SPIENfCE* 
 
 - x 
 
 RlES 
 
 THE CAU^E 
 
 OF AN ICE AGE 
 
 SIR ROBERT BALL 
 
EARTH 
 
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 Lir 
 
c 
 
 THE LIBRARY 
 
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 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
flfeobern Science Series 
 
 EDITED BY SIR JOHN LUBBOCK, BART., M.P. 
 
 AN ICE AGE 
 
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 I. The Cause of an Ice Age. 
 
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THE CAUSE 
 OF AN ICE AGE 
 
 BY 
 
 SIR ROBERT BALL, LL. D., R R. S. 
 
 ROYAL ASTRONOMER OF IRELAND 
 AUTHOR OF STARLAND 
 
 NEW YORK 
 
 D. APPLETON AND COMPANY 
 1891 
 
PALEONTOLOGY LIBRARY 
 Gift of C. A. Kofoid 
 
 COPYRIGHT, 1891, 
 
 BY D. AFPLETON AND COMPANY, 
 
 All rights reserved. 
 
B3 
 
 PKEFACE. 
 
 As I have written an Introduction to account for 
 the appearance of this book, I have nothing further to 
 say on that subject. 
 
 I have only to discharge here the pleasant duty of 
 expressing my thanks to the many friends who have 
 helped me in various ways and degrees by kindly coun- 
 sel and friendly remonstrance. Their names are Dr. 
 V. BALL, Rev. M. H. CLOSE, Rev. Dr. CROSSKEY, Prof. 
 G. F. FITZGERALD, Prof. C. LAPWORTH, Sir J. LUB- 
 BOCK, Mr. A. RAMBAUT, Prof. W. SOLLAS, and Mr. 
 
 L. STEELE. 
 
 ROBERT S. BALL. 
 
 OBSERVATORY, COUNTY DUBLIN, November 14, 1890. 
 
 M3359O5 
 
CONTENTS. 
 
 CHAPTER PAGE 
 
 INTRODUCTION xi 
 
 I. THE RECORDS OF THE ICE 1 
 
 II. THE SUCCESSION OF GLACIAL AND GENIAL PERIODS . 27 
 
 III. ATTEMPTS TO SOLVE THE PROBLEM OF GLACIATION . 39 
 
 IV. THE PERTURBATIONS OF THE EARTH'S ORBIT . . 60 
 
 V. THE ASTRONOMICAL THEORY OF THE ICE AGE . . 78 
 
 VI. THE REASON WHY THIS BOOK HAS BEEN WRITTEN . 112 
 
 VII. How THE ASTRONOMICAL THEORY is CORROBORATED . 138 
 
 VIII. CONCLUDING CHAPTER 150 
 
 APPENDIX 175 
 
 INDEX 179 
 
INTRODUCTION. 
 
 WE are to discuss in the following pages some of 
 those revolutions of which in ages past our earth has 
 been the scene. Perhaps it would be rather more cor- 
 rect to say that we are to discuss a whole series of 
 revolutions, each of which bears a general resemblance 
 to the rest. The date at which our story opens is a 
 very remote one judged by ordinary historical standards. 
 There is not even a tradition to which we might refer 
 which would give the slightest record of these great 
 events ; yet when viewed from a geological standpoint 
 the phenomena with which we shall have to deal assume 
 quite a modern character. Ages had already elapsed 
 since the surface of the earth had consolidated. Suc- 
 cessive layers of the stratified rocks, up to what we may 
 call the recent period, had been deposited. Numberless 
 races of animals and various tribes of plants had come 
 into being and had passed away before the times of 
 which we write. Indeed, the very animals and plants 
 which at present inhabit our globe had already occupied 
 it. The crust of the globe had approximated to what 
 we now find it, and in all probability the human epoch 
 had begun. 
 
 The study of the Ice Age is connected with the two 
 
xii INTRODUCTION. 
 
 greatest of the natural sciences. Geology has appealed 
 to Astronomy to aid her in the solution of a great prob- 
 lem, and we are now to discuss both the question which 
 the geologists have put, and the answer which the as- 
 tronomers have given. 
 
 I must at the outset express my recognition of the 
 labors of the eminent men who have rendered this in- 
 quiry possible. First of all I would mention the honored 
 name of Agassiz, to whose penetration we are so deeply 
 indebted for the means of interpreting the records 
 which the Ice Age has left for our instruction. Then 
 I must refer to Prof. James Geikie ; for who would 
 venture to write of the Glacial Period without having 
 studied and profited by his well-known work, Tfie Great 
 Ice Age. It would not be possible for me to mention 
 the names of all the other workers who have contributed 
 to elucidate the subject ; but of course the Astronomical 
 theory of the Ice Age must be especially associated with 
 its founder, Adhemar, and its laborious expounder, Dr. 
 James Croll, author of the remarkable work entitled 
 Climate and Time. 
 
 These acknowledgments having been made, I may 
 be permitted to add that I have ventured to think out 
 the subject for myself. It seems that a serious error has 
 crept into the Astronomical theory of the Ice Age as it 
 is usually expounded. I have shown in Chapter VI 
 how the mistake in the astronomy of the subject arose, 
 and how it came to affect much of the current teaching 
 on the cause of the Ice Age. 
 
THE CAUSE OP AN ICE AGE. 
 
 CHAPTER I. 
 
 THE RECORDS OF THE ICE. 
 
 THE beautiful county of Kerry is pierced by the 
 waters of the Atlantic in a long fiord known as the 
 Kenmare River. On the heather-clad mountain sides 
 which slope down to the water lies a great fragment of 
 rock more than a hundred tons in weight. A lady 
 friend of mine was good enough to take the photograph 
 of this object, from which the frontispiece to this vol- 
 ume has been engraved. The figure standing beside it 
 will suffice to show the proportions of the rock. Even 
 among the peasantry this Cloghvorra Stone, as they call 
 it, is regarded with much interest ; and legend assigns 
 to it a miraculous origin. 
 
 One who has not pondered on scientific matters 
 might be tempted to think that no special significance 
 attaches to such an object. Is not the whole earth made 
 of rocks ? Do not stones abound everywhere, and are 
 they not often bigger than this one ? Why should we, 
 then, think more about the Cloghvorra Stone than about 
 
2 THE CAUSE OF AN ICE AGE. 
 
 the rocks that we can see in the nearest quarry or the 
 stones which pave our streets ? A little reflection will, 
 however, show that the interest with which we regard 
 this object is not unreasonable. When we come to ex- 
 amine it we find that the materials from which it is 
 made are not the same as those of the rocks which lie 
 beneath it or immediately about it. Then, too, we 
 observe that lines of rock-bedding are distinctly marked 
 in the stone. They are indicated sufficiently in the 
 picture. We learn from its composition that this rock 
 is a fragment of a series of beds that were deposited 
 at the bottom of the sea untold ages ago. These rocks 
 were upheaved, and then in some way this particular 
 piece was broken from the rest of the mass, carried 
 to a distance, and left stranded where we now see it. 
 A close inspection made a few miles farther up the 
 valley of the Kenmare River will show the same 
 description of rock lying in situ. It is impossible 
 to doubt that the Cloghvorra Stone has migrated 
 from its home. 
 
 No second explanation of the position of this curious 
 object need be entertained. It were absurd, for ex- 
 ample, to say that the stone has lain on the shore of 
 the Kenmare River from all time ; we might with equal 
 reason assert that the monuments of Egypt have existed 
 from all eternity. In the case of the Pyramids as in 
 that of the Cloghvorra Stone, we have little or no direct 
 testimony as to their origin. We have to infer both 
 the mode in which the Great Pyramid was erected, and 
 
THE RECORDS OF THE ICE. 3 
 
 the mode in which the Cloghvorra Stone was deposited, 
 by indirect lines of reasoning. As we identify the 
 quarry from which the Egyptian masons drew their 
 supplies by the fossils which are characteristic of the 
 stones, so we identify the original home of the Clogh- 
 vorra Stone by the particular materials of which it is 
 composed. 
 
 The problem that first demands solution is to 
 discover the disruptive agent which would be potent 
 enough to rip the Cloghvorra Stone from its parent 
 bed, to bear it down the valley for miles, and to cast 
 it on the mountain side. Here again we may draw an 
 analogy between this question and the similar problem 
 that was presented in the study of the rearing of the 
 Pyramids. We find it hard to conceive how a host of 
 Egyptian toilers could have dragged great stones, each 
 weighing a couple of tons, across miles of desert, and 
 have fitted them together to form the wondrous edifice. 
 What transcendent powers must we attribute to that 
 agent which moved the Cloghvorra Stone ! It is fifty 
 times as big and fifty times as heavy as one of the 
 Great Pyramid stones. We should require elaborate 
 levers and screw-jacks to move it an inch ; but there 
 was once a power in the Ken mare valley which tossed 
 it like a plaything. 
 
 The picturesque site of the Cloghvorra Stone has 
 induced me to choose it as an illustration of a bowlder. 
 But there are multitudes of stones large and small all over 
 the kingdom which excite an interest of the same kind. 
 
4 THE CAUSE OF AN ICE AGE. 
 
 In these modern days we have discovered the agent 
 by which these bodies were quarried out and dropped 
 here and there over the country. The power is no 
 longer a mystery. The Cloghvorra Stone was not trans- 
 ported by any human agency ; it was not the freak of 
 a giant as local tradition has sometimes narrated ; it 
 was not dropped from the heavens as a meteorite, nor 
 was it a bomb projected from a volcano. There is no 
 occasion to regard a bowlder as the consequence of a 
 special effort of miraculous agency ; it was ice which 
 did the work. 
 
 Ice seems at the first glance to be a material so 
 hard and so unyielding that its behavior in the form 
 of a glacier is not a little surprising. Gigantic masses 
 of ice are not so rigid and unyielding as gigantic masses 
 of stone. We must rather regard ice as possessing 
 many of the properties of a semi-fluid substance. It 
 is more sluggish then either honey or treacle. Its be- 
 havior more nearly resembles that of pitch. It must, 
 however, be observed that I do not now enter into any 
 discussion as to the cause of the flow of the glacier. 
 Vexed questions are here involved, which lie, however, 
 wholly outside our argument. For the investigation of 
 the cause of an Ice Age it matters but little what view 
 of the nature of ice movement may be adopted. It 
 suffices for our present purpose to know that even 
 frozen water in vast masses does act somewhat like a 
 fluid, in so far that it creeps gradually downward when- 
 ever opportunity for doing so is afforded. A mimic 
 
THE RECORDS OF THE ICE. 5 
 
 glacier can be constructed by placing a block of pitch 
 at the upper end of a sloping board. In due course the 
 pitch will stealthily descend, thus imitating the move- 
 ment of the ice down an Alpine valley. 
 
 The Swiss glaciers present remarkable illustrations 
 of this property of ice when in sufficient mass. As the 
 more recently fallen snow accumulates in the higher 
 valleys of the great mountains, the weight of the super- 
 incumbent portions consolidates the earlier fallen snow 
 which lies beneath. With sufficient pressure the snow 
 passes into the more solid form of ice, and thus the 
 well-known glacier comes into being. Under the in- 
 tense pressure to which it is subjected the ice com- 
 mences to descend from the mountain tops to the valleys 
 beneath. The solid river wends its tardy way down- 
 ward between the mountains. In a river of water the 
 velocity of the flowing mass is greatest at the center and 
 gradually declines toward the sides, where the freedom 
 of motion is impeded by friction with the sides of the 
 channel. The same is the case with the glacier. 
 
 The river-like motion of the ice in a glacier has 
 been clearly demonstrated by driving a row of stakes 
 at regular intervals across the frozen current. Initially 
 these stakes may be placed in a straight line, but they 
 will gradually assume a curved form on account of the 
 increased velocity of the ice at the center over that near 
 the edges. 
 
 Although the glacier ice is thus seen to possess some 
 of the characteristics of a fluid, it yet retains sufficient 
 
6 THE CAUSE OF AN ICE AGE. 
 
 rigidity to form in its descent a potent agent for exca- 
 vation. The icy river in the course of ages has come to 
 occupy a deep ravine between two mountain slopes. It 
 will frequently happen that fragments of rock loosened 
 by frost or atmospheric influences thunder down these 
 mountain slopes and tumble on to the bosom of the 
 solid stream. In the lapse of time the glacier thus be- 
 comes freighted at each side with moraines composed 
 of blocks of rock, both large and small, which are often 
 borne in this manner to a distance of miles from their 
 original source. There is also another way in which 
 glaciers possess the power of conveying pieces of stone 
 far from the site in which they have lain since the days 
 in which their parent beds of sedimentary rock were de- 
 posited. The icy current flowing slowly over its rocky 
 bed exerts an almost irresistible power upon any obstacle 
 it may chance to encounter. At the bottom of the blue 
 and solid river pieces of stone are occasionally wrenched 
 from the rocky base. Incorporated in the solid ice these 
 blocks of course partake in the glacier motion ; nor is 
 their path always such a simple one as might be natu- 
 rally inferred from the general direction in which the 
 glacier is flowing. Here again we may revert to the 
 phenomena of a river of water for an illustration. In 
 such a river, although the general body of the water 
 tends downward parallel to the direction of the valley, 
 yet the irregularities of the channel will produce eddies 
 and subsidiary currents which occasionally assume a di- 
 rection opposite to that of the general mass of the water. 
 
THE RECORDS OF THE ICE. 7 
 
 We also observe that the water at some points has a ver- 
 tical component of motion. In that marvelous spot, 
 the whirlpool rapids of Niagara, there is no feature 
 which struck me more than the welling upward to the 
 surface of curious convolutions, due to local and sub- 
 sidiary currents which have ascended from the deep 
 water. 
 
 In a similar manner it would seem as if the equable 
 flow of a glacier down its rocky bed was checkered by 
 the presence of some cross currents which had a vertical 
 component in their motion. In such cases the ascent 
 of the ice in one part of the stream would be of course 
 more or less compensated by a descent at some other 
 points. We are thus able to account for the somewhat 
 paradoxical phenomenon, that though the whole drift 
 of the ice is from the higher level to the lower level, vet 
 it may occasionally happen that a block of stone dug 
 out by the glacier is made so much the sport of some of 
 these subsidiary currents that it ultimately attains an 
 elevation actually higher than that which it originally 
 occupied. 
 
 Two adjacent ice rivers will sometimes become con- 
 fluent, and then the rocky moraines on the two edges 
 that unite will coalesce to form a medial moraine on 
 the augmented glacier. At the present moment every 
 glacier - choked Alpine valley contains innumerable 
 stones, large and small, which have been carried for 
 many years, and have now arrived at considerable dis- 
 tances from their original home. 
 
8 THE CAUSE OF AN ICE AGE. 
 
 Let us imagine that the climate of Switzerland has 
 undergone a radical change. Let us suppose that the 
 winters are no longer severe enough to permit the accu- 
 mulation of the present incubus of snow on the mount- 
 ain tops. Let us suppose that the glaciers, stinted in- 
 their supply from the snowfields above, begin to shrink 
 in dimensions. Let us imagine that the same transfor- 
 mation goes on until the eternal snow has entirely for- 
 saken the Alps, and until a mountain torrent alone 
 occupies each channel down which a glacier now wends 
 its tedious way. When the ice has vanished from Switz- 
 erland, what would happen to the rocky moraines which 
 rested on the frozen surface ? Their materials would of 
 course lie scattered here, there, and everywhere down 
 the valleys which the glaciers had occupied. Doubtless, 
 as the glaciers shrank in size, many of the rocks which 
 happened to lie on the margin would be detained by 
 irregularities on the mountain slopes. Such blocks may 
 have any dimensions ; they may be small as grains of 
 sand, or they may be hundreds of tons in weight. 
 
 I have given this illustration for the purpose of 
 showing how we can completely account for the pres- 
 ence of the Cloghvorra Stone on the slopes of the 
 Kenmare River. There can not be a doubt that the 
 Kenmare River was once the channel down which a 
 glacier pursued its way from the highlands of Kerry to 
 the Atlantic Ocean. That glacier has long since van- 
 ished, but the Cloghvorra Stone is a monument on 
 which wo read its epitaph. The inscription tells us 
 
THE RECORDS OF THE ICE. 9 
 
 that the climate of Ireland has undergone a transfor- 
 mation like that which we just now imagined to have 
 been applied to Switzerland. The highlands of Kerry 
 are no longer crowned with eternal snow. No glacier 
 now encumbers the Kenmare River. The climate of 
 Kerry is in fact among the very mildest in the United 
 Kingdom ; it is in that county that fuchsias attain a 
 growth only to be described as arborescent, and it is in 
 the same neighborhood that the delicate Killarney fern, 
 the prized Trichomanes radicans, may be looked for. 
 But the Cloghvorra Stone assuredly tells us that in past 
 ages ice and snow in large quantities existed where now 
 this genial climate prevails. 
 
 If other evidence be required as to the past existence 
 of glaciers in the British Islands, it may be provided 
 from many different sources. Let us strive to imagine 
 the aspect which the Swiss valleys at this moment would 
 present were the glaciers to vanish by some transforma- 
 tion analogous to that which has already taken place in 
 Kerry. We have seen that it is in the power of a glacier 
 to rip stones from their bed, and to drag them down- 
 ward for miles. We can readily understand that such a 
 process must be necessarily accompanied with much vio- 
 lence. The stones forced onward will tear and scrape 
 against the rocky bed. Their corners, occasionally pro- 
 truding like colossal graving tools from the icy mass, 
 will score and mark the channel down which the exca- 
 vating machine has been forced. The bed of the glacier 
 must therefore retain abundant indications of the in- 
 
10 THE CAUSE OF AN ICE AGE. 
 
 tensity of the forces that the flowing ice has manifested. 
 Here we might expect to find scratches and rude en- 
 gravings parallel, generally speaking, to the direction of 
 motion. There we should expect to find projecting 
 edges rounded off and salient corners removed. Occa- 
 sionally we should find portions of the bed smoothed, 
 and sometimes even polished by the vigorous filing, 
 rasping, and sand-papering to which it has been sub- 
 jected. It therefore appears that for the detection of 
 ancient and now vanished glaciers we have not merely 
 to rely on the presence of erratic bowlders ; other in- 
 dications are provided by the engraving work which 
 the powerful glacier tools have been able to exe- 
 cute. 
 
 When regarded as the channels of ancient glaciers, 
 the valleys in the highlands of Great Britain receive a 
 greatly enhanced interest. Every feature that it would 
 be reasonable to expect the glaciers to carve out is ex- 
 hibited in profusion. The contours of the prominent 
 rocks or hillocks in such valleys are observed to be 
 rounded and smoothed in a way that is at once intelli- 
 gible if we assume the existence of an ancient glacier. 
 Here we find polished surfaces ; there we have long 
 fluted marks and scratches. The eye, instructed to 
 observe them, will find a variety of inscriptions which, 
 being interpreted, record that our islands must have 
 undergone a stupendous change of climate. It is cer- 
 tain that these valleys were once choked up by ice, 
 which has now entirely vanished. The existence of 
 
THE RECORDS OF THE ICE. 11 
 
 an Ice Age in former times has thus been demon- 
 strated. 
 
 I have cited Switzerland for the purpose of illus- 
 trating the phenomena of glaciers and of producing an 
 adequate comprehension of the competency of ice to 
 accomplish colossal tasks. We have now to show that 
 the distribution of ice which Switzerland at present 
 manifests, conveys only a very imperfect idea of the 
 severity of the Ice Age of which the Cloghvorra Stone 
 is a monument. For an illustration of the unhappy 
 condition of our country during the existence of the 
 great Ice Age, we must look to some far sterner clime 
 than that of the Alps at the present day. Switzerland 
 is charming, because it is only partially glaciated. Were 
 Switzerland totally glaciated, as the greater part of 
 these islands was once, the playground of Europe would 
 be transformed into an uninhabited region of utter 
 desolation. 
 
