
 
 In support of this view he has devised the following extremely in- 
 genious and remarkable experiment. A large Leyden jar, L, was ar- 
 ranged in such a manner that, while it received a steady charge from 
 
 INDUCTION EFFECT OF LEYDEN JAR DISCHARGE. 
 
 M Electrical Machine I A B Air Spaces between Brass Knobs. 
 
 L Leyden Jar. | C Conducting Wire. 
 
 an electrical machine, it discharged itself, at intervals, across the air 
 space at A, between two brass balls. The discharge had then two 
 alternative paths before it ; one through a conducting wire, C, the 
 other across a second air space, between two brass balls at B. During 
 the experiment, the two balls at A were kept at a fixed distance of 
 one inch apart ; but the distance between the two balls at B was 
 varied. The conductor, C, used in the first instance, was a stout cop- 
 per wire, about forty feet long, and having a resistance of only one- 
 fortieth of an ohm. 
 
 It was found that, so long as the distance between the B knobs was 
 less than 1.43 inches, all the discharges passed across between the 
 
CONTROVERSY ON LIGHTNING CONDUCTORS. 57 
 
 knobs, in the form of a spark. When the distance exceeded 1.43 
 inches, all the discharges passed through the conductor, C, and no 
 spark appeared between the balls at B. And when the distance was 
 exactly 1.43 inches, the discharge sometimes took place between the 
 knobs, and sometimes followed the conductor, C. The interpretation 
 given to these facts is that the obstruction offered by the conductor C 
 was about equal to the resistance of 1.43 inches of air ; and it is pro- 
 posed to call this distance, under the conditions of the experiment, 
 the critical distance. 
 
 Coming now to the application of these results, Professor Lodge 
 argues that the conductor C,-in his experiment, represents a lightning 
 rod of unimpeachable excellence ; and yet, in certain cases, the dis- 
 charge refuses to follow the conductor, and prefers to leap across a 
 considerable space of air, notwithstanding the enormous resistance it 
 there encounters. In like manner, he says, a flash of lightning may, 
 in certain cases, leave a lightning rod fitted up in the most orthodox 
 manner, and force its way to earth through resisting masses of mason 
 work and such chance conductors as may come across its path. 
 
 This conclusion, he admits, is altogether at variance with the re- 
 ceived views on the subject ; but he contends that it is perfectly in 
 accord with the scientific theory of an electrical discharge. The mo- 
 ment the discharge begins to pass in the conductor, it encounters the 
 obstruction due to self-induction ; and this obstruction is so great that 
 the bad conductors offer, on the whole, an easier path to earth. 
 
 Variation of the Experiment. When the experiment was va- 
 ried by substituting a thin iron wire for the stout copper wire at first 
 employed, a very curious result was obtained. The wire chosen was 
 of the same length as the copper, but had a resistance about 1,300 
 times as great ; its resistance being, in fact, 33.3 ohms. Nevertheless, 
 in this experiment, when the B knobs were at a distance of 1.43 inches, 
 no spark passed, which showed that the discharge always followed 
 the line of the conductor, and therefore that the conductor offered 
 less obstruction than 1.43 inches of air. The knobs were then brought 
 gradually nearer and nearer ; and it was not until the distance was con- 
 siderably reduced that the sparks began to pass between them. When 
 the distance was exactly 1.03 inches, the discharge sometimes passed 
 between the knobs, and sometimes through the conductor ; this was, 
 therefore, the critical distance, in the case of the iron wire. Thus it 
 appeared that the obstruction offered to the discharge by the iron 
 wire was much less than that offered by the copper, the one being 
 equal to a resistance of only 1.03 inches of air, the other to a resist- 
 ance of 1.43 inches. 
 