 The Arctic lands provide an instructive illustration 
 of a region which appears to be at present in a similar 
 condition to that which characterized a large part of 
 Europe at the time of the great Ice Age. Greenland 
 is the country to which we refer. It would seem that 
 ice is perennial in the interior of this desolate land, 
 which bears such an inappropriate name. The Green- 
 land ice is not confined to the summits of mountains 
 which lie above the general level ; it is a white sheet 
 overlying the greater part of the country. Here and 
 there along the coast there are some favored spots where 
 
12 THE CAUSE OF AN ICE AGE. 
 
 openings in the ice-sheet expose the soil, and permit a 
 few hardy inhabitants to maintain a precarious exist- 
 ence. As the winter snows accumulate on Greenland, 
 the glacial sheet becomes gradually thicker, until the 
 pressure of the superincumbent mass forces the sub- 
 jacent layers into tardy motion. From the high inte- 
 rior, the great ice -sheet gradually creeps downward 
 toward the coast, and thence extends into the ocean. 
 Once the ice-sheet has reached deep water, then, owing 
 to the buoyancy of its material, fragments are broken 
 off and float away as icebergs, which ultimately turn to 
 the condition of water under the influence of the hap- 
 pier climes into which they are conducted by the ocean 
 currents. Such is the origin of those vast icebergs that 
 we find in the North Atlantic. They tell us that a great 
 Ice Age still desolates Greenland and other solitudes of 
 the extreme North. 
 
 Little as we know of the Northern Polar regions, we 
 know still less about the dreary neighborhood of the 
 South Pole. It seems, however, that in its vicinity lies 
 an extensive tract which is crushed under an ice-sheet 
 far transcending, both in area and thickness, the pall 
 which lies over Greenland. From the dimensions of 
 the Antarctic icebergs it becomes possible to estimate 
 the thickness of the layer of ice, from the fringe of 
 which those icebergs have broken away. It is now gen- 
 erally believed that the layer of ice which submerges the 
 Antarctic continent must have a thickness amounting 
 to some miles. 
 
THE RECORDS OF THE ICE. 13 
 
 The study of the Polar ice-sheets, and of the influ- 
 ence which they have at present upon the land which 
 bears them, illustrates the characteristic phenomena of 
 an Ice Age. Think of the conditions under which an 
 ice-sheet exists. It is of course nourished by perennial 
 showers of snow, and as it grinds slowly onward, we 
 can realize the state of the country which lies beneath 
 its devastating influence. Our comprehension may be 
 quickened by the use of a numerical illustration. Let us 
 assume that 1,000 feet is the thickness of the ice. This 
 is a moderate estimate, for, as we have seen, the sheet 
 under which Greenland is buried has a much greater 
 depth, and the Antarctic sheet far exceeds that of 
 Greenland. Indeed, the ice which once desolated 
 Britain had sometimes a thickness two or three times 
 greater than that which we have supposed. Even with 
 the moderate thickness of 1,000 feet, an ice-sheet would 
 form a terrible engine. Every square inch of the floor 
 over which this frozen ocean plowed its way would have 
 to sustain a pressure of 400 Ibs. ; every square foot of 
 the country would on an average sustain a load of about 
 thirty tons. This is double as much as the pressure in 
 the boiler of a locomotive engine. Of course the mere 
 incubus of a load, so long as it remained at rest, would 
 not be capable of accomplishing geological work. We 
 are to think of this load, not as resting quietly, but as be- 
 ing thrust slowly onward with illimitable might. In the 
 Alps the speed of the glaciers may be taken as three feet 
 per diem. In the Arctic regions it is ten times as much. 
 
14 THE CAUSE OF AN ICE AGE. 
 
 How terrific an engine of destruction such an ice- 
 sheet would be may be shown by an illustration. The 
 base of the Great Pyramid has to support the weight of 
 stone which forms the ponderous edifice. No grinding 
 or destructive work is done on its floor by the pressure 
 of the superincumbent mass, because it lies at rest. But 
 suppose that, by the application of some power of great 
 intensity, the Great Pyramid were to be pushed onward 
 over the desert at the rate of, let us say, a foot a day 
 think of the awful friction that would be the result. 
 Imagine the grinding and crushing of the soil ; think 
 how the rocks would be torn up and pulverized by such 
 an operation. 
 
 This may afford some conception of the sufferings 
 endured by a country during the lapse of an Ice Age. 
 An ice -sheet, ponderous in mass and irresistible in 
 power, forms a destructive engine of almost illimitable 
 capacity ; by its influence fragments of the living rock 
 are ripped from their bed, crushed to pieces, reduced to 
 mud. Nor is the severity of this cruel treatment miti- 
 gated by the brevity of its action. During thousands 
 of years the ice supply is being renewed on the high- 
 lands, and the sheet sluggishly descends toward the 
 coast. 
 
 It will be noticed that there must be a remarkable 
 difference between the local action of an Alpine glacier 
 confined to a mountain valley and the extended action 
 of a wide-spread sheet of ice which stretches over an 
 entire country. As the Alpine glacier slowly winds its 
 
THE RECORDS OF THE ICE. 15 
 
 downward way, of course it must grind and pulverize 
 the materials that form the sides and the floor of its 
 rocky channel. Stupendous masses of material have 
 thus been operated on ; but in the ordinary glacier 
 such products of the icy mill are not permitted to ac- 
 cumulate. There is a vehicle incessantly at hand which 
 accomplishes their removal. Beneath the glacier a rush- 
 ing river hurries over the rocky bottom of the ravine. 
 Every visitor to Switzerland will remember the turbid 
 torrent which rushes out from beneath the termination 
 of the glacier. The pigment which has colored this 
 river is the mud which has been ground by the ice. 
 
 But the circumstances of a glaciated country over- 
 laid by an ice-sheet are very different from those of an 
 Alpine valley, so far as the destination of the materials 
 which have been ground by its action are concerned. 
 Wherever the ice moves over the higher grounds, the 
 products of the mill wiU be manufactured. Doubtless 
 there may be occasional rivers beneath the sheet, and 
 such rivers will be engaged in removing some of the 
 materials from their immediate neighborhood. How- 
 ever, the transportation by such rivers must be com- 
 paratively local. Only a small fraction of the entire 
 quantity of detritus produced in a country overlaid by 
 an ice-sheet can be removed by such agency. The ma- 
 terial is pushed along by the ice till it reaches the lower 
 plains, where it collects, and, except the local scaveng- 
 ing by streams, there is no vehicle by which the grow- 
 ing accumulations can be removed. The rocks offer no 
 
16 THE CAUSE OF AN ICE AGE. 
 
 hope of escape for the mud below, nor would an exit be 
 practicable through the ice above. We thus see that the 
 products of the stupendous grinding mill must steadily 
 accumulate, and thus in the lapse of ages a cushion of 
 clay, intermingled with stones, is formed between the 
 ice-sheet and the solid rocks of the earth's crust. 
 
 At the present moment there can be little doubt 
 that the manufacture of layers of clay by such a process 
 is in actual progress at many places both in the Arctic 
 and the Antarctic regions. The significance of this 
 action of an ice-sheet depends, however, on the fact 
 that such accumulations may serve as monuments to 
 testify to the former existence of an ice-sheet when the 
 ice itself has long since vanished. For suppose that 
 after a country has been thus glaciated for thousands of 
 years, the climate should begin to ameliorate, more ice 
 is melted in summer than is made in winter, so that the 
 sheet begins to lose its ample proportions. The glaciers- 
 shrink back to the mountain valleys, and the soil of 
 the low-lying lands is once again bared to sunshine. 
 Though the mild climate may suffice to chase away the 
 ice, yet it is a much more difficult matter to remove the 
 accumulated masses of clay. These will often, in part 
 at least, remain, and their character is so unmistakable 
 that we can point to them as infallible records of what 
 has actually taken place. 
 
 You will remember that the existence of erratic 
 bowlders afforded sufficient evidence to prove that 
 many valleys in the highlands of the United Kingdom 
 
THE RECORDS OF THE ICE. 17 
 
 were once incumbered with ice ; we have now learned 
 how to recognize the traces of a far more severe gla- 
 ciation. We have the best reasons for knowing that a 
 large part of the United Kingdom was once, or more 
 than once, buried beneath an ice-sheet. If I venture to 
 allude to the evidence on this subject which is afforded 
 in the highlands of Ireland, it is because I desire to 
 speak of phenomena which I have had frequent oppor- 
 tunities of observing. The records of the ancient pres- 
 ence of an ice-sheet over a great part of Ireland are 
 of such an unmistakable character that no one whose 
 attention has been directed to the matter can fail to 
 appreciate them. It is true that for the nice interpre- 
 tation of the more delicate parts of the evidence the 
 skilled eye and the experience of a trained geologist 
 are necessary. To such skill and training I can not 
 pretend, but the more obvious phenomena are so mani- 
 fest that he who runs may read, and it is to such phe- 
 nomena that I would specially direct attention. 
 
 To one who would study the records of glaciation a 
 visit to Clew Bay, in the west of Ireland, is full of in- 
 struction. At the time of the Ice Age an ice-sheet 
 broad and deep invaded the Atlantic. The mountains 
 in the neighborhood display evident marks of the ero- 
 sion to which they have been exposed. In some cases 
 they have been so much worn as to suggest that a vigor- 
 ous giant had actually smoothed off their flanks and 
 sometimes even their summits with a rasp. A vast 
 layer of ice-wrought material accumulated on the flanks 
 
18 TEE CAUSE OF AN ICE AGE. 
 
 of the mountains in the way we have described, and 
 much of that layer is there to this day. It is composed 
 of tough and dense clay, largely charged with multi- 
 tudes of fragments of rock of every dimension up to 
 masses which weigh several tons. It would seem that 
 the layer of the ice-wrought material must have origi- 
 nally overlaid a large part of the coast where Clew Bay 
 is now found. After the ice had vanished, the waters 
 of the ocean invaded the sheet and carved it into a 
 miniature archipelago. A cruise among these islands 
 shows many striking sections through the bowlder clay. 
 This material is so stiff that it will often form cliffs 
 nearly vertical from which characteristic stones of every 
 size and of diverse materials are protruding. If you 
 land on one of these islands you will find the shore 
 composed of bowlders that have obviously been derived 
 from the cliffs. These loose stones on the beach have, 
 of course, been cleansed from clay by the sea, and are 
 water-worn by the friction of the waves. But to study 
 the ice-manufactured material itself you must dig some 
 of its bowlders from the clay, and then carefully wash 
 away the finely wrought mud which adheres to them. 
 It will then be frequently found that the stones are 
 partly worn at the corners, and also engraved and stri- 
 ated in a remarkable manner. The glaciated stone rep- 
 resented in the figure on the following page was ob- 
 tained on one of these islands in Clew Bay. The mark- 
 ings which it displays can be accounted for if we bear 
 in min'd the origin of the bowlder clay. It can not be 
 
THE RECORDS OF THE ICE. 19 
 
 supposed that these marks were produced by the attri- 
 tion of the stone on the beach by the waves. The stone 
 
 FIG. 1. A GLACIATED STONE. 
 
20 THE CAUSE OF AN ICE AGE. 
 
 was extracted directly from the solid clay in the cliff, 
 and was not picked up from the beach. In, fact, the 
 water action on the shore would never have produced 
 marks of this description ; it would rather have tended 
 to obliterate them if they had previously existed. 
 Water-worn stones never retain that delicate tracery 
 characteristic of glaciated fragments in the bowlder 
 clay 
 
 Though the inscribed stones have obviously been 
 treated at some former time with no little violence, 
 they now repose in peace. Indeed, as they rest, and 
 have rested for thousands of years, undisturbed in their 
 bed of clay, the inscriptions they bear have been care- 
 fully protected from every source of injury. The soft 
 surrounding matrix has preserved the most delicate 
 tracery, just as the ashes of Vesuvius have preserved 
 the features of Pompeii. It is now our duty to inter- 
 pret the writing which these marked stones exhibit. In 
 the first place it is plain that such objects are frag- 
 ments which have been torn at some epoch from their 
 parent rocks. It is generally possible to point out the 
 actual districts from which certain of these fragments 
 have been derived. Both the material of the stones 
 themselves and the fossils or minerals which they some- 
 times contain will, in conjunction with other circum- 
 stances, enable them to be traced approximately to their 
 sources. We often discover that they must have been 
 dragged for miles, and we are thus enabled to pro- 
 pound the questions which the bowlder clay suggests. 
 
THE RECORDS OF THE ICE. 21 
 
 We have to explain how the upper layers of the solid 
 rock over which the ice-sheet moved became crushed 
 into fragments ; we have to account for the fact that 
 these fragments were borne for many miles across the 
 country, in the course of which journey they became 
 enveloped in vast sheets of plastic and tenacious clay ; 
 and we must show how in some cases they became en- 
 graved and scored like that stone we obtained at Clew 
 Bay. No reasonable explanation has ever been offered 
 of these phenomena except that provided by the exist- 
 ence of an ancient ice-sheet. The bowlder clay in Clew 
 Bay, as well as in other parts of the world, was manu- 
 factured by the grinding of a deep sheet of ice over the 
 surface of the unhappy regions which it laid waste. As 
 the ice advanced with deliberate movement for ages it 
 gradually interposed a substantial layer of clay, charged 
 with stones, between itself and the living rock. 
 
 The navvy often finds bowlder clay an extremely 
 tough material to excavate. Indeed, I believe that con- 
 tractors would frequently prefer to quarry solid rock 
 itself rather than deal with some descriptions of bowl- 
 der clay. The evidence, however, shows that under the 
 influence of the tremendous pressure to which it was 
 exposed by the superincumbent ice, the bowlder clay 
 did actually yield like dough to the forces brought to 
 bear upon it. It is well known in the mechanical arts 
 that many bodies which are ordinarily solid can be 
 made to flow when subjected to the influence of a suf- 
 ficiently intense pressure. Lead is assuredly a solid, yet 
 
22 THE CAUSE OF AN ICE AGE. 
 
 under the power of the hydraulic press this metal is 
 squeezed out into leaden pipes. The bowlder clay un- 
 der the ice-sheet evidently flowed in the same sluggish 
 and tardy manner as lead is made to do in the manu- 
 facture referred to. 
 
 During the kneading of the bowlder clay by the 
 advance of the overlying ice-sheet, the stones it con- 
 tained were rubbed and scratched against one another 
 or against the rocky floor. All the characteristic ap- 
 pearances which such stones present can thus be fully 
 accounted for. The asperities on the surface have been 
 frequently removed by wear and tear, and the rounded 
 edges are attributable to the same action. The extent 
 to which the glaciated stones will have suffered from 
 attrition vary in every degree. Some fragments which 
 seem to have been freshly broken off may bear but few 
 traces of the intense forces present in the torpid streams 
 down which they have been borne ; others, again, have 
 evidently been exposed to the full brunt of the torrent. 
 
 The evidence for the existence of an Ice Age has 
 now been briefly discussed. Three distinct classes of 
 phenomena have been brought into correlation. We 
 have, first, the existence of isolated bowlders like the 
 Cloghvorra Stone, which lie scattered broadcast over 
 the country. We have, secondly, the remarkable way in 
 which the contour of the hills and valleys in glaciated 
 districts have been shaped, while closer examination 
 shows the not less instructive groovings and polishings 
 to which the exposed surfaces of rock have been sub- 
 
THE RECORDS OF THE ICE. 23 
 
 jected. Finally, we have the presence of layers of bowl- 
 der clay covering vast areas in our Northern latitudes, 
 and charged with multitudes of engraved stones, which 
 require no other explanation than the former presence 
 of the ice-sheet. A theory which presents a satisfactory 
 method of accounting for a multitude of phenomena, 
 apparently so diverse in their nature, demands our care- 
 ful attention. Especially is this the case when it is 
 borne in mind that we can see even now great ice- 
 sheets in operation in other climes which are for the 
 present less favored than our own. 
 
 Provided with the means of tracing the footsteps of 
 the ancient and long-vanished ice-sheet, we can form 
 some conception of its extent and its thickness. Its 
 extent must have been far greater than even the present 
 areas of the bowlder clay, while its thickness is to be 
 inferred from the highest elevations on our lofty mount- 
 ains at which the indications of ice action have been 
 recognized. It would seem from all the evidence, that 
 a tremendous ice-sheet covered Scandinavia and choked 
 up the North Sea, while the greater part of the British 
 Islands was also buried under ice. Beneath this ocean 
 of ice even the outlying islands of our western coasts 
 were submerged, and beyond them the great sheet ex- 
 tended to some unknown distance into the Atlantic. 
 There, no doubt, an ice cliff opposed a front to the bil- 
 lows of the great ocean, and from thence icebergs broke 
 off and floated away just as they do from the coasts of 
 Greenland or Alaska at the present hour. Into the 
 
24 THE CAUSE OF AN ICE AGE. 
 
 minuter details of the subject I do not here presume to 
 enter ; let it be sufficient to say that the southern parts 
 of England seem to have been spared the fearful visita- 
 tion by which the greater part of Britain was over- 
 whelmed. In other parts of Europe phenomena of a 
 like description have been observed ; and the ice-sheet 
 seems to have extended from the North into Central 
 Europe. The New World also has not been exempted 
 from the desolation which an ice-sheet is competent to 
 produce. Over a great part of North America huge 
 glaciers broad and deep, as befitted the proportions of 
 the continent where they originated, submerged the 
 Northern regions, occupied the basins of the great 
 lakes, and spread as far toward the South, as is fully 
 described in that most interesting work, Wright's Ice 
 Age in North America. But we shall not attempt to 
 give many particulars ; let us rather present a connected 
 summary of the phenomena, which will be more suit- 
 able to our purpose. 
 
 On our earth, as we find it to-day, we have at the 
 North Pole an intensely glacial condition, which extends 
 for hundreds of miles around. So severe is the climate 
 that all attempts to penetrate the icy solitude and reach 
 the Pole have been hitherto unavailing. Encircling the 
 Arctic regions we have the temperate regions of our 
 globe, which manifest great varieties of climate, even 
 along the same circles of latitude. Farther South we 
 come to the sunny regions, which seem never to have 
 been invaded by the desolation of an Ice Age. In a 
 
THE RECORDS OF THE ICE. 25 
 
 similar manner we subdivide the Southern Hemisphere. 
 There is intense glaciation at the South Pole, and there 
 is a temperate region intervening between it and the 
 tropics. Owing, however, to the large proportion of the 
 Southern Hemisphere which is occupied by ocean, and 
 owing also to the imperfection of our knowledge of the 
 geological phenomena of the lands in the south temper- 
 ate regions, we are not able to speak with the same de- 
 gree of confidence with regard to the glaciation of the 
 Southern Hemisphere which we have felt when speak- 
 ing of the Northern. Such is at the present moment 
 the condition of our globe so far as glacial phenomena 
 are concerned. It may be described as the normal con- 
 dition. We shall presently have to contrast it with the 
 two extreme conditions which appear at some times to 
 have prevailed. 
 
 Putting the information together which we have 
 procured, we are now enabled to sketch in some degree 
 the circumstances attending the advent of an Ice Age. 
 We must imagine that the cap of ice and snow which is 
 normal at the Pole commences to enlarge its boundaries. 
 It refuses to remain confined within that Arctic circle, 
 which we are now fortunately permitted to regard as its 
 legitimate home, and it creeps downward and invades 
 the temperate latitudes. Centuries and perhaps even 
 thousands of years must have elapsed before the ice had 
 completely occupied the area destined for glacial devas- 
 tation. Doubtless the progress was intermittent. The 
 ice-sheet advanced during the winter, it retreated dur- 
 
26 THE CAUSE OF AN ICE AGE. 
 
 ing the summer, but each advance exceeded the subse- 
 quent retreat, until at last the southern limits of the 
 glaciated districts were reached. Then some of the 
 fairest regions of our globe lay crushed for ages beneath 
 a superincumbent load of ice hundreds or thousands of 
 feet in thickness. The life, both animal and vegetable, 
 which had formerly abounded in these countries in the 
 days when their climate was still temperate, had neces- 
 sarily retreated before the encroaching ice, and now 
 sought for generations the hospitalities of southern 
 climes. It was, however, not designed that the desolat- 
 ing presence of an ice-sheet should be eternal. Amelio- 
 ration began at last to take place in the climate. The 
 ice withdrew each summer more than it had advanced 
 during the preceding winter. Doubtless the transfor- 
 mation was a very slow one. Centuries must have been 
 required before the ice-sheet had completely retreated 
 within its original boundaries, and the climates of the 
 entire hemisphere had regained their normal condition. 
 
CHAPTER II. 
 
 THE SUCCESSION OF GLACIAL AND GENIAL PERIODS. 
 
 THE records of tho rocks, however, sometimes sug- 
 gest a far more agreeable picture. We have learned 
 that our hemisphere was once covered with ice, but we 
 may also learn that there have been times when con- 
 ditions widely different have prevailed. The climate 
 has once been more mild and more genial over the 
 Northern Hemisphere than that which it now enjoys. 
 Sunshine and warmth have abounded within the Arc- 
 tic circle, and, it would seem, as far as the Pole itself. 
 There have been times when Greenland merited the at- 
 tractive name it bears. A luxuriant verdure has clothed 
 its shores, and stately forests have adorned the land now 
 smothered beneath a thousand feet of ice ; even in the 
 vicinity of the dreary North Pole itself, as far as the 
 hardihood of Arctic explorers has carried them, the re- 
 mains of what seems to have been a charming vegetation 
 have been discovered ; plants which are too tender to 
 withstand the rigors of an ordinary British winter with- 
 out protection, appear at one time to have flourished 
 within a few hundred miles of the Pole. In the Arctic 
 solitudes there were once fresh- water lakes, whose mar- 
 
28 THE CAUSE OF AN ICE AGE. 
 
 gins were fringed with large reeds, while water-lilies, 
 which demanded a mild climate, floated on their sur- 
 face. Just as the ice-sheet, when it prevailed during 
 the glacial epoch, suggests a climate incomparably more 
 severe than that which we now possess, so these genial 
 conditions indicate that our hemisphere must have also 
 enjoyed intervals in which the climate was far more 
 benign than that which we at present experience. 
 
 I would, then, have you think of our earth, or more 
 accurately of one hemisphere of the earth, as robed from 
 time to time in three different garbs. There is, first, 
 the garb of extreme glaciation which prevails during an 
 Ice Age ; the second is one when what may be described 
 as a perennial summer prevails in the land, and extends 
 even to the vicinity of the Pole itself ; this we may call 
 the genial garb. There is, thirdly, an intermediate garb, 
 where the climate has neither the severity of the Ice 
 Age nor the mildness of the Genial Age. This inter- 
 mediate state may be illustrated by the condition in 
 which we find our earth to-day, and in which it has 
 been during the centuries known to human history. 
 
 It is necessary to mention at this stage of our dis- 
 cussion that the question has been sometimes debated 
 among geologists as to the simultaneity of the existence 
 of the ice-sheet over different continents of the North- 
 ern Hemisphere. No one can reasonably hesitate to be- 
 lieve that Northern Europe was once desolated by an 
 ice-sheet, and no one can doubt but that a similar visita- 
 tion once made a wilderness of a large part of the North 
 
GLACIAL AND GENIAL PERIODS. 29 
 
 American continent. It has, however, been doubted 
 whether the centuries during which the ice inundated 
 Europe, were also the centuries during which the frozen 
 deluge inundated America. It has been sometimes con- 
 tended that the icy invasions were indeed not simul- 
 taneous. It has been thought that the ice-sheet some- 
 times ravaged one continent, and was subsequently 
 transferred to another. The testimony of the rocks is 
 notoriously imperfect ; indeed, it is difficult to imagine 
 what the nature of that evidence would be by which 
 such a point could be summarily decided. I think, 
 however, I may say that the weight of opinion, among 
 those best qualified to judge, inclines to the belief that 
 the glacial invasion of the Northern continents took 
 place at the same time. Great Britain and Scandinavia 
 were not alone at the time of their desolation ; around 
 Europe and America, around the Old World and the 
 New, the great Ice Age wrapped its deadly mantle. 
 
 I may so far anticipate the theory of the Ice Age, 
 which I shall shortly enunciate, as to say that it is of 
 the essence of that theory that the glaciation over a 
 hemisphere shall be simultaneous. In fact, the strong- 
 est argument in favor of the simultaneity appears to be 
 deduced from the theory itself. Indeed, if the Ice Ages 
 be not simultaneous if, for instance, Canada had been 
 in a genial garb while Great Britain was in a glacial 
 one it would seem wholly impossible to offer an ex- 
 planation of the phenomena by any physical cause 
 known to us. 
 
30 THE CAUSE OF AN ICE AGE. 
 
 It is also an essential doctrine of the Astronomical 
 theory of the Ice Age that the glaciation of the South- 
 ern Hemisphere shall be simultaneous over the Polar 
 parts of that hemisphere, just as we have understood it 
 to have been in the Northern regions. Viewing, how- 
 ever, the two hemispheres each as a whole, it is most im- 
 portant to observe that their respective glaciations were 
 not simultaneous. This is, at least, the necessary con- 
 sequence of the Astronomical doctrine, and we look 
 with anxiety to the geological data for any verification 
 of so crucial a point in the theory. The geological facts 
 that can be appealed to on such a question are, it must 
 be admitted, of a somewhat meager character. Indeed, 
 if we will but reflect on the conditions of the problem, 
 we shall find it rather hard to conceive what those 
 facts should be which would demonstrate that when the 
 Northern Hemisphere was glacial the Southern Hemi- 
 sphere was genial, or when the Southern Hemisphere 
 was glacial the Northern Hemisphere was genial. 
 
 It is obvious that the advent of an Ice Age must 
 enormously affect the welfare of organized beings. We 
 have often read how animals flee in terror before the 
 prairie fires in North America ; we can equally under- 
 stand how the sheet of ice, trespassing beyond its accus- 
 tomed home in the Arctic regions, would drive to more 
 genial climes the plants and animals that inhabited the 
 temperate zones subjected to such an invasion. Whither 
 are these organisms to go? If it should happen that 
 the opposite hemisphere was simultaneously desolated 
 
GLACIAL AND GENIAL PERIODS. 31 
 
 by an ice-sheet, then, indeed, all animals to whom tem- 
 perate climes were congenial would find their natural 
 habitats very seriously restricted. They would verily 
 lie between the upper and the nether millstone. Sup- 
 pose, on the other hand, that while one hemisphere was 
 experiencing the rigors of an Ice Age the opposite hemi- 
 sphere was not glaciated ; nay, further, let us suppose 
 that it was not even in its normal state, but that it had 
 assumed that genial garb in which verdure seems to 
 clothe the land from the Pole to the Equator; then 
 this delightful region would seem almost designed to 
 offer hospitality to those creatures which were flying 
 before the ice in the desolated hemisphere in search of 
 a suitable abode. Again, after the lapse of ages another 
 change of condition takes place : the glaciated hemi- 
 sphere experiences an amelioration of climate. The 
 ice-sheet shrinks back within its normal confines in the 
 Arctic regions, and the contraction of margin and dimi- 
 nution of bulk continues until the ice nearly or totally 
 vanishes, and a genial climate returns to revivify the 
 newly uncovered earth. The reappearance of the former 
 conditions on land and at sea will invite the return to 
 their original home of the plants and animals whose an- 
 cestors had been chased away so many centuries ago. 
 "We may further suppose that the recall of these emi- 
 grants would be accelerated by the fact that the land 
 which had cherished them during their exile had at 
 last become persistently inclement ; had begun, in fact, 
 to fabricate an ice-sheet before which organized life 
 
32 THE CAUSE OF AN ICE AGE. 
 
 must necessarily flee. Thus it would come to pass, 
 when sufficient time had elapsed, that the plants and 
 animals, or rather their descendants, would find them- 
 selves re-established in their old home. 
 
 It is essentially characteristic of such phenomena 
 that they are periodic. When the fullness of time has 
 arrived, the conditions which produce an ice-sheet are 
 again restored ; an ice-sheet is consequently formed, a 
 general emigration again takes place, to be followed, as 
 ages roll on, by yet another immigration. In accord- 
 ance with this view of the order in which Ice Ages and 
 Genial Ages alternate on our globe, we should expect to 
 find indications of an oscillation of characteristic or- 
 ganic forms between one hemisphere and another. In 
 Darwin's pages will be found some singularly interest- 
 ing phenomena connected with the distribution of Al- 
 pine plants and animals which lend much support to 
 this view. 
 
 I have found it necessary to assume the existence of 
 several Ice Ages. It is now convenient to state a little 
 more fully the grounds on which we assert that such re- 
 markable vicissitudes of climate have actually occurred. 
 
 The records of the rocks assure us that our temper- 
 ate zones have not suffered from the icy invasion on a 
 single occasion only. Distinct evidence on this subject 
 is available from several localities. Nor is the interpre- 
 tation of that evidence a difficult matter. We are in 
 the first place to recollect that after the ice-sheet has 
 melted away, it leaves a deposit of bowlder clay extend- 
 
GLACIAL AND GENIAL PERIODS. 33 
 
 ing far and wide. A more unkindly soil than that same 
 bowlder clay can hardly be conceived ; but with a gra- 
 cious climate, vegetation of some kind will gradually 
 begin to cover the surface, so that, as years roll on, a 
 humus of nutritious soil, charged with organic matter, 
 will establish itself, and verdure will beautify the un- 
 lovely material. In the course of time great forests 
 may spring up, and a luxuriant underwood may provide 
 the shelter and sustenance for numerous animals both 
 great and small. A long and glorious genial period has 
 succeeded to the dread ages of glaciation. Again, after 
 the lapse of thousands of years, the scene begins to 
 change. The contrast between the seasons becomes 
 more marked, the severity of the winter becomes sig- 
 nificant, and the ice-sheet again begins to creep on with 
 irresistible motion. In its desolating progress it often 
 plows away the stately timber, the underwood, and even 
 the accumulations of rich soil that have appeared after 
 centuries of incessant vegetable growth. Over the sur- 
 face thus fearfully devastated another layer of bowlder 
 clay will be deposited by the ice-sheet which has laid 
 the land waste. 
 
 Historians are often indebted to merely fortuitous 
 circumstances for the preservation of facts of the high- 
 est value, and an incidental reference by some ancient 
 writer will frequently in later ages attain an importance 
 that could never have been suspected at the time it was 
 penned. But no historian is more indebted to the casual 
 preservation of facts than is the geologist. The ice- 
 
34: THE CAUSE OF AN ICE AGE. 
 
 sheet plows away the superficial features of the country, 
 and if this work were perfectly done we should be de- 
 prived of information testimony of paramount conse- 
 quence for our present purpose. But it occasionally 
 happens that by some odd chance the disturbance of 
 the soil by the progress of the ice-sheet has been locally 
 impeded. It may have been merely the obstruction of 
 some larger piece of rock, which has slightly checked 
 the local force of the icy current. At all events we find 
 that patches of the surface lie unmolested beneath the 
 bowlder clay which the new ice-sheet has deposited. In 
 a railway cutting, or other section through such a mass, 
 the heterogeneous substance will be occasionally inter- 
 rupted by some layers of materials more or less stratified, 
 and consisting of a soil charged with organic matter. 
 Such beds sometimes inclose fragments of the plants 
 and specimens of the animals which inhabited the dis- 
 trict during the genial period which intervened between 
 the glaciations indicated by the two sheets of bowlder 
 clay. From evidence of this kind we learn that suc- 
 cessive Ice Ages have been often separated by genial 
 intervals. Unfortunately, the number of such instances 
 is necessarily very limited. But certain it is, however, 
 that in some places there have been preserved to us suc- 
 cessive layers of bowlder clay, duly interrupted by strati- 
 fied beds, bearing remains of organisms whose place in 
 such a sequence can only be interpreted as evidence of 
 the intervention of genial periods. 
 
 It will be convenient if I here set forth a summary 
 
GLACIAL AND GENIAL PERIODS. 35 
 
 of the order of succession of the glacial phenomena of 
 which our earth has been the scene. I have already 
 pointed out that the imperfect geological record does 
 not afford us adequate information on the subject ; we 
 are compelled to resort for guidance to the Astronomical 
 theory. It is clear that if this theory be accepted, then, 
 within a period certainly to be reckoned in hundreds of 
 thousands of years, and probably attaining to millions 
 of years, the succession of Ice Ages and Genial Ages, 
 and their relations to the normal condition of the 
 earth, must have been somewhat as follows. 
 
 After our globe had continued in a normal state for 
 an extended period of time, during which both hemi- 
 spheres wore that same garb which they have at this 
 moment, a changed climatic condition supervenes. Over 
 one of the hemispheres, let us say the Northern, the se- 
 verity of the winters begins to increase, the ice-sheets 
 radiate from the highlands, and the rigors of a glacial 
 period set in. But while the Northern Hemisphere is 
 thus desolated, the Southern does not maintain its nor- 
 mal state ; a change of an opposite character is there 
 experienced, so that by the time extreme glaciation has 
 seized the Northern zones in its deadly grip, the South- 
 ern have assumed that delightful aspect which we call 
 the Genial Age, in which the rigors of an ordinary win- 
 ter have been abated, or, indeed, totally cleared away. 
 
 But such a state of things is not eternal ; the great 
 oscillation continues. The Northern Hemisphere will 
 exchange its climate for that of the Southern. Here I 
 
36 THE CAUSE OF AN ICE AGE, 
 
 should specially note that we are enabled to offer some 
 numerical facts, which are all the more acceptable inas- 
 much as they are unhappily rather scarce in the Astro- 
 nomical theory of Ice Ages. We have sufficient reasons 
 for asserting that when one of these critical periods is 
 in progress, the interval of time between the maximum 
 of an Ice Age in one hemisphere and the next ensuing 
 maximum of an Ice Age in the opposite hemisphere is 
 about 10,500 years. Astronomical data of unchallenged 
 accuracy provide us with these figures. The Ice Age 
 can, however, have no longer reign in the Southern 
 Hemisphere than it was permitted to have in the North- 
 ern; another period of 10,500 years will suffice for a 
 restoration of the ice from South to North. Again, 
 another similar period, and the ice will have quitted the 
 North to resume its tyranny over the South. Under 
 such circumstances there will be a succession of Ice 
 Ages, and it must be noticed that while the earth is in 
 this somewhat critical condition, the normal garb of 
 climate is never assumed. Each hemisphere is plunged 
 alternately into extremely glacial and axtremely genial 
 conditions, and though no doubt during the transition 
 there may be centuries during which intermediate con- 
 ditions will prevail, yet such periods can hardly be said 
 to have resembled the normal condition of the globe, 
 such as that which it now enjoys. 
 
 A certain cluster of Genial Ages and Ice Ages thus 
 succeed each other in a fashion which we can almost 
 describe as rapid when we bear in mind the extremely 
 
GLACIAL AND GENIAL PERIODS. 37 
 
 protracted character of other great geological periods. 
 At last, however, a change sets in ; the intensity of each 
 succeeding glaciation declines, and, doubtless, the alter- 
 nating genial epochs will have lost some of their charm, 
 until the normal condition of the globe will have become 
 restored. In this state, when the winters and the sum- 
 mers are much as we find them at present, our globe 
 continues for a period that is doubtless to be measured 
 by some hundreds of thousands of years, and Ice Ages 
 and Genial Ages will be things of the far past. After a 
 protracted interval of time, compared with which the 
 duration of an Ice Age or of a cluster of Ice Ages is 
 comparatively brief, the climatic conditions of the earth 
 will again relapse into that acute phase in which a fresh 
 cluster of glacial and genial periods will swing alter- 
 nately from one hemisphere to the other. Again these 
 acute climatic symptoms will pass away ; again will 
 ensue an enormously long period of a perfectly normal 
 climate, until the next recurrence of active conditions. 
 
 Thus we see that the ordinary normal climate of the 
 earth is interrupted at long intervals by comparatively 
 brief fits, during which Ice Ages and Genial Ages inter- 
 change between the hemispheres. Such is at all events 
 the sequence which the Astronomical theory assigns to 
 the phenomena. I need hardly add that it is necessary 
 to examine with scrupulous care all the definitely ascer- 
 tained geological facts which can throw any light on 
 the actual laws of the succession of glacial periods. So 
 far as I have been able to learn, the evidence presented 
 
 4 
 
38 THE CAUSE OF AN ICE AGE. 
 
 by bowlder clay and other traces of the Ice Age can be 
 reconciled with a sequence of phenomena such as we 
 have just indicated. 
 
 We can now enunciate in its full significance the 
 magnificent problem which lies before us. It is worthy 
 of every effort we can put forth to obtain its solution. 
 We have to discover why a huge sheet of ice grew over 
 a large part of our hemisphere. We have to show how 
 its existence was sufficiently prolonged to enable that 
 ice-sheet to become hundreds and often thousands of 
 feet thick. We have to show how the forces in action 
 were powerful enough for the ice to plane away the 
 slopes of mountains, and to knead together sheets of 
 bowlder clay often scores of feet in thickness. We have 
 to explain the advent of more genial conditions, and to 
 show how the ice-sheet was chased away and replaced by 
 the charms of a perennial summer. We have to show 
 how the great Ice Age was again, and even yet again, 
 permitted to hold the hemisphere in its lethal embrace, 
 and was again, and yet again, driven back during the 
 vicissitudes of an eternal conflict. Was ever a grander 
 problem presented to the student of Nature ? 
 
CHAPTER III. 
 
 ATTEMPTS TO SOLVE THE PROBLEM OF GLACIATION. 
 
 IT has been one of my objects in the preceding chap- 
 ter to indicate as forcibly as I can the ample extent of 
 the ice-sheets which cover a large part of a glaciated 
 hemisphere. The wide extent of the region laid deso- 
 late during an Ice Age is a consideration of the utmost 
 importance when we come to examine the theories 
 which have been offered as an explanation of the phe- 
 nomenon. In fact, the dimensions of the problem may 
 be regarded as affording a test that can be readily ap- 
 plied to any proposed theory, with the not unfrequent 
 result of consigning to oblivion the theory submitted to 
 the ordeal. ]STo cause can account for an event so stu- 
 pendous as an Ice Age, unless it contains promise of 
 magnificent accomplishment commensurate with the 
 work to be done. 
 
 It is necessary at the outset to glance at the view 
 which is the natural outcome of the revolution in geol- 
 ogy wrought by the labors of Lyell. In his ever memo- 
 rable work, Lyell has sought to account for the great 
 changes in the climates of the earth, by oscillations be- 
 tween the relative positions of the continents and the 
 
40 THE CAUSE OF AN ICE AGE. 
 
 oceans. If the land on our globe were generally re- 
 stricted to the Northern and Southern regions, while 
 the oceans were disposed in a broad girdle encircling the 
 earth at the equator, the climates of the globe would 
 doubtless be widely different from those which it would 
 possess if the relative positions of the land and the water 
 were interchanged. There can be no doubt that during 
 past geological periods the disposition of the continental 
 masses relatively to the surrounding oceans has under- 
 gone many changes. In the interior of our great conti- 
 nents, and at considerable altitudes on mountain sum- 
 mits, we find unquestionable evidence, in the presence of 
 marine fossils, that sea once occupied what is now dry 
 land. Such interchanges have occurred again and again 
 within geological times, and they were doubtless associ- 
 ated with, and were the causes of, marked modifications 
 in terrestrial climate. It would, however, be unnecessary 
 for our present purpose to enter into any discussion as 
 to the extent to which the temperature or other climatic 
 elements could be affected by any mere rearrangement 
 of the land and water on the globe. This question can 
 hardly be said to be relevant to the theory of the Ice 
 Age, for we must recollect that the times of which we 
 speak, when discussing glacial phenomena, are but of 
 yesterday when placed in their true geological perspec- 
 tive. The stratified rocks indicative of the great geo- 
 logical periods had all been consolidated for unnum- 
 bered ages. The strange flora with which the earth had 
 been clothed during remote geological periods had van- 
 
THE PROBLEM OF GLACIATION. 41 
 
 ished, and the plants that at present adorn our globe 
 had generally taken their places. To a great extent, 
 also, the animals which dwelt on the earth during the 
 last Glacial Period are the animals that dwell with us 
 now, only we should not forget what Mr. Wallace has so 
 forcibly stated, that we live at present in a zoologically 
 impoverished age, from which many of the largest and 
 the finest animals, such as the mammoths and the 
 saber-toothed tigers, have but recently vanished. We 
 should, however, be probably correct in asserting, that 
 all the animals now inhabiting this globe, man himself 
 not excepted, survived through the last Glacial Period, 
 if not through one or more similar antecedent periods. 
 The outlines of the continents were also much the same 
 in glacial times as they are at present, and consequently 
 the cause of climatic change which Lyell has so ably 
 discussed can not now be appealed to ; we are therefore 
 compelled to look for the cause of the Ice Age in some 
 other direction. 
 