 It does not appear that Professor Lodge undertakes to offer any 
 satisfactory explanation of this result. He has come to the conclu- 
 sion, from his various experiments, that, in the case of a sudden 
 
5 8 APPENDIX. 
 
 discharge, difference of conducting power between fairly good con- 
 ductors is a matter of practically no account ; and that difference of 
 sectional area is a matter of only trifling account. But he does not 
 see why a thin iron wire should have a smaller impedance than a 
 much thicker wire of copper. He proposes to repeat the experiments 
 so as to confirm or to modify the result, which for the present seems 
 to him anomalous. 1 
 
 The Outer Shell only of a Lightning Rod Acts as a Con- 
 ductor. As a consequence of self-induction or electrical inertia, 
 Professor Lodge contends that a lightning discharge in a conductor 
 consists of a series of oscillations. These oscillations follow one an- 
 another with extraordinary rapidity there may be a hundred thou- 
 sand in a second, there may be a million. Now it has been shown 
 that, when a current starts in a conductor, it does not start at once all 
 through its section ; it begins or. the outside, and then gradually, but 
 rapidly, penetrates to the interior. From this he infers that the ex- 
 tremely rapid oscillations of a lightning discharge have not time to 
 penetrate to the interior of a conductor. The electricity keeps surg- 
 ing to and fro in the superficial layer or outer shell, while the interior 
 substance of the rod remains inert and takes no part in the action. A 
 conductor, therefore, will be most efficient for carrying off a flash of 
 lightning if it present the greatest possible amount of surface ; a thin, 
 flat tape will be more efficient w than a rod of the same mass ; and a 
 number of detached wires more efficient than a solid cylinder. As for 
 existing lightning conductors, the greater part of their mass would, 
 in many cases, have no efficacy whatever in carrying off a flash of 
 lightning. 
 
 The Discussion. The discussion at the meeting of the British 
 Association was opened by Mr. William H. Preece, F.R.S., Electrician 
 to the Post Office, who claimed to have 500,000 lightning conductors 
 under his control. He expressed his conviction that a lightning rod, 
 properly erected and duly maintained, was a perfect protection against 
 injury from lightning ; and in support of this conviction he urged very 
 strongly the report of the Lightning Rod Conference. This report 
 represented the mature judgment of the most eminent scientific men, 
 who had devoted years to the study of the question ; and he wished 
 particularly to bring before the meeting their clerrand decisive asser- 
 tion an assertion he was there to defend that -'there is no authen- 
 tic case on record where a properly constructed conductor failed to do 
 its duty." 
 
 The new views put forward by Professor Lodge were based, in 
 great measure, on his theory that a lightning discharge consisted of a 
 series of rapid oscillations. But this theory should be received with 
 great caution. It seemed to be nothing more than a deduction from 
 
 1 See paper read at the meeting of the British Association, in Bath, 1888, published in the Elec- 
 trician^ page 607. September 14. 
 
CONTROVERSY ON LIGHTNING CONDUCTORS. 59 
 
 certain mathematical formulas, and was not supported by any solid 
 basis of observation or experiment. Besides, there were many facts 
 against it. They all knew that a flash of lightning magnetized steel 
 bars, deranged the compasses of ships at sea, and transmitted signals 
 on telegraph wires. But such effects could not be produced by a series 
 of oscillations, which, being equal and opposite, would neutralize each 
 other. It was alleged that these rapid oscillations occurred in the 
 discharge of a Leyden jar. That might be true, and probably was 
 true ; but they were not dealing with Leyden jars, they were dealing 
 with flashes of lightning. If there was any analogy between the dis- 
 charge of a Leyden jar and a flash of lightning, it was to be found, 
 not in the external discharge employed by Professor Lodge in his ex- 
 periments, but in the bursting of the glass cylinder between the two 
 coatings of the jar. 
 
 Lord Rayleigh thought the experiments of Professor Lodge were 
 likely to have important practical applications to lightning conductors. 
 But though these experiments were valuable as suggestions, they did 
 not furnish a sufficient ground for adopting any new system of pro- 
 tection. It was only by experience with lightning conductors them- 
 selves that the question could be finally settled. 
 
 Sir William Thomson hoped for great fruit from the further investiga- 
 tion of self-induction in the case of sudden electrical discharges. He 
 warmly encouraged Professor Lodge to continue his researches ; but 
 he expressed no decided opinion on the question at issue. Incident- 
 ally he observed that the best security for a gun-powder magazine 
 was an iron house ; no lightning conductor at all, but an iron roof, 
 iron walls, and an iron floor. Wooden boards should, of course, be 
 placed over the floor to prevent the danger of sparks from people 
 walking on sheet-iron. This iron magazine might be placed on a dry 
 granite rock, or on wet ground ; it might even be placed on a founda- 
 tion under water ; it might be placed anywhere they pleased ; no 
 matter what the surroundings were, the interior would be safe. He 
 thought that was an important practical conclusion which might safely 
 be drawn from the consideration of these electrical oscillations and 
 the experiments regarding them. 
 