 So long as we merely think of agencies in operation 
 on this globe itself, it seems hardly possible to conjecture 
 what cause would be sufficiently potent for the produc- 
 tion of phenomena so extraordinary. The creation of an 
 ice-sheet which extended over a large part of an entire 
 hemisphere would not admit of the explanation of hav- 
 ing been caused by any insignificant agency. But when 
 we come to regard the subject in its true light as hav- 
 ing an astronomical origin, we find abundantly adequate 
 forces at our disposal. An ice-sheet is a very great phe- 
 
42 THE CAUSE OF AN ICE AGE. 
 
 nomenon from one point of view, and a very small phe- 
 nomenon from another. The geographer who found 
 regions devastated by its influence, would naturally re- 
 gard it as an awful catastrophe. To the astronomer, 
 however, an Ice Age will only present itself as a trifling, 
 though no doubt an interesting, incident. 
 
 It is surprising to find how the difficulties of the 
 subject vanish when once the solution of the problem 
 is sought in astronomy rather than in geography. The 
 whole perspective becomes changed. No longer do we 
 remember that our earth is a gigantic globe sustaining 
 over a thousand millions of inhabitants ; for astrono- 
 mers regard it merely as a small particle which, with a 
 number of other similar particles, are all performing 
 their voyages through space under the guidance of a 
 great central luminary. In the solar system we already 
 find a potency amply sufficient for such a trifling effort 
 as that implied in the chilling of one hemisphere of a 
 little globe so small that it could hardly ever be sighted 
 from many of the other globes belonging to the same 
 family. The relations between the several bodies in the 
 solar system are such that a very small change, astro- 
 nomically speaking, might be competent to effect a stu- 
 pendous transformation in terrestrial climate. Several 
 different astronomical solutions of the problem have 
 been offered, but, with the exception of that with which 
 the names of Adhemar and Croll are so honorably, 
 though somewhat erroneously, associated, they all ap- 
 pear to contain some vital defect. 
 
THE PROBLEM OF GLACIATION. 4.3 
 
 It is, however, almost unnecessary to add that the As- 
 tronomical theory has still some opponents. Doubtless 
 this is partly due to the peculiar position of this sub- 
 ject, as it were on the border-land of two great sciences. 
 There are still some, I believe, who think that the Ice Age 
 has arisen from the interchange of position between land 
 and water. There are some who assert that the Ice Age 
 was caused by excessive heat, just as there are others 
 who say that there was never any Ice Age at all. With 
 every respect for the views of those who hold different 
 opinions, I must be permitted to say that the Astro- 
 nomical theory seems to offer the only intelligible ex- 
 planation of the phenomenon in question. It is no 
 doubt true that there must have been many circum- 
 stances which tended to modify the climatic changes 
 that have followed from astronomical causes ; it is no 
 doubt true that many difficult subsidiary questions arise. 
 There are questions about the laws of radiation and of 
 cooling, questions as to the distribution of land and 
 water, and questions as to the effects of winds and 
 clouds and ocean currents. But none of these details, 
 however important they may be, should be permitted to 
 obscure the broad features of the question. In fact, it 
 might almost be said that the Astronomical theory must 
 be necessarily true, inasmuch as it is a strictly mathe- 
 matical consequence from the laws of gravitation. Per- 
 haps it would hardly be an exaggeration to assert that 
 even if geologists had not hitherto discovered the Ice 
 Age from its records on the globe's surface, astronomers 
 
44 THE CAUSE OF AN ICE AGE. 
 
 would have demonstrated by calculation that Ice Ages 
 must have happened, and would even now be urging 
 the geologists to go and look for their traces. 
 
 Once we have determined to search for an astro- 
 nomical solution we can hardly hesitate as to which 
 of the heavenly bodies must first be reckoned with. 
 Considering that the warmth which preserves our life 
 on the earth's surface is due entirely to the sun's 
 beams, it seems inevitable that any vast climatic change 
 such as that of an Ice Age must be connected with 
 some irregularity or variation in the daily supply of 
 sunbeams to warm the earth. Imagine for a moment 
 that our beneficent sun was to withhold his radiation 
 from us. If the genial beams no longer warmed the 
 earth, the temperature of our globe, on its surface, at 
 all events, would begin to fall. Down will the tem- 
 perature sink until whatever store of heat the earth 
 may have retained on its surface from past sunbeams 
 would rapidly flow away, and in return we should only 
 receive such miserable succor as is afforded by the 
 radiation from the stars. How paltry this would be 
 will be obvious when we reflect on the feebleness of 
 star light as contrasted with that of sun light. Star 
 heat bears in all probability an equally insignificant pro- 
 portion to sun heat. Down, therefore, the earth's tem- 
 perature must sink to 100 or 200 or 300 below zero ; 
 down, in fact, to the temperature of space ; to a point 
 which can hardly be much above the absolute zero of 
 temperature itself. Such would be the unhappy con- 
 
THE PROBLEM OF GLACIATION. 45 
 
 dition of our globe if the sun no longer communicated 
 warmth to us. Now we shall be able to appreciate the 
 full significance of the benefits which the sun confers. 
 His radiation maintains the temperature of our globe 
 at a point which is at least 300 higher than that to 
 which we should be forced to submit if the sun ceased 
 to share with us his warmth. It might indeed be justi- 
 fiable to say that the temperature of the earth is 400 
 above that which it would have if the sun were absent. 
 This would of course strengthen the argument on 
 which I am about to enter. I shall, however, to avoid 
 any semblance of exaggeration, be content to take 300 
 as the basis of my calculation. The degrees are of 
 course Fahrenheit. 
 
 The point that I have just mentioned is of such 
 great significance in the Astronomical theory of the Ice 
 Age that I desire to draw particular attention to it. 
 We ordinarily estimate the benefits derived from the 
 sun by the excess of the genial warmth of summer 
 over the ordinary rigor of winter. A temperature of 
 zero Fahrenheit is naturally viewed in our climate as an 
 indication of a winter of extreme severity ; but we must 
 remember that, cold as this is, we should be infinitely 
 worse off if it were not for the sun's heat. In fact, the 
 range between summer and winter only corresponds to 
 a very small proportional change in the actual tempera- 
 ture at which our earth is maintained by the sun. Let 
 me endeavor to illustrate this point. I can not assert 
 that the figures I mention are rigorously correct, but 
 
46 THE CAUSE OF AX ICE AGE. 
 
 there is no doubt that they are at all events sufficiently 
 so for the purpose of enforcing my argument. 
 
 Suppose that the average winter temperature in a 
 cold climate were 30, and that the average summer 
 temperature in the same country were 60, we might 
 hastily assert that twice as much benefit was received 
 from the sun in summer as in winter. A little reflec- 
 tion will, however, show that this is not at all true ; we 
 are led into error, and perhaps excusably so, by having 
 used in our thermometers a zero of temperature which 
 is entirely arbitrary. 
 
 The natural zero to be used in discussions on climate 
 would be the temperature to which our earth would fall 
 were the heat of the sun withdrawn entirely. There is, 
 of course, much uncertainty as to the extent to which 
 this point is below the ordinary zero that we employ. 
 "We have, however, decided that for our present argu- 
 ment it may lie at, let us say, 300 below that of 
 Fahrenheit. As the summer temperature of the cli- 
 mate we have been describing is 60 above the arbitrary 
 zero, it will of course be 360 above the natural zero. 
 In like manner, the winter temperature of that climate, 
 being 30 above the zero of Fahrenheit, will stand at 
 330 above the natural zero, which we are now assum- 
 ing. The summer and winter temperatures are thus 
 360 and 330 instead of 60 or 30, as they would be 
 when reckoned by the more familiar standards. We 
 therefore see that the difference between summer and 
 winter corresponds to a comparatively small change 
 
THE PROBLEM OF GLACIATION. 4.7 
 
 when viewed in proportion to the whole range of tem- 
 perature at which the earth is sustained by the sun's 
 radiation. With sufficient accuracy for the purpose of 
 illustration, we may perhaps estimate the heat which 
 we receive from the sun, by supposing it to be propor- 
 tional to the temperature at which the earth is sus- 
 tained when measured from the natural zero. Thus 
 taking the heat received from the sun in summer to be 
 360, we find that received in winter is 330, or only 
 one twelfth less. Of course I do not maintain that the 
 heat received can really be measured in quite so simple 
 a fashion; I merely wish to illustrate the important 
 point that such a range of temperature as we experience 
 between summer and winter only corresponds to a very 
 small proportional change in the total supply of heat 
 which is received from the sun. This point is apt to 
 be lost sight of when we merely express the tempera- 
 ture by units in which the ordinary zero is employed. 
 A fall from 60 to 30 will seem to correspond to a 
 decline of heat amounting to 50 per cent from the 
 source of supply; whereas when we place the zero 
 where it ought to be, it would seem that a deficiency 
 of about 8 per cent in the heat supply would be quite 
 large enough to account for the change between sum- 
 mer and winter. 
 
 I have laid particular stress upon this illustration 
 because it is impossible to comprehend the character 
 and the origin of an Ice Age unless the idea which it 
 involves be thoroughly understood. It shows us that 
 
18 THE CAUSE OF AN ICE AGE. 
 
 though for the phenomena of glaciation a fall in 
 peratare to a considerable number of degrees may bo 
 required, jet that such a fall does not necessarily in- 
 volve any wide departure from the uniformity of our 
 present heat supply. In brief, the argument on which 
 I have been here insisting amounts to that 
 
 changes in the intensity of the solar radiation of rela- 
 tive unimportance to the sun, may suffice to pro 
 climatic changes of startling, and even appall 
 nitnde on the earth. 
 
 These illustrations show that the expla: 
 an Ice Age assumes a comparatively easy aspect when 
 viewed from an astronomical standpoint A small pro- 
 portional change in the daily receipt of sunbeams 
 in its climatic effects be competent to transform this 
 earth from the normal state to the glacial state, while 
 another small change would dissipate the ice-sheet, and 
 make the whole hemisphere glow with the charms of a 
 Genial Period. 
 
 We have now to discuss what the cause of the 
 changes in solar radiation may have been. The inten- 
 sity of the heat which we receive from the sun depends 
 partly on the intrinsic heat of the great luminary and 
 partly on the distance by which we are separated from 
 him. If either the distance of the earth from the sun, 
 or the intrinsic heat of the sun, be altered even 
 small extent from its normal value, climatic changes 
 wfll result; and these changes may seem altogether dis- 
 proportionate to the cause which has originated them. 
 
THE PROBLEM OF GLACIATttMf. 4^ 
 
 In oar search for the cause of the Ice Age, we hare 
 therefore to examine the possible fluctuation* which 
 the son's beat may experience, and we most also con- 
 sider the possible fluctuations of the earth's orbit upon 
 which the quantity of the heat which we receive from 
 the sun is dependent 
 
 Is, then, the sun a capricious source of heat? Does 
 he really discharge for us a supply which is at some 
 periods more copious than at others? At a first glance 
 we might think that this may indeed be the case, for 
 there are no doubt some mysterious periodic phe- 
 nomena associated with the sun which are at present 
 but little understood. The outbreak of sun-spot* in 
 certain years attains a mATimnm, and an undoubted 
 rhythmic succession of such years of sun-spot mariim 
 has been discovered. But it is quite evident that the 
 phenomena here referred to are so brief in their period 
 that they are wholly without any influence on ages so 
 immense as those which glacial phenomena require. 
 
 We must also remember the great astronomical 
 truth that our sun is only a star, and we are well 
 aware that among the stars there are not a few which 
 are properly called variables. There are stars whoso 
 brilliancy waxes and wanes with more or less regular- 
 ity; some of them diminish occasionally to invisibility 
 while at their brightest they are eminently conspicuous 
 objects. Certain classes of variable stars go through a 
 systematic series of changes, and repeat those changes 
 during thousands of cycles with undeviating uniform- 
 
50 THE CAUSE OF AN ICE AGE. 
 
 ity. It lias therefore been sometimes thought that as 
 our sun is a star, he may also be a variable star. But 
 there is really no substantial ground for such a surmise. 
 It seems almost certain that what we call variable stars 
 are only variable in their light, in much the same way 
 as the beams from a revolving lighthouse exhibit peri- 
 odic changes to the mariner. The variable star is 
 doubtless in rotation itself, and is attended by other 
 objects which revolve around it. The variableness 
 which it presents to us is certainly in some way a con- 
 sequence of these rotations, though it cannot be denied 
 that a good deal of obscurity still surrounds the sub- 
 ject. 'There is not the slightest reason to believe that 
 the alterations are due to any actual shrinkage in the 
 volume of the radiation. What happens is, that a por- 
 tion of the light is sometimes intercepted or sent else- 
 where; thus the variable star appears to change its 
 luster, or sometimes becomes altogether invisible. 
 
 This analogy is really of no service in the attempt 
 to explain the cause of the Ice Age. It is perfectly 
 certain that our sun is not at all a variable star in the 
 sense in which we speak of Algol or Mira Ceti as vari- 
 ables. It is also certain that even if the sun did possess 
 a character of this description, the circumstance would 
 give us no aid whatever toward the solution of the par- 
 ticular problems presented by the Ice Age. 
 
 I do not, however, assert that from age to age, from 
 one million of years to the next million of years, the 
 radiation which the sun dispenses must be absolutely 
 
THE PROBLEM OF GLACIATIOX. 51 
 
 uniform. Indeed, we know that this can hardly be the 
 case ; the sun must obey the same laws of heat which 
 we have studied in our laboratories. Radiation involves 
 loss, and large though the sun is, and intensely heated 
 though it is, our luminary has still only a certain capi- 
 tal of heat available for all purposes. The crash of 
 meteors will, no doubt, serve to supply heat which will 
 compensate, in a small degree, for the torrent which is 
 perennially dispersed by radiation. It can not, how- 
 ever, be doubted that the sun discharges more than it 
 receives, so that at the close of each century it contains 
 much less heat, actual and potential, than it had at the 
 beginning. It seems almost a paradox to say the sun is 
 losing heat, and then to add that it may yet not be fall- 
 ing in temperature. It may be, indeed, that the sun 
 during a large part of its career actually rises in tem- 
 perature notwithstanding that it is continually losing 
 its heat. I do not now dwell on this matter. I have 
 only introduced it for the purpose of showing that there 
 is no intrinsic variation in the sun's radiation which 
 would account for such phenomena as Ice Ages. The 
 solution of the problem must account for the disap- 
 pearance as well as for the appearance of Ice Ages, and 
 for the appearance as well as the disappearance of those 
 genial intervals by which successive glaciations have 
 been divided. The changes that are possible in the 
 sun's radiation will not explain such a succession of 
 contrasted phenomena. There can be no doubt that in 
 stupendous periods of time past the intensity of the 
 
52 THE CAUSE OF AX ICE AGE. 
 
 sun's radiation has performed a deliberate progress. 
 Compared, however, with the changes in the heat-emit- 
 ting power of the sun through past uncounted millions 
 of years, the coming of Ice Ages and the going of Ice 
 Ages can only he described as the merest flutter. So 
 far as our present inquiry is concerned we may conduct 
 our arguments on the supposition that the intrinsic 
 radiation from the sun has remained at its present 
 value throughout that recent chapter in geological his- 
 tory which treats of the distractions of our globe by 
 glaciation. 
 
 I ought also to mention, that there are some other 
 astronomical considerations which have occasionally 
 been supposed to account for the recurrence of Ice 
 Ages. It has been conjectured that while the solar 
 system voyages through space, it may occasionally saun- 
 ter through warm regions, which will cause it to glow 
 with a genial climate. It seems also to have been 
 thought possible that the path of our system may occa- 
 sionally have traversed parts of space where the tem- 
 perature is so much below that found elsewhere, that 
 our globe received a chill which produced a glacial 
 epoch. I confess that these appear to be fanciful no- 
 tions wholly unsupported by the known facts of astron- 
 omy. The temperature of space is rather a vague ex- 
 pression, and I do not feel able to attach any meaning 
 to it, except in so far as it is an indication of the heat 
 received from the stars. If, therefore, our solar system 
 has at any time experienced a space temperature differ- 
 
THE PROBLEM OF GLACIATIOX. 53 
 
 ent from that of the region through which it is now 
 passing, it follows that there must have been at that 
 epoch a supply of star heat communicated to the system 
 different from that which it now receives. Let us sup- 
 pose, for instance, that the system did enter a part of 
 space where the temperature was sensibly higher than 
 we find it now to be. Some of the stars must then 
 have been either intrinsically much hotter or else very 
 much closer to the sun than they are at present. Does 
 there seem any probability that this has been the case ? 
 Here is, indeed, an interesting question which touches 
 some of the grandest truths of astronomy. 
 
 At the outset of our inquiry we are unhappily 
 rather baffled by a dearth of precise information. We 
 have little or no knowledge of the amount of heat 
 which we at present receive from the stars, though no 
 doubt indications of such heat have been recognized by 
 instruments in the hands of skilled observers. If, how- 
 ever, we regard the heat which each star sends forth as 
 being on the average proportional to the star's bright- 
 ness, we can make an estimate which will suffice for our 
 purpose. The luster of several of the stars has been 
 carefully measured by photometers, and it has been 
 found possible to express the relation numerically be- 
 tween the light radiated from one of the stars and the 
 light dispensed by the sun himself. It has been shown 
 that the most brilliant star in the whole heavens, the 
 peerless Sirius, sends to us an amount of rays which, 
 
 though much in excess of those emitted by any other 
 5 
 
54 THE CAUSE OF AN ICE AGE. 
 
 star, require multiplication by a formidable array of 
 figures before they can be brought to equality with 
 those from the sun. Sun light transcends the light 
 from Sirius as received by us in the ratio of 20,000,- 
 000,000 to unity. Let us suppose that the sky contains 
 2,000 stars, each of which transmits to the earth as 
 much heat as Sirius sends us (this is a liberal supposi- 
 tion, for we have exaggerated the total amount of star 
 heat, which is not at all likely to be nearly so much as 
 2,000 times that of the heat due to Sirius), and even 
 with this assumption it would seem that the total 
 amount of star heat is not the ten-millionth part of 
 that which we receive from the sun. I have shown that 
 a fluctuation of sun heat to the extent of one tenth 
 would be amply sufficient, so far as mere questions of 
 temperature are concerned, either to produce an Ice 
 Age or to disperse it. As, however, the star heat is 
 only one ten-millionth part of the sun's heat, it would 
 seem that unless the capacity of the stars for radiating 
 heat were a million times as great as it is at present, it 
 would not be a factor sufficiently potent to dispel an 
 Ice Age by its presence, nor could the advent of an Ice 
 Age be the consequence of its absence. 
 
 It is, therefore, obvious that, with the present rela- 
 tion of our sidereal system to the sun, the stars give no 
 aid in our search for the cause of the great Ice Age. 
 
 It would be proper to inquire whether this relation 
 has always been as it is at present. No doubt the 
 twinkling stars are in these days incompetent to exer- 
 
THE PROBLEM OF GLACIATION. 55 
 
 cise any climatic influence. Can it, however, have been 
 possible that a temporary increase in the splendor of 
 particular stars might have been an appreciable source 
 of heat? 
 
 Stars of exceptional luster occasionally burst forth 
 in the sky, and they sometimes attain no little brill- 
 iancy. Indeed, it has happened that a temporary star 
 has so far outshone all ordinary stars, as to be con- 
 spicuous with the unaided eye in broad daylight. But 
 such phenomena, however interesting, render no service 
 to the present inquiry. There has never been an out- 
 break among the stars, so far as we can tell, sufficiently 
 important to produce the slightest possible effect upon 
 climate ; we can not, therefore, look to their variable- 
 ness for any aid in explaining tho occurrence of a great 
 Ice Age. 
 
 There is, however, another way in which it might be 
 contended that star heat may have occasionally risen to 
 be a far more important climatic element than it ap- 
 pears to us to be at present. Can our system, in the 
 course of its journey through space, chance to have 
 voyaged into the vicinity of other stars? We know 
 that many of the stars are in actual motion. Indeed, 
 out of all the myriad hosts that the sky contains, it 
 seems inconceivable that even a single one should be 
 found which lies actually at rest. It is no doubt true 
 that many of the stars, or I might say most of the 
 stars, have not as yet exhibited to us an amount of 
 motion which is appreciable by our measurements. As, 
 
56 THE CAUSE OF AN ICE AGE. 
 
 however, years roll on, one after another exhibits suf- 
 ficient displacement of its position to arrest the as- 
 tronomer's attention, and thus the so-called fixed stars 
 are gradually being discovered to be endowed each with 
 its proper motion. 
 
 Although the sun is of such unrivaled importance 
 to the earth as the source of our light and our heat, 
 still among the general host of heaven the sun has no 
 more importance than is possessed by thousands of 
 other suns of equal and in many cases of far greater 
 splendor. The sun is but a star around which the 
 planetary system is clustered ; nor has the sun a fixity 
 in the heavens which is denied to the rest of the sidereal 
 system. As the other stars are moving, so also the sun 
 is following a definite route through space, and is ac- 
 companied in its journey by the earth, and the other 
 planets which own its sway. 
 
 When we attempt to express the velocities of star- 
 travel by the ordinary standards of miles per hour, no 
 doubt they seem enormously great. These movements 
 are not, however, to be described as rapid, when we 
 remember the size of the theatre in which such grand 
 phenomena are developed. There can be no doubt that 
 as the stars are moving one and all over the face of the 
 sky, the sidereal heavens must be undergoing a con- 
 tinuous transformation. Its progress is, however, very 
 tardy when estimated by ordinary chronology. The 
 constellations seem to have existed from the remotest 
 antiquity of which any record is preserved. The Great 
 
THE PROBLEM OF GLACIATION. 57 
 
 Bear of which Homer sang is the Great Bear of our 
 skies to-night. The constellations are as old as the 
 hills, no doubt, but have the hills themselves existed 
 from all antiquity ? In the progress of geological pe- 
 riods of time, hills and mountains have been upheaved, 
 and hills and mountains have vanished away. The 
 noblest and loftiest of mountain ranges, the everlasting 
 mountains as they seem to our view, are but transient 
 objects, the mere sport of geological forces. The fitful 
 ocean, which ripples to every breeze, preserves its gen- 
 eral form unaltered from age to age, while the mighty 
 mountain reared of solid granite, with its foundations 
 deep in the earth, is upheaved in one age only to be 
 dissolved in the next. 
 
 The constellations in the sky may be permanent so 
 far as man's observation has extended, but when ade- 
 quate time is allowed for the stellar hosts to manoeuvre, 
 we see that our constellations have no more permanence 
 than have our mountain chains. The family groups of 
 the Great Bear and Orion, as well as the other striking 
 collocations of stars, are undergoing ceaseless transfor- 
 mation. Sometimes the old forms are being relaxed, 
 sometimes doubtless new and picturesque arrangements 
 are being produced. In the course of ages, those stars 
 which adorn the heavens at the present moment must 
 be transformed, nay, must sink into invisibility and 
 be replaced by other stars, forming new constellations 
 which will have no greater permanence. 
 
 Considering the transitory nature of all the objects 
 
58 THE CAUSE OF AN ICE AGE. 
 
 in the heavens, no less than of the objects on the earth, 
 it has been thought that great climatic changes may 
 thus find their explanation. Suppose that, in the lapse 
 of ages, that star which we call the sun shall have 
 adventured so near some other star as to have experi- 
 enced an increase of temperature ; may not great alter- 
 nations of climate have arisen ? It is, indeed, hard to 
 see how such an event, even if it could have happened, 
 would give promise of throwing much light on the 
 occurrence of an Ice Age, though it might conceivably 
 be invoked to provide an explanation of a warm period. 
 It is, however, useless to discuss the matter, for astrono- 
 mers will not admit the possibility of any approxima- 
 tion between our sun and another star sufficiently close 
 to account for any considerable derangement of the 
 conditions of temperature on this globe. The astrono- 
 mer remembers that a body can not radiate heat in 
 vast quantities unless that body also possesses consid- 
 erable mass. Any body possessing mass will exercise 
 attraction on all other objects around. If, therefore, 
 the supposed body were near enough and large enough 
 to radiate appreciable heat, it must also have been both 
 near enough and massive enough to effect quite an 
 appreciable derangement in the movements of the 
 planetary system. The several motions of the planets 
 around the sun are adjusted in delicate relationship. 
 We can not, indeed, assert that external agency has 
 never created a disturbance of the organized arrange- 
 ments, but we may feel confident that the motions of 
 
THE PROBLEM OF GLACIATION. 59 
 
 the planets exhibit no trace of having suffered, even 
 during geological times, any great interference from 
 outside influence. We are, therefore, assured that dur- 
 ing such periods as those to which the Ice Age has 
 reference, there has been no close approach of our 
 solar system to any large stellar body. We can not, 
 therefore, believe that any exaltation of the earth's tem- 
 perature has taken place by approach to a star. We 
 conclude, then, that the circumstances attending the 
 voyage of the solar system through space oifer us no aid 
 in attempting to account for the climatic revolutions 
 of the Ice Ages. 
 
 We are thus compelled to dismiss the supposition 
 that the changes of geological climate can be attributed 
 to variations in the supply of star heat, still less can we 
 account for such changes by the assumption of fluctua- 
 tions in the intrinsic intensity of the heat radiated 
 from our own star which we call the sun. Such expla- 
 nations would certainly, if they could be substantiated, 
 offer a bold and at all events a readily intelligible solu- 
 tion, but there is no such heroic method of dealing 
 with the problem. 
 
 We are therefore constrained to look in another 
 direction for the source of the power by which the 
 ice- sheet was generated, and at last we obtain the solu- 
 tion. We shall find it to lie in the fluctuations which 
 from time to time have affected the shape of the path 
 in which the earth performs its annual journey around 
 the sun. 
 
CHAPTER IV. 
 
 THE PERTURBATIONS OF THE EARTH'S ORBIT. 
 
 A FAMOUS law of Kepler declares that a planet 
 revolves around the sun in an ellipse, and that the sun 
 occupies one of the foci of the ellipse. Did our system 
 consist of but a single planet, this body would at each 
 revolution describe an ellipse of precisely the same size 
 and shape as it had done in each preceding revolution, 
 and as it would do in each succeeding one. The laws 
 of dynamics show that the dimensions of the orbit 
 which the planet pursued could not vary by the smallest 
 measurable quantity. Were our earth a solitary planet 
 revolving around the sun, there could be no astronomi- 
 cal explanation of an Ice Age, and assuredly there 
 would be no Ice Age seeking for explanation. 
 
 It is, however, well known that the earth is only 
 one of a considerable number of bodies which revolve 
 as planets around the sun ; nor is our globe even one 
 of the more important planets in so far as size is con- 
 cerned. There are some much larger, such as the 
 great outer planets Jupiter, Saturn, Uranus, and Nept- 
 une. Venus is a globe of nearly the same size as, 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 61 
 
 while Mars and Mercury are considerably smaller than, 
 the earth, and there are some hundreds of planets much 
 smaller still. The earth occupies not only an inter- 
 mediate position as to size, but is also intermediate 
 as regards the situation in which its orbit is placed. 
 There are some planets nearer to the sun, but the 
 greater number, including the more gigantic objects of 
 this class, perform their journeys in paths exterior to 
 that in which the earth moves. 
 
 The great discovery of Newton provided a reason 
 for the elliptic motion of the planets. He showed that 
 the attraction of the sun on a planet, in virtue of the 
 law of gravitation, would wholly account for the laws 
 of Kepler. He further demonstrated that no law ex- 
 cept that of an attractive force, varying inversely as 
 the square of the distance, could explain the elliptic 
 movement. The force of gravitation was found to 
 account for the movements of the moon as well as for 
 the fall of an apple. Stated generally, the law of uni- 
 versal gravitation asserts, that the attraction between 
 any two masses varies directly as their product, and 
 inversely as the square of the distance by which they 
 are separated. All the principal phenomena of the 
 motions of the planets admit of complete explanation 
 by this theory. 
 
 In the solar system the sun attracts the planet, but 
 we must be careful to remember that the latter also 
 attracts the sun. In fact, if the sun and the planet 
 were bodies of equal mass, we could not say that the 
 
G2 THE CAUSE OF AN ICE AGE. 
 
 planet moved around the sun more than the sun moved 
 around the planet; in such a case each body would 
 perform its own elliptic path, and the focus of the 
 ellipse would be midway on the line joining the two 
 bodies, If the masses of the two bodies be not equal, 
 the motion of each will still be in its own ellipse of 
 which the focus lies at the center of gravity of the 
 bodies. In the case of the sun and of the planetary 
 system generally the mass of the central body enor- 
 mously exceeds that of any of his planets. The sun, 
 for example, is 1,047 times as heavy as Jupiter, the 
 heaviest of the planets; while if the luminary were sub- 
 divided into a million equal pieces, the mass of each one 
 of them would be greater than the mass of the earth. 
 It therefore follows that the center of gravity of the sun 
 and of the earth lies close to the sun's center. 
 
 The universal law asserts that every body attracts 
 every other body, and therefore there is attraction not 
 alone between planet and sun, but also between planet 
 and planet. Jupiter not only is attracted by the sun, 
 and retaliates by attracting the sun, but Jupiter also 
 attracts the earth, and is in turn attracted by the earth. 
 In like manner there is a mutual attraction between 
 every pair of planets, the intensity of which is measured 
 by the product of the masses of the two planets, divided 
 by the square of their distance apart. 
 
 The determination of the motions in the solar 
 system is thus an excessively complicated problem. 
 The path of each planet is no doubt determined chiefly 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 63 
 
 by the attraction of the sun, but it is affected to an 
 appreciable degree by the attraction of other planets. 
 Fortunately for the labors of those astronomers who 
 attempt to compute the heavenly motions, the problem 
 in the case of our solar system has been simplified. 
 The enormous preponderance of the sun's mass over 
 the aggregate of all the other masses in the system, 
 gives to his attraction so great a potency that the plan- 
 etary movements are nearly those which would result 
 were the other planets absent. A principle with which 
 mathematicians are familiar is here brought into use. 
 There are many problems which can not be solved with 
 absolute mathematical accuracy such as we are accus- 
 tomed to find in a proposition in Euclid, but which 
 yet admit of a solution which shall possess any desired 
 degree of accuracy short of actual perfection. As a 
 familiar instance of such a problem, I may mention the 
 famous one of determining the circumference of a cir- 
 cle. Every mathematician knows that it is impossible 
 to draw in a mathematical sense a straight line which 
 shall be equal to the circumference, but every mathe- 
 matician knows also that this limitation of his power 
 involves no practical inconvenience. We can calculate 
 the length of the circumference to a far greater degree 
 of accuracy than would ever be required in any practi- 
 cal application. That would be a large circle of which 
 the radius was equal to the distance from the earth to a 
 fixed star, and yet the circumference of that circle, if 
 its radius be given, can be determined with such pre- 
 
64 THE CAUSE OF AN ICE AGE. 
 
 cision that the error is under the millionth of an inch. 
 For all practical purposes this solution may be regarded 
 as possessing absolute accuracy. 
 
 In a somewhat similiar spirit we attack the problem 
 of gravitation in the solar system. We can compute by 
 successive approximation the effect which one planet 
 exercises upon another, and we can obtain with any 
 required degree of accuracy the changes produced in 
 their elliptic paths by such influences. We speak of 
 the effects thus arising as the planetary perturbations. 
 
 Among the most famous problems which have ever 
 exercised the skilled mathematicians, there is none 
 more important than that of the "three bodies." 
 Given two attracting globes, the problem is easily 
 solved, and Kepler's laws afford a complete solution ; 
 but when a third body is introduced into the system it 
 commences to attract and to be attracted by each of the 
 other two bodies. The symmetry of the movements 
 entirely vanishes Each body, instead of pursuing an 
 ellipse, will move along a curve of the utmost com- 
 plexity, which the mathematician finds it extremely 
 difficult, if not quite impossible, to follow. But sup- 
 pose that one of the three bodies is enormously greater 
 than either of the others, then certain artifices can be 
 employed which bring the problem within the power of 
 human ingenuity to solve. We first determine an ap- 
 proximate form for the track which one of the planets 
 will pursue, and from this we can obtain a more accu- 
 rate one. A further repetition of the same process will 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 65 
 
 exhibit an orbit still more close to that actually fol- 
 lowed, until at last, by what is known as successive 
 approximation, it becomes possible to determine the 
 figure with a degree of accuracy that will suffice for 
 all the needs of astronomy. 
 
 In the actual solar system there are also some other 
 conditions which specially tend to the convenience of 
 the mathematical computer. It fortunately happens 
 that all the orbits in which the planets revolve lie 
 nearly in the same plane. It is a further matter of 
 congratulation to the practical astronomer, that the 
 movements of all these planets take place in the same 
 direction. This involves, in fact, a cardinal doctrine in 
 the organization of the solar system. By availing our- 
 selves of these favorable conditions we have been en- 
 abled to discover with sufficient accuracy the influence 
 which each planet is capable of exercising on the orbits 
 of the rest. 
 
 The subject of planetary perturbations is naturally 
 one of the most difficult in the whole range of science. 
 Every artifice of the skilled mathematician is employed 
 to simplify the work, and the results obtained have 
 amply justified the labor that has been expended. 
 When precise examination of the planetary orbits began 
 to be made with telescopes of optical perfection, and 
 when the lapse of time had enhanced the value of early 
 observations, then it was found that the movements of 
 the planets were not absolutely in accordance with the 
 simple elliptic laws which Kepler announced. It was 
 
66 THE CAUSE OF AN ICE AGE. 
 
 found that the planet was sometimes a little in advance, 
 was sometimes a little in arrear of the calculated posi- 
 tion. It was also discovered that the path which the 
 planet followed was not again exactly traversed during 
 the next revolution. In fact, it was shown by observa- 
 tion that though on the whole the planet moved as 
 Kepler said it did, yet that when we examined mi- 
 nutely the movements which were followed, we found 
 innumerable small departures from the simplicity of 
 elliptic motion. 
 
 Such discrepancies are always of value in scientific 
 investigation. We calculate what a magnitude is ex- 
 pected to be, and then we measure what is actually pre- 
 sented to us. The difference between the two can only 
 be regarded as an indication of the presence of some 
 agent which was either overlooked or not accurately 
 allowed for in the process of making the calculation. 
 
 The deviations in the places of the planets from 
 those which would be taken by elliptic motion alone 
 have been shown to arise from the mutual attraction 
 between one planet and another. One after another 
 these problems have all, or nearly all, yielded to the 
 repeated attacks of those astronomers who wield the 
 potent weapons of mathematical analysis; their dis- 
 coveries form some of the most striking chapters in the 
 history of astronomy, and they have a special interest 
 because they illustrate the great law of gravitation, and 
 confirm in a striking manner the evidence on which 
 our belief in that law is founded. 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 67 
 
 Lagrange, who has been happily termed the poet of 
 mathematicians, has discovered certain beautiful prin- 
 ciples in connection with the theory of planetary per- 
 turbations. He has shown that the extent to which 
 the perturbations of one planet by the remaining planets 
 can affect its motion, is limited in their extent. This 
 is a proposition of singular interest inasmuch as it in- 
 volves what is often called the stability of the planetary 
 system. When first the doctrine of perturbations began 
 to receive attention, fears were sometimes entertained 
 lest the irregularities of the earth's orbit should grow 
 to such an extent that a total disorganization of the 
 conditions of life would result. It was thought that 
 possibly the attraction of the great planets, lying as 
 they do outside the earth's path, might succeed in with- 
 drawing the earth altogether from the enjoyment of 
 the sun's benefit. It was also conjectured that a great 
 change of an opposite description might possibly take 
 place by which the earth's orbit would be so contracted 
 that life would be impossible from excessive heat. 
 That neither extreme was to be reasonably feared 
 should be obvious to any one who would look at the 
 evidence of palaeontology, which shows that for untold 
 millions of years this globe has been the abode of life. 
 During all that time it has never been exposed to such 
 extremely violent conditions as have been sometimes 
 apprehended. To Lagrange, however, is due the merit 
 of having explained how it is that perturbations have 
 never been able, even in millions of years, to accumu- 
 
68 THE CAUSE OF AN ICE AGE. 
 
 late in the earth's path changes so disastrous. He has 
 demonstrated that the incompetency of perturbations to 
 produce such tragical phenomena is due to a certain 
 organization of the movements of the bodies in our 
 system. 
 
 There is a delicate beauty about these celebrated 
 theorems which is sometimes, perhaps, not appreciated 
 to the extent it deserves. The stability of the system 
 results from a particular adaptation of the movements 
 of the planets. The salient features of our system, in so 
 far as the present phenomena are concerned, are three- 
 fold. In the first place, the orbits of all the important 
 planets, though undoubtedly to some extent elliptic, 
 approximate very closely in their forms to circles. In 
 the second place, the planes of all these orbits are very 
 nearly coincident, or are, at all events, inclined at very 
 small angles. In the third place, the directions in 
 which the planets move are all the same. No doubt 
 there are many of the minor planets whose orbits do 
 not conform to these conditions, and the host of comets 
 have orbits which are anything but circles, while their 
 planes are inclined at every angle. Comets and minor 
 planets are, however, bodies of very inconsiderable 
 mass ; indeed, it is a significant fact that all the really 
 massive planets in our system, including the earth itself, 
 move subject to the conditions already stated. It has 
 been shown that the stability of our system as a mathe- 
 matical proposition depends for its proof on the fact that 
 the planetary movements are subject to these conditions. 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 69 
 
 With respect to a system like ours, it follows from 
 the laws of dynamics that the mutual attractions wheer- 
 by each planet seeks to draw aside the other planets 
 from their orbits can never succeed in effecting a per- 
 manent alteration either of enlargement or contraction 
 in the diameters of the orbits. The length of the 
 elliptical orbit remains invariable, notwithstanding the 
 influence of other planets. It has also been shown that 
 though the planes of the orbits in which the planets 
 revolve undergo some slight fluctuations in consequence 
 of perturbations, yet no one of the planes can ever 
 attain any large departure from its original position. 
 Finally, and this is the most important point for our 
 present purpose, it has been demonstrated that though 
 the eccentricities of the several orbits fluctuate in re- 
 sponse to the perturbing attractions, yet that in no 
 case can the eccentricity of a planetary orbit exceed a 
 certain ascertainable limit. 
 
 Thus we see that the competency of perturbations 
 to effect changes in the planetary system is confined 
 within definite bounds. The orbits are now nearly 
 circular ; they can never depart more than a certain ex- 
 tent from that form. The dimensions of those orbits 
 as measured by the longest diameters remain unalter- 
 ably constant, and the situations of the planes of the 
 orbits must always remain in the vicinity of their pres- 
 ent situations. 
 
 We shall best appreciate the significance of the 
 celebrated mathematical principles which I am here 
 
70 THE CAUSE OF AN ICE AGE. 
 
 enunciating, if we observe how comparatively slight 
 would be the alterations in our system which would 
 suffice to deprive it of protection against catastrophe. 
 Let us conceive a system consisting of a sun and 
 planets equal in their several bulk and weight to ours. 
 Let these planets all revolve in orbits severally identical 
 with the orbits which our planets pursue. Let the 
 planes of the orbits in the ideal system be situated both 
 as to inclination and as to aspect precisely as the planets 
 of our system ; while, to complete the resemblance, we 
 may conceive the ideal system to be supplied with 
 troops of comets and with satellites which revolve 
 around the planets and coincide in every respect with 
 their actual prototypes. It might be supposed that the 
 theorems which guarantee immunity from the accumu- 
 lation of perturbation to an injurious extent, would also 
 take under their shelter the imaginary system I have 
 supposed. This is, however, not necessarily the case. 
 It may be noted that in our stipulations as to the re- 
 semblance of the two systems, no mention has been 
 made of the direction in which the planets are to pur- 
 sue their revolutions. In the actual system all the 
 movements, at least of the important bodies, observe 
 the same direction. If, however, in the ideal system, 
 the direction of motion of even a single great planet, 
 such as Jupiter, were altered, then our expectations of 
 the permanence of the system might vanish. Lagrange's 
 theorems only apply to the case where the movements 
 are all performed in the same direction. 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 71 
 
 I may give an instance of the remarkable reasoning 
 by which results of this kind are obtained. Let us 
 simplify the system until it consists but of a sun which 
 guides the circulation of two planets not very widely 
 differing from each other in mass. Let the orbits of 
 these planets be nearly circular, and for still further 
 simplicity we may suppose that they lie in the same 
 plane. AVe first multiply each of the squares of the 
 eccentricities by a certain number, and then add them 
 together. It follows from the laws of dynamics that no 
 matter how much each planet may disturb the other, 
 this total remains absolutely unaltered. If, therefore, the 
 eccentricities of the two orbits were small, either in the 
 beginning, or, indeed, at any time, then this total will 
 be small. It follows that the sum of the squares of the 
 eccentricities, each having been previously prepared for 
 the operation by having numerical factors applied to it, 
 must always remain small. The consequence of this 
 principle is irresistible. If the sum of two things, both 
 essentially positive, is a small quantity, it necessarily 
 follows that each of the quantities by itself must be 
 small ; therefore the eccentricity in each of the two 
 orbits, which was small at the beginning, will continue 
 forever small. Hence we receive assurance of stability 
 in the shapes of the great planetary orbits. It appears 
 that they can never depart to any considerable degree 
 from their present nearly circular form. They can 
 never, for example, have their paths transformed into 
 long ellipses like those in which some comets revolve. 
 