 Professor Rowland, of the Johns Hopkins University, America, said 
 that the question seemed to be whether the experiment of Professor 
 Lodge actually represented the case of lightning. He was very much 
 disposed to think it did not. In the experiment almost the whole cir- 
 cuit consisted of good conductors ; whereas, in the case of lightning, 
 the path of the discharge was, for the most part, through the air, and 
 therefore it might be an entirely different phenomenon. The air be- 
 ing a very bad conductor, a flash of lightning might, perhaps, not 
 consist of oscillations, but rather of a .single swing. Moreover, it was 
 not at all clear that the lejigth of the spark, in the experiment, could 
 
60 APPENDIX. 
 
 be taken as a measure of the obstruction offered by the conductor. 
 Professor George Forbes was greatly impressed with the beauty and 
 significance of Professor Lodge's experiments, but he did not think 
 the result so clear that they should be warranted in abandoning the 
 principles laid down by the Lightning Rod Conference. 
 
 M. de Fonvielle, of Paris, supported the views of Mr. Preece. He 
 cited the example of Paris, where they had erected a sufficient num- 
 ber of lightning conductors, according to the received principles, and 
 calamities from lightning were practically unknown. He suggested 
 that the Eiffel Tower, which they were now building, and which would 
 be raised to the height of a thousand feet, would furnish an unrivalled 
 opportunity for experiments on lightning conductors. 
 
 Sir James Douglass, Chief Engineer to the Corporation of Trinity 
 House, had a large experience with lighthouse towers. The light- 
 ning rods on these towers had been erected and maintained during 
 the last fifty years entirely according to the advice of Faraday. They 
 never had a serious accident ; and such minor accidents as did occur 
 from time to time were always traced to some defect in the con- 
 ductor. They had now established a more rigid system of inspection, 
 and he, for one, should feel perfectly safe in any tower where this 
 system was carried out. 
 
 Mr. Symons, F.R.S., Secretary to the Meteorological Society, had 
 taken part in a discussion on lightning conductors as long ago as 1859. 
 It had been a hobby with him all his life to investigate the circum- 
 stances of every case he came across in which damage was done by 
 lightning, and the general impression left by his investigations entirely 
 coincided with the views just expressed by Sir James Douglass. He 
 had been a member of the Lightning Rod Conference, and was the 
 editor of their report ; and he wished to enter his protest against the 
 idea of rejecting all that had hitherto been done in connection with 
 lightning conductors on the strength of mere laboratory experiments. 
 
 Professor Lodge, in reply, said he could perfectly understand the 
 position of those who held that a lightning rod properly fitted up 
 never failed to do its duty, because, whenever it failed, they said it 
 was not properly fitted up. The great resource in such cases was to 
 ascribe the failure to bad earth contact. He thought a good earth 
 contact was a very good thing, but he could not understand why such 
 extraordinary importance should be attached to it. A lightning rod 
 had two ends an earth end and a sky end and he did not see why 
 good contact was more necessary at one end than at the other. If a 
 few sharp points sticking out from the conductor were sufficient for a 
 good sky contact, why were they not sufficient also for a good earth 
 contact ? 
 
 Besides, though a bad earth contact might explain why a certain 
 amount of disruption should take place at the earth where the bad 
 
CONTROVERSY ON LIGHTNING CONDUCTORS. 61 
 
 contact existed, he did not see how it accounted for the flash shooting 
 off sideways half-way down the conductor. Again, what does a baci 
 earth contact mean ? If an electrical engineer finds a resistance of a 
 hundred ohms, he will rightly pronounce the earth contact to be very 
 bad indeed. But why should the lightning flash leave a conductor 
 with a resistance of a hundred ohms in order to follow a line of non- 
 conductors where it encounters a resistance of many thousand 
 ohms ? 
 