72 THE CAUSE OF AN ICE AGE. 
 
 Let us note in what particulars our argument would 
 have broken down if the planets did not move in the 
 same direction around the sun. Instead of the asser- 
 tion that the sum of the squares of the eccentricities 
 was constant when each was modified by an appropriate 
 factor, we should now have to admit that it was the 
 difference between these two quantities which remained 
 unchanged. No inference as to stability would be 
 possible from such a theorem. It is true that each of 
 the two quantities is at this moment small, and there- 
 fore their difference is now small, and small it must 
 ever remain. This, however, offers no assurance that 
 the two quantities themselves will remain small ; it is 
 indeed obvious that the difference between two quanti- 
 ties might still be insignificant while both of the quan- 
 tities were enormously great. If, therefore, the direc- 
 tions of the planetary movements were not coincident, 
 there would be nothing, so far as these theorems are 
 concerned, to prohibit the formation of monstrous 
 orbits which would be quite incompatible with the 
 permanence of our system. It can be similarly shown 
 that each of the other articles of the planetary conven- 
 tion is a necessary condition for stability. In fact, there 
 seems every reason to believe that the mere existence of 
 our earth, with its teeming life, for past millions of 
 years would be alone sufficient to require that the orbits 
 must have been conformable to the regulations of the 
 convention. It is hardly too much to assert that if 
 there should be any other great planets besides those 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 73 
 
 already recognized, we might fairly expect from dy- 
 namical reasoning alone to find that the orbits of those 
 planets were nearly circles, that their planes were in- 
 clined at very small angles, and that the direction in 
 which these planets revolved was identical with that of 
 the planets with which we are familiar. Indeed, such 
 laws were found to be observed by the two new planets, 
 Uranus and Neptune, which have been discovered in 
 the sun's system within modern times 
 
 We have now to concentrate our attention on the 
 variations to which the earth's orbit is subject in virtue 
 of the perturbations. The efficiency of the different 
 planets in their capacity to produce disturbance will 
 differ according to their masses and their distances. 
 The disturbance produced by the same planet is more- 
 over always varying both in magnitude and direction. 
 It is, however, important for us to acquire some dis- 
 tinct notion of the magnitudes of the several disturb- 
 ing agencies ; we shall, therefore, for the sake of illus- 
 tration, express the disturbing force of each planet at 
 the maximum value of which it is capable. 
 
 Unparalleled in its luster, the planet Venus is un- 
 exampled in the intensity of the pull with which it 
 seeks to make the earth swerve from its revolution 
 around the sun. I have calculated the magnitude of 
 this force, and when expressed in tons the figures that 
 are required baffle our powers of comprehension. The 
 tons in the attraction, or rather, I ought to say, in 
 the disturbing force, of Venus are comparable with 
 
74 THE CAUSE OF AN ICE AGE. 
 
 the miles in star distances. The force is indeed 
 130,000,000,000,000 tons. We must, however, impair 
 the effect of this array of figures by comparing it, as 
 we are bound to do, with the central force by which 
 the motion of the earth is guided. Compared with 
 the attraction of the sun the perturbing force ex- 
 erted by Venus is reduced to insignificance, it merely 
 amounts to the twenty-seven-thousandth part of the 
 sun's attraction. This shows in a striking manner 
 how inconsiderable are the greatest effects of planetary 
 perturbation when the earth is controlled by a force of 
 such colossal intensity as that which pulls it toward 
 the sun. 
 
 Next in importance to the perturbations produced 
 by Venus are those which are due to the attraction of 
 the planet Jupiter. The great planet is so much more 
 distant from us than Venus that its efficiency as a 
 disturber is largely reduced. On the other hand, the 
 preponderating mass of Jupiter, which is a thousand 
 times that of Venus, tends to swell the magnitude of 
 his influence. It is sufficient for our present purpose 
 to say that the greatest disturbing effect of Jupiter is 
 about half that of Venus. It would be unnecessary to 
 examine what the other planets can severally do; the 
 disturbances they create are much less than those of the 
 two planets we have named. Yet it may be not with- 
 out advantage to remember that even the most distant 
 planet, or the smallest planet, tugs at our earth with a 
 disturbing force which, when expressed in tons, is often 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 75 
 
 stupendous and always considerable. Long ere Adams 
 and Leverrier had ever thought of undertaking the 
 calculations of the perturbations which the then un- 
 known Neptune could produce, Neptune had been pull- 
 ing our earth with a disturbing force which can only 
 be expressed by thousands of millions of tons. Nor does 
 the comparative minuteness of some of the planets ren- 
 der them incapable of generating disturbance, which, 
 though imperceptible when applied to so vast a mass 
 as the earth, still seems of great intensity when com- 
 pared with more ordinary standards of force. Take, 
 for example, the most insignificant of these bodies so 
 far as size is concerned ; it is number 217 in the now 
 ample list of the minor planets, and it bears the name 
 of Eudora. This little object has received very scanty 
 attention from astronomers ; it is the merest point of 
 light just visible in a powerful telescope. Like its 
 congeners, Eudora revolves in that broad zone between 
 the orbits of Mars and Jupiter which is tenanted 
 by the numerous host of little planets. Eudora is 
 doubtless only a few miles in diameter, but yet that 
 little body transmits to our earth a disturbance which 
 would seem of startling magnitude if we expressed it in 
 tons. Think for a moment of that superb suspension 
 bridge which stretches from New York to Brooklyn; 
 each of the main cables by which this bridge is sup- 
 ported is composed of a multitude of steel wires, and is 
 capable of withstanding a force amounting to thou- 
 sands of tons ; yet one of these cables would snap like 
 
76 THE CAUSE OF AN ICE AGE. 
 
 a piece of packthread if exposed to a strain equal to the 
 disturbance produced by Eudora on the earth. 
 
 It may also be desirable to glance at the possible 
 disturbances to which our earth may be exposed from 
 bodies lying wholly exterior to the solar system. There 
 are, of course, objects possessing this character in un- 
 told millions. They form the host of stars. The mag- 
 nitude and masses of these stars are such that they 
 would be competent to originate frightful perturbations 
 were it not that they are fortunately separated from us 
 by stupendous distances. Indeed, it may very probably 
 be that the enormous distances of the stars is a neces- 
 sary condition to the well-being of our system. It is, 
 however, possible in those cases where we know both 
 the mass and the distance of a star to compute the dis- 
 turbing effect that it can produce. In performing such 
 a calculation it has, however, to be borne in mind that 
 the efficiency of the disturbing force does not follow 
 the simple law of the inverse square that is so familiar 
 in the enunciation of the law of gravitation. We have 
 here to deal not with the absolute amount of the attrac- 
 tion, but rather with the difference between the attrac- 
 tion of the sun by the disturbing planet and the attrac- 
 tion of the earth by the disturbing planet. The dis- 
 turbing force varies according to the inverse cube of 
 the distance; this, fortunately for our welfare, places 
 the more distant bodies at a special disadvantage in 
 their capacity for producing disturbance. 
 
 To examine the possible effect of the stars we natu- 
 
PERTURBATIONS OF THE EARTH'S ORBIT. 77 
 
 rally take the most brilliant of them Sirius which is a 
 million times as distant from us as the sun. If Sirius 
 were but twenty times as heavy as the sun, then it is 
 easy to calculate that the disturbing force that the star 
 will exert on the earth is equivalent to about a hundred 
 tons. This perhaps would be a large force in many ter- 
 restrial matters, though engineers nowadays have often 
 to deal with forces far greater. 
 
 The mass of our earth is so enormous that it re- 
 quires a correspondingly gigantic force to produce an 
 appreciable change in its motion. The consequence is 
 that the consideration of many of these minor pertur- 
 bations may be neglected, at all events for our present 
 purpose; the chief disturbances of the earth's path 
 must be attributed to the influence of Jupiter and Ve- 
 nus. It will therefore be seen that these planets are 
 largely responsible for the production of the Ice Ages. 
 
CHAPTER V. 
 
 THE ASTRONOMICAL THEORY OF THE ICE AGE. 
 
 OUR investigation of the perturbations has now con- 
 ducted us to this stage. We see that the earth de- 
 scribes an elliptic path around the sun, and that the 
 major axis of that ellipse preserves its length unaltered 
 in so far at least as the more important class of per- 
 turbating effects are concerned. The shape of the orbit, 
 however, undergoes certain changes. Sometimes, from 
 a nearly circular form, the orbit passes by an extremely 
 gradual process into an elipse, and then, in periods of 
 time that are to be told in hundreds of thousands of 
 years, the ellipse gradually again assumes a nearly cir- 
 cular shape. Thus in the lapse of ages we perceive a 
 slow waxing and waning in the ellipticity of the earth's 
 orbit. But let us repeat that even when the orbit has 
 departed to the utmost extent from a circle, it is still 
 not to be regarded as a long ellipse, like, for example, 
 one of those in which a periodic comet would be found 
 to revolve. I represent in the figures on pages 100 and 
 101 sketches of the path of the earth at a time when 
 the eccentricity has reached the highest degree which it 
 
THE ASTRONOMICAL THEORY. 79 
 
 is possible for it to attain in consequence of planetary 
 perturbations. 
 
 These changes appear to have profound significance 
 when viewed in connection with those climatic vicissi- 
 tudes through which our globe has passed, and of which 
 Ice Ages appear to be among the latest manifestations. 
 It would, therefore, be necessary to examine carefully 
 the possible thermal changes that can result from varia- 
 tions in the eccentricity. It is the more desirable to 
 attend specially to this matter, because at a first glance 
 it would appear to be an utterly inadequate explanation 
 of any considerable climatic effect. There can be no 
 doubt that when the eccentricity is at its highest point 
 the earth is, on the whole, rather nearer to the sun, 
 because while the major axis of the ellipse is unaltered 
 the minor axis is least. It can be demonstrated that 
 the total quantity of heat which the earth receives dur- 
 ing each complete revolution will be inversely propor- 
 tional to the minor axis of the ellipse. 
 
 Considering the extremely small changes in the 
 minor axis, it is evident that the total quantity of heat 
 received by the earth during each revolution can only 
 vary by a very small fraction of the whole. The ther- 
 mal effect of this variation seems so insignificant that 
 it has often been concluded, not perhaps, without 
 plausibility that changes in the eccentricity of the 
 orbit were inadequate to account for such stupendous 
 phenomena as Ice Ages. There can be no doubt that 
 the total heat received during the year by the earth 
 
80 THE CAUSE OF AN ICE AGE. 
 
 does undergo only an insignificant variation in direct 
 consequence of the changes in the eccentricity. The 
 material point for our present purpose is not, however, 
 the total quantity of heat which the earth receives in a 
 year ; of far more significance is the distribution of that 
 heat during the various seasons and over the different 
 parts of the globe. It might, for instance, happen that 
 during one season there might be a great exaltation in the 
 daily receipt of heat, but that at the opposite season the 
 heat might be received so sparingly that the average for 
 the whole year would remain unaltered. It might also 
 happen that in certain seasons one hemisphere would 
 receive far more than its daily share of sunbeams and 
 the other correspondingly less, and yet this inequality, 
 so potent as a climatic agent, might not be at all per- 
 ceptible in the total quantity of heat received during 
 an entire year. It is, therefore, incumbent on us to 
 scrutinize narrowly the effect which changes in the 
 earth's eccentricity can have on the supply of heat at 
 different seasons, and on the shares of that heat which 
 each hemisphere shall receive. We shall presently find 
 that the inequality of the distribution of heat, arising 
 from this cause, attains an importance which would 
 hardly have been anticipated from the very narrow 
 limits within which the fluctuations of the eccentricity 
 are confined. It fortunately happens that a little ge- 
 ometry will serve to pilot us through the mazes of a 
 subject that would be very difficult to trace with a less 
 certain guide. 
 
THE ASTRONOMICAL THEORY. 81 
 
 Let us first discuss the question as to the amount of 
 heat which the earth receives at the several parts of its 
 orbit. We shall here speak of the earth as a whole, 
 reserving to later paragraphs the important considera- 
 tions which arise when we examine the distribution of 
 the heat between the two- hemispheres. If the earth's 
 orbit were truly a circle, then the earth's movement 
 would be uniform around the circumference ; and, the 
 distance from the sun being unaltered, the heat received 
 on one day would be the same as that received on every 
 other day. The earth's orbit is, however, not a circle 
 but an ellipse, and therefore it is by no means obvious 
 what the total daily receipt of heat should be. At 
 some seasons the earth is nearer to the sun than it is at 
 others, and, of course, the nearer the sun the larger is 
 the share of heat which the earth obtains. The fig- 
 ure on page 82 may be taken to represent the path of 
 a planet around the sun at the focus F. Draw any two 
 rays, A x and B Y, through that focus, and let us study 
 the heat which the planet receives while it passes from 
 A to B and also while it moves from x to Y. It will be 
 noticed that I have so placed these lines that the part 
 A B is comparatively near the sun, while the other part, 
 x Y, is as far as possible from the sun. During its 
 passage from A to B the planet is submitted to ex- 
 cessive radiation on account of the sun's proximity. 
 On the other hand, while the body is moving from x to 
 Y it is very much farther from the sun, and the warmth 
 which it receives is correspondingly less. We might, 
 
82 THE CAUSE OF AN ICE AGE. 
 
 therefore, rather hastily conclude that the total quantity 
 of heat received by the planet while passing from A to 
 B would exceed that which it would gather up between 
 x and Y. This might be true if the planet were ex- 
 posed between A and B for as long a time as that which 
 it takes in passing from x to Y. But this is not the 
 
 FIG. 2. AN ELLIPTIC ORBIT. 
 
 case. The planet moves more quickly when near the 
 sun than when more remote ; it therefore sweeps across 
 from x and Y in much less time than it takes to pass 
 from A to B ; in consequence it is submitted to the high- 
 er temperature for a shorter time than it is to the lower. 
 Indeed, it can be proved from Kepler's second law, 
 that the intense radiation experienced in the passage 
 between A and B affords to the planet precisely so 
 much heat and no more than that which it obtains 
 during the journey from x to Y. "We can express this 
 law still more simply by saying that the heat received 
 while the planet is traversing the arc A B equals that 
 
THE ASTRONOMICAL THEORY. 83 
 
 which it receives in traversing x Y. This proposition 
 can be extended to show the following general principle : 
 Draw any straight line, A x, through the focus of an 
 ellipse, then the heat received by a planet in passing 
 round the part of the ellipse on one side of the line is 
 equal to that which it obtains while passing around 
 that on the other. 
 
 From this theorem we are enabled to deduce the 
 first important principle which makes clear the effect 
 of eccentricity on climate. It is convenient, in the dis- 
 cussions connected with this subject, to divide a twelve- 
 month into two seasons instead of into four. We shall 
 regard the entire year as composed of summer and 
 winter. By summer, in the Northern Hemisphere, we 
 mean the time during which the sun's center is above 
 the Equator, while winter in the same hemisphere is 
 denned to be the period when the sun's center lies be- 
 low the Equator. When summer prevails in one hemi- 
 sphere there is, of course, winter in the other. Let 
 the line A x in Fig. 2 represent the line of the equi- 
 noxes, then it is summer in the Northern Hemisphere 
 while the earth moves around through A B x, and it is 
 winter in the same hemisphere while the earth moves 
 back to its original place through x Y A. Hence we 
 learn the important fact that the total heat received by 
 the earth from equinox to equinox while moving round 
 one part of the orbit, is equal to that received by our 
 globe while completing its journey round the remaining 
 part. To illustrate this principle by a somewhat ex- 
 
84: 
 
 THE CAUSE OF AN ICE AGE. 
 
 treme case, let us suppose the line of equinoxes to be 
 represented by A B in the adjoining Fig. 3. The high 
 degree of eccentricity in this ellipse makes the length 
 of the part A x B very much less than that of the re- 
 mainder, A Y B. The total heat received by a planet 
 in its passage around A x B will equal that received 
 during the passage around A Y B. 
 
 There is another important result which follows 
 from strict geometrical reasoning. It relates to the 
 distribution of heat between the two hemispheres, and 
 
 FIG. 3. AN EXTREME CASE FOB ILLUSTRATION. 
 
 may be thus stated : the total amount of heat received 
 on the Northern Hemisphere in the year i. e., in sum- 
 mer and winter taken together is equal to that re- 
 ceived on the Southern Hemisphere. 
 
 Of course, if our earth were revolving in a circle 
 around the sun this theorem would be obviously true ; 
 the simplest consideration of symmetry would show 
 that both hemispheres had in the course of the year 
 
THE ASTRONOMICAL THEORY. 85 
 
 gone through precisely the same relations to the sun. 
 When, however, we remember that the orbit may be 
 elliptical, and that the position of the line of equinoxes 
 may lie across the ellipse in any direction, the result is 
 by no means obvious ; it is rather to be regarded as a 
 striking and significant theorem. Refer, for example, 
 to Fig. 3 ; the Northern Hemisphere will, in that case, 
 have a short and intensely hot summer and a long and 
 intensely cold winter; the Southern Hemisphere will 
 enjoy a long and mild summer and a short winter, the 
 severity of which will be mitigated by proximity to the 
 sun. Notwithstanding these great differences between 
 the seasons and the climates of the two hemispheres, 
 the total quantity of heat which each receives would be 
 absolutely the same. If we take 200 to represent the 
 share of the sun's radiation which the entire earth is 
 able to grasp during the course of the year, then, what- 
 ever be the eccentricity and whatever be the line of 
 equinoxes, 100 will be the share of heat received by the 
 Northern Hemisphere and 100 the share received by 
 the Southern. 
 
 Up to the present it would almost seem as if we 
 had been striving to illustrate how slight was the ef- 
 fect which changes in the earth's eccentricity were 
 competent to produce on climate. Have we not shown 
 that the total quantity of heat received by the earth in 
 the course of the year is practically invariable ? Have 
 we not also shown that the total heat received by the 
 earth during one season (equinox to equinox) is equal 
 7 
 
86 THE CAUSE OF AN ICE AGE. 
 
 to that which it receives in the other, no matter what 
 be the eccentricity, or where the equinoxes may lie? 
 Nay, have we not further shown that no degree 
 of eccentricity and no position of the equinox can 
 disturb that fundamental law which asserts that each 
 hemisphere receives its due half of the total sunbeams 
 of the year? It seems, therefore, hardly surprising 
 that some should have drawn the erroneous inference 
 that changes in the eccentricity could produce no ap- 
 preciable effect on climate. Closer examination will 
 reveal the irregularities of heat distribution which 
 changes of eccentricity are capable of originating. 
 
 At this point I have to introduce a theorem which 
 throws much light on the subject. The demonstration 
 can not be obtained without the use of mathematical 
 symbols, and I have accordingly relegated it to the 
 Appendix ; but we shall here enunciate the important 
 result to which it leads us. It being granted that 
 100 is the expression for the total quantity of heat 
 received on, let us say, the Northern Hemisphere in a 
 year, it is required to determine the shares into which 
 it is to be divided, if they are to represent the heat re- 
 ceived during summer and winter respectively. At 
 first sight it might seem that the distribution would 
 depend upon the eccentricity of the orbit, and would 
 also be affected by the situation in which the line of 
 equinoxes was placed with reference to the axis of the 
 ellipse. It fortunately happens that these elements are 
 found to disappear in the course of the calculation. 
 
THE ASTRONOMICAL THEORY. 87 
 
 The shares into which the total heat of the hemisphere 
 is divided between summer and winter remain unaf- 
 fected both by eccentricity and by the position of the 
 line of equinoxes. The datum in our system on which 
 the distribution does depend is the obliquity of the 
 ecliptic. It will be, indeed, obvious that the obliquity 
 must form a significant factor; for if we should sup- 
 pose the obliquity to be zero, so that the ecliptic coin- 
 cides with the equator, the distinction of seasons as we 
 now understand them would vanish altogether. 
 
 Amid so much that is changeable in the planetary 
 system, it is fortunate that the obliquity of the ecliptic 
 may, for our present purpose, be regarded as practically 
 constant. It is, no doubt, true of the obliquity as of 
 every other astronomical element, that absolute con- 
 stancy is not to be expected. But it will be easy to 
 show that, so far as the present investigation is con- 
 cerned, we are entitled to overlook the operation of 
 the minute changes of the obliquity which arise from 
 planetary perturbation. At the present time it is ad- 
 mitted that the obliquity of the ecliptic is slowly de- 
 clining. According to Powalky, the magnitude, which 
 at present we take to be 23 27' 22-3*, is decreasing at 
 the rate of 47* per century ; that is to say, very nearly 
 half a second per annum. Small as this rate of change 
 undoubtedly is, it might yet be capable of accomplish- 
 ing immense alterations in the element it affects during 
 the vistas of time that geology opens up to us, and con- 
 sequently in the climatic conditions of our globe. It is, 
 
88 THE CAUSE OF AN ICE AGE. 
 
 therefore, of importance to observe that the diminution 
 of the obliquity of the ecliptic does not proceed con- 
 stantly ; it is, indeed, in the nature of a periodical 
 fluctuation, which goes through its changes with such 
 extreme slowness that for practical purposes we may 
 speak of the rate of alteration as remaining uniform 
 during the mere centuries or thousands of years with 
 which human affairs are concerned. Laplace has shown 
 that the obliquity of the ecliptic fluctuates through a 
 range whose limits are about three degrees apart. Fur- 
 ther investigation has been made on this subject, espe- 
 cially by Stockwell, who was enabled to use much more 
 accurate data than those which were available to La- 
 place. Not only were the masses of the planets better 
 determined when Stockwell's work was accomplished, 
 but the influences of Neptune could also be taken into 
 account. The result of his inquiry tended to contract 
 the limits which Laplace had assigned to the fluctua- 
 tions of the obliquity of the ecliptic. According to 
 Stockwell's determination this element can never sink 
 below 21 58' 36", and it can never rise above 24 35' 
 58" ; so that the range according to this astronomer has 
 been reduced to 2 37' 22". It will thus be seen that at 
 present the obliquity of the ecliptic lies not far from 
 midway between the extreme limits which it is capable 
 of attaining. 
 
 I entreat the particular attention of the reader at 
 this stage of the argument. Especially would I warn 
 him from falling into the error by which some previous 
 
THE ASTRONOMICAL THEORY. 89 
 
 writers have been led sadly astray. What I have shown 
 is, that the heat received on the whole earth from vernal 
 to autumnal equinox equals that received on the whole 
 earth from autumnal to vernal equinox. Do not read 
 this as if it asserted that the heat received during sum- 
 mer is equal to that received during winter. This is 
 not true, but it has been often asserted. It is, in fact, 
 the fundamental error which has vitiated the Astro- 
 nomical theory of the Ice Age as previously presented, 
 and which it is the chief object of this little book to 
 expose. As, however, this is the great stumbling-block, 
 I shall spare no pains to elucidate the point, so as to 
 distinguish as clearly as I can between the remarkable 
 law which is true, and the incorrect statement which 
 bears such a specious resemblance to the truth that the 
 two have been frequently confounded. It is obvious 
 that summer in one hemisphere is winter in the other ; 
 therefore, when we speak of the heat received during 
 summer, we must, of course, have one particular hemi- 
 sphere in view. Think, then, of the Northern Hemi- 
 sphere. It receives a certain quantity of heat during 
 the passage from the vernal equinox to the autumnal 
 equinox that is, during the summer in the Northern 
 Hemisphere. It also receives a certain quantity of 
 heat between the autumnal equinox and the next ver- 
 nal equinox that is, during the winter in the Northern 
 Hemisphere. But these quantities are NOT equal, as 
 has been frequently supposed. The true Astronomical 
 theory is mainly dependent on the accurate calculation 
 
90 THE CAUSE OF AN ICE AGE. 
 
 of the figures in this part of the subject, so, on the first 
 allusion to them, I request the special attention of the 
 reader. As the work proceeds, he will find the requi- 
 site detail added, which it would be hardly convenient 
 to insert in this place. I shall, however, at once enun- 
 ciate a fundamental truth, which I commend to the 
 earnest consideration of those who do not wish to miss 
 the cardinal feature of this book. 
 
 Of the total amount of heat received from the sun on 
 a hemisphere of the earth in the course of a year, 63 per 
 cent, is received during the summer and 37 .per cent, is 
 received during the winter. 
 
 There must be no mistake about these numbers; 
 they are sixty-three and thirty-seven ; nor must there 
 be any mistake as to their significance. They depend 
 on the mathematical calculation given in the Appendix ; 
 they admit of no dispute ; they are perfectly determi- 
 nate and certain elements in the theory. If it should 
 prove that the facts which these cumbers imply have 
 not been given by any previous writer, then their an- 
 nouncement is the novelty in this book the one central 
 feature by which it is to be judged. My chief object is 
 to emphasize the relation of these figures to the Astro- 
 nomical theory. This theory will be entirely misunder- 
 stood unless the facts signified by these numbers are 
 borne in mind. No one can discuss the Astronomical 
 theory of the Ice Age unless the figures 63 and 37 form 
 a portion of his consciousness and the refrain of his 
 every argument. 
 
THE ASTRONOMICAL THEORY. 91 
 
 Imagine any elliptic orbit around the sun, no matter 
 what may be its eccentricity ; imagine a planet revolving 
 in this orbit with an inclination between its equator and 
 the plane of its orbit equal to the obliquity of the eclip- 
 tic at the present moment. There is, of course, an infi- 
 nite number of varieties in the arrangement of the sea- 
 sons on such a globe. They correspond to the various 
 positions in which the plane through the center of the 
 sun parallel to its equator intersects the plane of the 
 ecliptic. 
 
 For instance, we might represent the line of equi- 
 noxes as lying along the major axis of the orbit. The 
 two seasons of summer and winter would then be equal 
 in duration, and the two hemispheres would have pre- 
 cisely parallel conditions as to climate; while it was 
 summer in one it would be winter in the other. The 
 summer in the Northern Hemisphere would be iden- 
 tical in character with the ensuing summer in the 
 Southern Hemisphere. Regarding, as before, the total 
 radiation received by one hemisphere in the course of 
 a year as 100, then the shares received during summer 
 and winter respectively would be 63 and 37. This 
 would necessitate a considerable climatic difference be- 
 tween different parts of the year, for the two seasons 
 are of equal length, and yet one of them receives but 37 
 measures of heat, while the other obtains 63. 
 
 A wholly different case is exhibited in Fig. 3, page 
 84. In this case the line of equinoxes is perpendicular 
 to the axis major of the ellipse, and divides the path 
 
92 THE CAUSE OF AN ICE AGE. 
 
 into two very unequal portions ; and we shall suppose 
 that the Northern Hemisphere is turned toward the 
 sun, while the planet is moving through A x B. Sum- 
 mer will, therefore, be enjoyed by the Northern Hemi- 
 sphere during the time occupied by the journey from 
 A to B, when winter sets in and continues during the 
 completion of the rest of the orbit. The Northern 
 Hemisphere will, in this case, have a brief summer and 
 a long winter. Indeed, we can offer a double reason 
 why the durations of the two seasons are so very un- 
 equal. In the first place, the length of the journey 
 which the planet has to perform during the winter is 
 longer than that which it pursues during the summer. 
 Secondly, according to Kepler's law, by which the 
 radius from the sun to the planet sweeps over equal 
 areas in equal times, the pace at which the planet winds 
 round the end of the ellipse near the sun is much more 
 rapid than that at which it traverses the distant parts 
 of its path. Both causes will thus protract the dura- 
 tion of winter as compared with that of summer. Yet, 
 notwithstanding the wide difference between such a 
 movement and that previously considered, it still re- 
 mains true that 63 per cent, of the sun's heat is received 
 by each hemisphere in summer, leaving only 37 per 
 cent, for the winter. It will be noticed that though 
 the summer was brief and the winter was long, yet the 
 planet was so near to the sun during the summer of the 
 Northern Hemisphere that the increased proximity to 
 the source of heat compensated for the brevity of that 
 
THE ASTRONOMICAL THEORY. 93 
 
 season, and thus permitted that law to be observed 
 which declares that G3 per cent, of the total year's heat 
 on a hemisphere will be received by that hemisphere 
 during its summer, leaving the remainder to suffice for 
 the winter as it best may. 
 
 It will be also instructive to ponder on the condition 
 of the Southern Hemisphere in the case last considered. 
 At the extremity, x, of the orbit the Southern Hemi- 
 sphere is turned away from the sun, and winter will 
 reign during the movement around A x B, while the 
 same hemisphere will enjoy summer so long as the 
 planet is completing its orbit by moving through B Y A. 
 The seasons in the Southern Hemisphere will therefore 
 be of a wholly different type to those of the Northern ; 
 the winter will be short, the summer will be long. The 
 severity of the brief winter will be mitigated by the cir- 
 cumstance that the planet at that season is compara- 
 tively near to the sun. The heat of the long summer 
 will be tempered by the circumstance that it is radiated 
 from a sun at a distance above the average. To obtain 
 the figures a certain numerical estimation of the par- 
 ticular facts of the case has to be made. The diagram 
 in Fig. 3 represents an orbit far more elliptical than the 
 orbit which is actually that of the earth. The true 
 shape of the earth's orbit will be seen in Figs. 4 and 5. 
 However, if the obliquity of the ecliptic be 23 27', as 
 it actually is for the earth, then, no matter how extreme 
 the eccentricity of the ellipse, the law of distribution of 
 the total sun heat between the two seasons will remain 
 
94 THE CAUSE OF AN ICE AGE. 
 
 unaltered. The length of the summer and the brevity 
 oi the winter, accompanied by the unusual mildness of 
 both seasons, will preserve the observance of the law 
 that 63 measures of the sun's heat are conferred during 
 the summer on the Southern Hemisphere, while 37 are 
 left for winter. 
 
 Finally, let us take the case represented in Fig. 2, 
 page 82, where the line of equinoxes, A x, crosses the 
 ellipse, in a direction which we may describe as being 
 at random. During the passage around A B x it will 
 be summer in the Southern Hemisphere and winter in 
 the Northern. The climatic conditions, too, will be 
 somewhat intermediate between those sketched in the 
 last two cases. The contrast between summer and 
 winter will neither be specially accentuated nor will 
 it be well-nigh removed ; one feature, and one alone, 
 remains constant during all varieties of circumstance, 
 and that is, the oft-repeated statement that, so far as 
 direct radiation from the sun is concerned, each hemi- 
 sphere can only obtain 37 measures of heat during its 
 winter, and 63 in summer. 
 
 So far as the present argument is involved, the chief 
 effect of eccentricity on the orbit is to assign the lengths 
 of the seasons. As the earth passes from one equinox 
 to the other, there is summer in the Northern Hemi- 
 sphere and winter in the Southern ; while as the voyage 
 is completed which brings the earth round to where it 
 was at first, there is winter in the Northern and sum- 
 mer in the Southern Hemisphere. 
 
THE ASTRONOMICAL THEORY. 95 
 
 For the theory of the Ice Age we require to exam- 
 ine particularly the cases in which the disparity in the 
 lengths of the seasons shall be the utmost possible. 
 This will occur when the line of equinoxes is perpen- 
 dicular to the axis major of the ellipse. This line di- 
 vides the ellipse into two unequal segments, while the 
 fact that the earth moves with a quicker pace round the 
 small segment than round the large one further tends 
 to increase the inequality between the number of days 
 of summer and the number of days of winter respect- 
 ively. We can give expression to the difference be- 
 tween the lengths of the seasons in this highly spe- 
 cialized case in a simple manner. The number 465, 
 multiplied by the eccentricity, expresses the difference 
 in days between the length of the two seasons. For ex- 
 ample, with the present eccentricity of the earth's orbit, 
 the greatest possible difference between summer and 
 winter would amount to 33 days. I do not mean that 
 the actual disparity between summer and winter at the 
 present moment is so much as this; it only, in fact, 
 amounts to 7 days, because at present the line of equi- 
 noxes does not happen to be adjusted in the manner 
 described. 
 
 The line of equinoxes does not, however, occupy a 
 constant position. The well-known phenomenon of 
 precession arises from the circumstance that the line 
 of equinoxes revolves around the plane of the ecliptic. 
 It will, therefore, happen from to time that the line of 
 equinoxes will come to occupy every conceivable posi- 
 
96 THE CAUSE OF AN ICE AGE. 
 
 tion with regard to the axis major of the earth's path. 
 If the position of the ellipse itself in its plane were a 
 constant one, the recurrence of the epochs when the 
 line of equinoxes was perpendicular to the axis major 
 would, of course, be equal to that of the precession of 
 the equinoxes. The matter is, however, somewhat com- 
 plicated by the movement of the major axis itself. As 
 planetary perturbation affects the eccentricity of the 
 ellipse, so also it affects the situation of its axis. The 
 direction of this motion is opposite to that of the line 
 of equinoxes, and consequently the time of a complete 
 revolution relatively to the ellipse is somewhat reduced. 
 
 It follows that about every 21,000 years the line of 
 equinoxes is so placed with reference to the elliptic 
 path of the earth that tho difference in duration be- 
 tween the two seasons attains a maximum ; what that 
 maximum is depends, of course, upon the eccentricity 
 of the orbit at the time. 
 
 It is of importance at this stage of our inquiry to 
 investigate the range of the fluctuations to which the 
 eccentricity is liable. We here touch upon a mathe- 
 matical inquiry of no little difficulty; it is, however, 
 possible to solve the problem which is thus stated. 
 Given all the masses and the other necessary data con- 
 nected with the solar system, it is required to deter- 
 mine the greatest eccentricity that the earth's orbit 
 can assume under the disturbance arising from the in- 
 fluence of the other planets. This problem has been 
 solved, and the mean between the different determina- 
 
THE ASTRONOMICAL THEORY. 97 
 
 tions of its maximum value may be adopted. This 
 shows that, when all circumstances combine to accentu- 
 ate as much as possible the difference in the lengths of 
 the seasons, one of them may be 199 days long, and the 
 other 166 ; the two together making up, of course, the 
 entire year of 365 days. 
 
 It is not, however, to be supposed that whenever 
 the eccentricity attains its greatest value the difference 
 between the lengths of the seasons must necessarily be 
 so large as 33 days. It is only when the requisite direc- 
 tion of the line of equinoxes and the maximum degree 
 of eccentricity conspire, that so great a difference be- 
 tween the length of summer and that of winter can be 
 obtained. The changes in the eccentricity take place 
 with such extreme slowness, that by comparison with a 
 cycle of such changes the period of 21,000 years which 
 suffices for the rotation of the line of equinoxes rela- 
 tively to the axis major will seem inconsiderable. 
 Doubtless, when a period of high eccentricity has been 
 reached, the orbit will remain in practically the same 
 condition long enough to permit of two or even more 
 than two passages of the line of equinoxes through that 
 critical position which produces the maximum differ- 
 ence between the seasons. 
 
 Special attention must now be given to those epochs 
 in the earth's history when these two agencies concurred 
 to produce the greatest difference in length between sum- 
 mer and winter. It seems hardly possible to doubt that 
 such epochs have been those of the Ice Ages. At first 
 
98 THE CAUSE OF AN ICE AGE. 
 
 it might seem that a difference of 33 days between the 
 lengths of the seasons would be hardly of sufficient sig- 
 nificance to give rise to phenomena so colossal as those 
 of the Ice Age. I think, however, we shall be able to 
 show that the forces at our disposal are amply sufficient 
 to account for the effects which have been observed. 
 
 I must here recall the fundamental theorem to 
 which I have so often referred, which states that 63 per 
 cent, of the total sun heat of the year on either hemi- 
 sphere is received during summer, and only 37 per cent, 
 is left with which to eke out the winter. Fortunately 
 for the simplicity of our calculation, we have seen that 
 these figures are independent both of the eccentricity of 
 the orbit and of the position ot' the line of equinoxes. 
 We are therefore entitled to apply these figures to those 
 critical epochs in past time when, by a confluence of 
 causes, the maximum difference in duration of seasons 
 had been reached. There was therefore an epoch, or 
 doubtless more than one, when the seasons were 199 
 and 166 days, and when the shares of sun heat received 
 on each hemisphere during those seasons were 63 and 
 37 respectively. "We may express the matter a little 
 more clearly by describing more particularly the case of 
 the Northern Hemisphere. There were epochs when 
 the Northern Hemisphere had a summer of 199 days 
 and a winter of 166 days ; there were also epochs when 
 in the same hemisphere the winter endured for the long 
 period of 199 days, while the summer only lasted for 
 166. In each case, however, the figures 63 and 37 are 
 
THE ASTRONOMICAL THEORY. 99 
 
 to represent the proportional quantities of heat which 
 that hemisphere received in summer and winter respect- 
 ively. We have to suppose two cases, that in which 
 the long season was the summer and the short season 
 the winter, and that in which the long season is the 
 winter and the short season the summer. The climatic 
 conditions of these two cases are profoundly different. 
 In the first place the long summer and the brief winter 
 would certainly afford a much more uniform distribu- 
 tion of the sun's benefits than would be found in 
 the opposite case. Seeing that 63 measures of heat 
 come in summer and 37 in winter, it would seem equi- 
 table that the 63 measures should extend over the long 
 season of 199 days, leaving the 37 measures to do the 
 best they could for the brief winter of 166 days. But 
 while the Northern Hemisphere was enjoying the be- 
 neficent climate which these figures indicate, the cli- 
 matic condition of the Southern Hemisphere would be 
 totally different. There, too, the seasons had the same 
 lengths of 199 and 166 days respectively, but of course 
 the summer in the Northern Hemisphere was the winter 
 in the Southern ; and consequently while the 63 meas- 
 ures of heat, which the Southern Hemisphere received, 
 were all poured in during its brief summer of 166 days 
 only, the remaining 37 were left to supply the pro- 
 tracted winter of 199 days. 
 
 In Fig. 4 the true shape of the earth's orbit is shown 
 at a time of greatest eccentricity. The earth is, of 
 course, represented on a monstrous scale, in order to 
 
100 
 
 THE CAUSE OF AN ICE AGE. 
 
 distinguish the hemispheres. The Northern is glacial, 
 and the Southern genial. In Fig. 5, which represents 
 the state of things 10,500 years later, the Northern is 
 genial, and the Southern is glacial. 
 
 Fia. 4. A GLACIAL NORTHERN HEMISPHERE. 
 
 There is another mode of expressing the same result 
 which possesses some advantages. If the daily average 
 receipt of heat from the sun on a hemisphere be taken 
 as unity, then of course the receipt during the whole 
 year will be represented by 365 ; this is not, however, 
 imparted uniformly, for 229 of these units are con- 
 tributed during summer and 136 during winter. 
 
THE ASTRONOMICAL THEORY. 
 
 101 
 
 This exhibits in a rather instructive manner the ex- 
 treme contrasts between the climates of the two hemi- 
 spheres, that have occasionally arisen. 
 
 We express them as follows : 
 
 FIG. 5. A GENIAL NORTHERN HEMISPHERE. 
 
 LNTERGLACIAL. 
 
 229 heat measures spread over 199 days. 
 136 heat measures spread over 166 days. 
 
 GLACIAL. 
 
 229 heat measures spread over 166 days. 
 136 heat measures spread over 199 days. 
 
102 THE CAUSE OF AN ICE AGE. 
 
 From these figures we are enabled to deduce the 
 average daily receipt of sun heat by each hemisphere 
 under the several conditions as follows : 
 
 PRESENT (NORTHERN HEMISPHERE). 
 Mean daily sun heat in summer (186 days) 1*24. 
 Mean daily sun heat in winter (179 days) -75. 
 
 INTERGLACIAL. 
 
 Mean daily sun heat in summer (long) . 1-16. 
 Mean daily sun heat in winter (short) . *81. 
 
 GLACIAL. 
 
 Mean daily sun heat in summer (short) . 1-38. 
 Mean daily sun heat in winter (long) . -08. 
 