 He accepted the statement of Mr. Preece that his whole theory de- 
 pended on the existence of oscillations in the lightning discharge ; 
 but there was good reason to believe they existed, because they were 
 proved to exist in the discharge of a Leyden jar. Mr. Preece objected 
 that an oscillating discharge could not produce magnetic effects, as a 
 flash of lightning was known to do. He confessed he was unable to 
 explain how an oscillating discharge produced such effects; ' but that 
 it could produce them there was no doubt whatever, for the discharge 
 of a Leyden jar produces magnetic effects, and we have ocular dem- 
 onstration that the discharge of a Leyden jar is an oscillating dis- 
 charge. 
 
 As to the assurances we had received from electrical engineers that 
 a properly fitted lightning conductor never fails, he should like to ask 
 them how the Hotel de Ville, in Brussels, had been set on fire by 
 lightning on the ist of last June. The system of lightning conduct- 
 ors on this building had been erected in accordance with the received 
 theory, and had been held up by writers on the subject as the most 
 perfect in Europe. Unless some explanation were forthcoming to 
 account for its failure, we could no longer regard lightning conductors 
 as a perfect security against danger. 
 
 The President of Section A, Professor Fitzgerald, in bringing the 
 discussion to a close, observed that one result of this meeting would 
 be to give a new interest to the phenomena of static electricity and its 
 practical applications. He was inclined himself to think that the ex- 
 periments of Professor Lodge were not quite analogous to the case of 
 a flash of lightning. In comparing the discharge of a Leyden jar 
 with a flash of lightning they should look for the analogy, not so 
 much in the external discharge through a series of conductors, but 
 rather, as Mr. Preece had observed, in the bursting of the glass be- 
 tween the two coatings of the jar. As regarded the oscillations in a 
 Leyden jar discharge, he did not think such oscillations were at all 
 necessary to account for the phenomena observed in the experiments. 
 Many of the results which Professor Lodge seemed to think would 
 require some millions of oscillations per second would be produced by 
 a single discharge lasting for a millionth of a second. Improvements, 
 perhaps, were possible in our present system of lightning conductors, 
 
 1 See a very ingenious hypothesis, to account for this phenomenon, suggested by Professor Ewing in 
 the Electrician^ p. 712. October 5, 1888. 
 
62 APPENDIX. 
 
 but practical experience had shown, however we might reason on the 
 matter, that, on the whole, lightning conductors had been a great 
 protection to mankind from the dangers of lightning. 
 
 Summary. I will now try to sum up the results of this interesting 
 discussion, and state briefly the conclusions which, as it seems to me, 
 may be deduced from it. First, I would remind my readers that a 
 lightning rod has two functions to fulfill. Its first function is to pro- 
 mote a gradual, but rapid, discharge of electricity according as it is 
 developed, and thus to prevent such an accumulation as would lead to 
 a flash of lightning. Its second function is to convey the flash of 
 lightning, when it does come, harmless to the earth. Now, the new 
 views advanced by Professor Lodge in no way impugn the efficiency 
 of lightning rods as regards their first function ; and it is evident that 
 the greater the number of lightning rods distributed over a given area, 
 the more perfectly will this function be fulfilled. This is a point of 
 great practical importance which seemed to me, in some degree, lost 
 sight of during the progress of the discussion. 
 
 Secondly, it was practically admitted by the highest authorities that 
 the experiments and reasoning of Professor Lodge afford good grounds 
 for reconsidering the received theory of lightning conductors as re- 
 gards their second function that of carrying the lightning flash 
 harmless to the earth. But there was undoubtedly a general feeling 
 that it would be rash to set aside, all at once, the received theory on 
 the strength of laboratory experiments made under conditions widely 
 different from those which actually exist in a lightning discharge. 
 Experiments are wanted on a larger scale ; and, if possible, experi- 
 ments with lightning rods themselves. 
 
 Thirdly, the testimony of electrical engineers who have had large 
 experience with lightning conductors seems almost unanimous that a 
 lightning conductor erected and maintained in accordance with the 
 conditions prescribed by the Lightning Rod Conference gives perfect 
 protection. It was certainly unfortunate that the Hotel de Ville, in 
 Brussels, which was reputed the best protected building in Europe, 
 should have been damaged by lightning just two months before the 
 discussion took place ; but no certain conclusion can be drawn from 
 this catastrophe until we know exactly the conditions under which it 
 occurred. 
 
 So the matter stands, awaiting further investigation. 
 
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