 A little consideration will show that these figures 
 must import wide climatic differences between the dif- 
 ferent seasons, for we must never forget the magnitude 
 of the thermal unit which we have employed. What 
 the actual measure of that unit may be in the conven- 
 tional language of physics does not here concern us. 
 To appraise its value it will be sufficient to allude to its 
 effects. We have already pointed out how the heat re- 
 ceived from the sun suffices to maintain our earth at 
 a temperature which is 300, or more, degrees above that 
 to which it would sink if the sun's heat were with- 
 drawn. We are therefore provided with a notion of the 
 value of our unit by the excess of temperature above 
 that of space, at which the earth is maintained. Looked 
 
THE ASTRONOMICAL THEORY. 103 
 
 at in this way we can readily believe that a fall of one 
 tenth in the supply of sun heat might correspond to 
 quite an alarming fall in temperature. We can hardly 
 pretend to express the range of the fall accurately, for a 
 multitude of conditions are involved which are only im- 
 perfectly known to us. It will, however, be obvious 
 that if a certain supply of sun heat is required to keep 
 our earth at a temperature 300 above that to which it 
 would otherwise drop, then a reduction of one tenth in 
 the supply would involve a fall of many degrees in the 
 temperature. The strict numerical proportion would 
 of course indicate a fall of 30, but we should certainly 
 not be justified in supposing that this exact proportion 
 would be observed. 
 
 It is useful to contrast the extreme conditions of 
 our globe with the present condition. In the Northern 
 Hemisphere, for example, we now have a summer of 
 186 days and a winter of 179 days. As the summer is 
 seven days longer than the winter, we enjoy at the pres- 
 ent moment what may be regarded as a faint approxi- 
 mation to an interglacial or genial period. In the 
 Southern Hemisphere, on the other hand, the winter 
 exceeds the summer, so that the tendency in that hemi- 
 sphere is toward but only a short way toward a gla- 
 cial epoch. At present, in the Northern Hemisphere, 
 the mean daily receipt of sun heat during summer is to 
 be represented by 1-24; in a glacial condition of the 
 same hemisphere the mean daily receipt of heat through 
 the short summer would be 1-38. Thus we see that the 
 
104: THE CAUSE OF AN ICE AGE. 
 
 summer days at the height of a glacial period would be 
 as much warmer than those of our present summers as 
 the daily receipt of 14 per cent, of an additional heat 
 unit would make them. Bearing in mind the magni- 
 tude of the heat unit which is involved, a unit suffi- 
 cient, as we have so often said, to maintain a tempera- 
 ture 300, or more, above that which would be other- 
 wise maintained, it is obvious that an increase of 14 
 per cent, in sun heat must necessitate a large increase 
 of temperature; the brief summer, therefore, in the 
 glaciated hemisphere will be intensely hot, so far as 
 direct solar radiation is concerned. 
 
 At present the mean daily receipt of heat in our 
 Northern Hemisphere during winter is expressed by 
 75 ; the corresponding figure during the glacial epoch 
 would be -68. It therefore follows that the mean daily 
 receipt of heat during the long and dreary glacial win- 
 ter will be 7 per cent, of a heat unit below our present 
 average daily supply in the Northern Hemisphere dur- 
 ing the same season. At the first glance it might ap- 
 pear that a mere decline of 7 per cent, in the sun heat 
 received would not be very momentous from a climatic 
 point of view, but this first impression will be modified 
 when we consider two other factors in the question. In 
 the first place, the winter was then 33 days longer than 
 the summer, instead of being 7 days shorter, as it is at 
 present. The effects of intense cold had, therefore, a 
 longer time in which to accumulate, while the brevity 
 of the succeeding summer would tend to impair its 
 
THE ASTRONOMICAL THEORY. 105 
 
 power of dissipating the ice and snow which gathered 
 in the long winter. Then, too, we must remember in 
 what units the 7 per cent, is expressed. A certain aver- 
 age daily supply of heat is continuously required to 
 maintain our temperature at an elevation of at least 
 300 above the temperature of space ; a decline in that 
 supply to the extent of 7 per cent, must involve a con- 
 siderable fall in the temperature which that heat was 
 capable of sustaining, unless in so far as its effect may 
 be neutralized by the operation of other agencies. 
 
 We are now in a position to describe the astronomi- 
 cal theory of the Ice Age. So long as the earth's orbit 
 retains a form which is nearly circular there can be no 
 extensive glaciation; the seasons must succeed each 
 other according to the same laws that they observe at 
 present. When, however, by the disturbing effect of 
 the other planets the earth's orbit passes from the 
 nearly circular to the elliptic shape (such a state of 
 things is only attained after immense intervals of time), 
 then it endures during a period which is long enough 
 for several Ice Ages. But this is only the first condi- 
 tion ; another astronomical adjustment must conspire 
 with a high degree of eccentricity. When the line of 
 equinoxes happens to be perpendicular to the axis ma- 
 jor of the ellipse, we have the second necessary condi- 
 tion for the establishment of an Ice Age. Nor is there 
 anything arbitrary in the assumption that the adapta- 
 tion of the line of equinoxes to this critical position 
 shall take place once, or more than once, during the 
 
106 THE CAUSE OF AN ICE AGE. 
 
 continuance of a high degree of eccentricity in the 
 orbit. The changes in the shape of the orbit require 
 enormously longer periods of time than those which 
 suffice for the revolution of the equinoxes. The changes 
 in the eccentricity proceed with such extreme slowness 
 that the ages during which the eccentricity remains in 
 the vicinity of its maximum value are long enough to 
 admit of more than a single revolution of the line of 
 equinoxes. Indeed, it would seem to have happened 
 not infrequently that several successive revolutions of 
 the line of equinoxes with respect to the axis major 
 have had time for their completion before the eccen- 
 tricity had sufficiently declined to render glaciation im- 
 possible. 
 
 Let us suppose that the Northern Hemisphere is so 
 placed that the summer it enjoys lasts for 166 days and 
 the winter for 199 days. The Northern Hemisphere 
 will then be exposed to a brief and intensely hot sum- 
 mer, during which the sun is at its least possible dis- 
 tance, and to a long and excessively cold winter, during 
 which the sun is at its greatest possible distance. This 
 is the condition required for the development of glaci- 
 ation. During the rigors of the winter the ice and 
 snow accumulate, while the succeeding brief summer is 
 not able to thaw as much water as has been solidified 
 during the winter. Thus the ice grows from year to 
 year, and an ice-sheet is produced which extends far be- 
 yond the limits within which ice-sheets are at the pres- 
 ent time confined. All this time the Southern Hemi- 
 
THE ASTRONOMICAL THEORY. 1Q7 
 
 sphere enjoys a climatic condition widely different from 
 that of the Northern. It has no doubt a summer and a 
 winter, but the winter has been short, and its severity 
 has been further mitigated by the circumstance that 
 the earth was comparatively near the sun. The for- 
 mation of ice and snow during winter must have been 
 carried on on a very restricted scale, or, indeed, wholly- 
 prevented. On the other hand, the summer in this 
 delightful climate would contain as great a number of 
 days as it was possible for that season to possess, but 
 the fierce heat of the sun would be abated from its 
 average amount, because the sun during all that season 
 would be placed at the greatest distance from the earth 
 which it was possible to attain. The astronomical ele- 
 ments of the problem are so definite, and the necessary 
 climatic changes that they involve seem so obvious, that 
 on purely astronomical grounds alone we seem almost 
 entitled to assert that the earth must have passed 
 through a repetition, and many a repetition, of great 
 climatic changes. 
 
 It would, however, be a mistake to suppose that 
 extreme cold was the sole requisite for the formation of 
 an Ice Age. That is certainly not the case, for a large 
 supply of heat somewhere or other is also a necessary 
 element in the production of an ice-sheet. For what is 
 an ice-sheet but an accumulation of the snow which 
 has fallen ? That snow has of course come from the 
 clouds in which the moisture which forms the snow was 
 suspended as vapor. The supply of vapor to the atmos- 
 
108 THE CAUSE OF AN ICE AGE. 
 
 phere has to be continuously maintained in order to 
 provide for the perennial showers of rain or falls of 
 snow with which the clouds are charged. The moisture 
 which is elevated to the upper regions of the air has, of 
 course, been derived from the seas which cover this 
 globe. As the sunbeams pour down on the wastes of 
 sea they evaporate the water, which ascends in the form 
 of invisible vapor. The extent to which this operation 
 daily goes on is astounding when expressed in figures. 
 The daily quantity of water which ascends in vapor 
 from the surface of the oceans would, when condensed 
 again, occupy a volume of some hundreds of cubic 
 miles. Before a pound weight of ice can be manu- 
 factured for a glacier a pound weight of snow must 
 have fallen; that pound weight of snow necessitates 
 that the sun shall have raised a pound of water in the 
 form of vapor from the surface of the ocean. We are 
 now enabled to see how important a factor heat must 
 have been even in the formation of the ice-sheet. Pro- 
 fessor Tyndall has remarked that the heat which would 
 be required to evaporate enough water to form a glacier 
 would be sufficient to fuse and transform into glowing 
 molten liquid a stream of cast iron five times as heavy 
 as the glacier itself. 
 
 The astronomical theory of the Ice Age shows at 
 once where to look for the heat which is so necessary to 
 the accomplishment of glaciation. It is important to 
 observe that even when the earth's orbit has its highest 
 degree of eccentricity, and when the severity of an Ice 
 
THE ASTRONOMICAL THEORY. 109 
 
 Age is at a maximum, the total quantity of heat which 
 the earth receives during the year has not been lessened ; 
 indeed, it is somewhat greater than when the orbit was 
 more nearly a circle. If, therefore, the glaciated hemi- 
 sphere received during its winter a deficient supply of 
 heat to which the glaciation owes its origin, the defect 
 of heat upon this hemisphere will have to be compen- 
 sated by a superabundance elsewhere. More favored 
 regions of the globe will accordingly enjoy a more 
 copious supply of sunbeams, and thus those volumes of 
 aqueous vapor will be provided which, put in circulation 
 by the winds, are deposited in the form of snow where 
 the ice-sheets are growing. 
 
 It will thus be seen that the explanation we have 
 offered of the Ice Age depends upon the consequences 
 of an irregularity in the supply of heat from season to 
 season, notwithstanding that the total amount remains 
 unaltered. It can hardly be denied that such great 
 irregularities in the supply as we have shown to be the 
 consequences of a suitable combination of astronomical 
 conditions, must correspond to great climatic changes. 
 But it has been doubted whether these climatic changes 
 will necessarily be the same as those required for the 
 production of extreme glaciation. For instance, it has 
 been contended that no Ice Age can be produced in the 
 Northern Hemisphere if the total quantity of heat in 
 that hemisphere received during the course of the year 
 remains unchanged. At first sight it must be admitted 
 that the view here taken is a plausible one, nor can it be 
 
HO TIIE CAUSE OF AX ICE AGE. 
 
 denied that there are considerable difficulties connected 
 with this aspect of the matter. A little attention may 
 tend to lessen the difficulty. The formation of an ice- 
 sheet is an operation of such magnitude that it is wholly 
 impossible for it to arise as the product of a single sea- 
 son. We can not believe that the ice-sheet is manu- 
 factured afresh each winter and dissolved away during 
 the ensuing summer ; it must slowly grow from year to 
 year, perhaps we might rather say from century to cent- 
 ury, the net annual increase being determined by the 
 excess of the ice produced during the winter over the 
 ice melted during the ensuing summer. If a certain 
 daily average of heat given uniformly be quite incom- 
 patible with the accumulation of ice, we have to explain 
 how the ice-sheet does wax great when the same annual 
 total of heat is given, but in such a way that each day 
 in winter receives much less than its average and each 
 day in summer much more than its average. Suppose 
 a horse has a daily allowance of a stone of oats, which is 
 sufficient to keep him in good health. The condition 
 of that horse would be surely very seriously affected if 
 he received a stone and a half for the first six months 
 and but half a stone per day for the remainder of the 
 year. Yet in this case, as in the former one, his daily 
 average of a stone per diem has not been interfered with. 
 In a similar way the irregularity in the supply of heat 
 to the earth is capable of producing great climatic 
 changes. No doubt if the earth had some absolutely 
 perfect agency for storing up the excess of heat during 
 
THE ASTRONOMICAL THEORY. HI 
 
 the summer, and carrying it over to the next winter, it 
 might be possible in that way so to equalize the climate 
 that glaciation might not take place. Nature does not, 
 however, provide any completely effective method for 
 storing up the heat of summer; a great deal of it is 
 radiated back again into space from the soil which it 
 has heated, while much of the summer heat that falls 
 on the surface of the ice and snow is reflected back 
 again into space, and is not captured and stored away as 
 heat would be, which was employed in melting the ice 
 and snow. The sunbeams, in the brief and fiercely hot 
 summer of the Glacial Period, failed to melt as much ice 
 as had been accumulated during the preceding winter, 
 Thus it was that the ice-sheet was permitted to grow 
 from year to year until the stringency of the astronom- 
 ical conditions became relaxed. 
 
CHAPTER VI. 
 
 WHY THIS BOOK HAS BEEN WRITTEN". 
 
 IT will be convenient at this stage to offer some ex- 
 planation of the circumstances which led me to write 
 this book. The most suitable method of doing so will 
 be to give some account of my connection with the 
 theory of the Ice Age. 
 
 Geology has always been a subject in which I have 
 felt a special interest, chiefly on account of the many 
 points of contact between that science and astronomy ; 
 it was therefore natural that I should have studied Dr. 
 Croll's famous book on Climate and Time. I was 
 greatly struck by this work when I first read it many 
 years ago. Subsequent acquaintance with this volume, 
 and also with his second work, Climate and Cosmology, 
 has only increased my respect for the author's scientific 
 sagacity, and my admiration for the patience and the 
 skill with which he has collected and marshaled the 
 evidence for the theory that he has urged so forcibly. 
 
 I was invited to deliver three scientific lectures be- 
 fore the Royal Institution of Great Britain in January, 
 1886. The Astronomical theory of the Ice Age seemed 
 
WHY THIS BOOK HAS BEEN WRITTEN. H3 
 
 to me to offer a suitable subject, and as the theory had 
 specially engaged my attention already, I accordingly 
 chose it. Thereupon I commenced diligent prepara- 
 tion for my work, and I made a special examination 
 of the astronomical doctrine on which the theory is 
 founded. It soon appeared that the astronomical part 
 of the subject depended mainly upon the investigation 
 of the following problem : How much of the annual sun 
 heat does a single hemisphere of the earth receive in 
 summer, and how much does it receive in winter? I 
 worked this out by the calculation which is given in 
 the Appendix, and I discovered the law of distribution 
 of sun heat on a hemisphere between the two seasons 
 into which the year is divided by the equinoxes. This 
 shed much light on the subject. I felt confident that 
 the Astronomical theory gave the true solution to the 
 Ice Age, and my lectures were duly delivered at the 
 Royal Institution. 
 
 A few months later I delivered two lectures on the 
 same subject before the Royal Dublin Society. It was 
 thought desirable by some of my friends to have a more 
 ample discussion of the mathematical part of the sub- 
 ject than would have been suitable for my lectures, 
 which were addressed to a general audience. It was in 
 compliance with this wish that I read a short paper 
 before the Royal Irish Academy on May 24, 1886, en- 
 titled, Note on the Astronomical Theory of the Great 
 Ice Age. In this I enunciated and proved that law of 
 the distribution of sun heat between the two seasons 
 
THE CAUSE OF AN ICE AGE. 
 
 which I have already referred to as the cardinal feature 
 of this little book. 
 
 At the time I wrote the paper I merely desired to 
 bring the matter prominently forward for discussion; 
 I did not pretend to put forward any novelty, except, 
 perhaps, in the manner in which the subject was treat- 
 ed. I thought that the law as I expressed it would be 
 a compendious method of conveying what must have 
 been familiar to those who had given any special atten- 
 tion to the subject. I may mention that I was desirous 
 of diffusing among my scientific friends an interest in 
 the doctrine, and I thought that a special method of 
 calculating the astronomical part of the subject would 
 conduce to that end. There, probably, my connection 
 with the question would have terminated had it not 
 been for something of which I afterward became aware. 
 No doubt I had made out for myself and for my own 
 information the law of the distribution of heat between 
 the two seasons, but every one who is engaged in scien- 
 tific work knows that matters like this are discovered 
 over and over again by persons working independently 
 at the same subject. It is therefore rash to claim nov- 
 elty in such a matter. At this moment I think it pos- 
 sible that the law, or what is equivalent to it, may have 
 been discovered and perhaps even published before ; I 
 have, however, never been able to meet with any account 
 of it. I have not even observed in any of the writers 
 with whom I am acquainted any consciousness of the 
 existence of such a law. I confess that this has not a 
 
WHY THIS BOOK HAS BEEN WRITTEN. H5 
 
 little surprised me. The law is so pregnant with sig- 
 nificance that the theory of the Ice Age can not indeed 
 be rightly understood unless by those who are acquaint- 
 ed with the facts to which it gives expression. It seems, 
 however, certain that the true principle of the distribu- 
 tion of heat between the seasons is not generally known, 
 if known at all ; it is therefore the more necessary that 
 it should be pressed on the consideration of those who 
 have paid special attention to the connection between 
 terrestrial climate and the shape of the earth's orbit. 
 
 I must, however, go farther than this. Not only 
 did I find that the true law had been overlooked, but 
 it became apparent that sometimes another law had 
 been adopted which was absolutely incorrect. I found, 
 furthermore, that the Astronomical theory of the Ice 
 Age as ordinarily stated was contaminated with absolute 
 blunders in the simple mathematical questions which 
 are involved. When the true law of the distribution 
 of heat between the seasons is introduced, it places 
 the astronomical doctrine as to the cause of the Ice 
 Age in an impregnable position. It appeared to me 
 that this point had not been properly dealt with. The 
 more reflection I gave to the subject, the more impor- 
 tant did it seem, that the astronomical side of the ques- 
 tion should at all events be stated with whatever mathe- 
 matical accuracy it possessed, and hence it was that I 
 conceived the idea of writing this book. 
 
 No one can feel a warmer admiration than I do for 
 the splendid talents displayed by Sir John Herschel. 
 
116 THE CAUSE OF AN ICE AGE. 
 
 As an all-round astronomer he seems never to have had 
 a rival. He was equally successful in observing with 
 his telescope, in performing abstruse mathematical cal- 
 culations, or in describing his discoveries in a style now 
 regarded as classical. In the whole literature of astro- 
 nomical research there is not a nobler work than that 
 grand volume in which Sir John Herschel records the 
 discoveries that he made at the Cape of Good Hope. A 
 striking illustration of the great astronomer's powers as 
 an expounder is exhibited in his volume The Outlines 
 of Astronomy ) which every one with a taste for science 
 cherishes in a favored corner of his library. 
 
 In this book we find a remarkable paragraph, in 
 which Herschel sets forth the relations between the 
 changes in climates on the earth and the correspond- 
 ing changes in the eccentricity of the earth's orbit. 
 It runs as follows; I quote from the edition of 1875 
 p. 233 : 
 
 " In what regards the comfort of a climate and the 
 character of its vegetation the intensity of the summer 
 is more naturally estimated by the temperature of its 
 hottest day and that of a winter by its sharpest frosts 
 than by the mere duration of those seasons and their 
 total amount of heat. Supposing the eccentricity of 
 the earth's orbit were very much greater than it actu- 
 ally is, the position of its perihelion remaining the same, 
 it is evident that the characters of the seasons hi the 
 two hemispheres would be strongly contrasted. In the 
 Northern we should have a short but very mild winter 
 
WHY THIS BOOK HAS BEEN WRITTEN. H7 
 
 with a long but very cold summer, i. e., an approach to 
 perpetual spring, while the Southern Hemisphere would 
 be inconvenienced, and might be rendered uninhabit- 
 able, by the fierce extremes caused by concentrating 
 half the annual supply of heat into a summer of very 
 short duration, and spreading the other half over a long 
 and dreary winter, sharpened to an intolerable intensity 
 of frost when at its climax by the much greater remote- 
 ness of the sun." 
 
 I have italicized certain words in this passage which 
 have unfortunately been the means of diffusing an en- 
 tirely erroneous impression. Herschel has always been 
 understood in this passage to say that at a time of 
 maximum eccentricity, when the difference between 
 the lengths of the seasons has attained a considerable 
 value, there would be a difference in the climate, be- 
 cause 50 per cent, of the total heat of the hemisphere 
 was poured in during the short season, while the re- 
 maining 50 had to be eked out over the long season. 
 Indeed, his words seem to admit of no other interpreta- 
 tion. It is somewhat remarkable that Herschel has 
 made a considerable error in this critical subject. It 
 must be remembered that as the point in question is 
 purely a mathematical one, there can be no room for 
 doubt. He states that the heat is equally divided be- 
 tween the two seasons, and he considers that even in 
 this case the difference between their lengths would 
 necessitate a profound climatic disturbance. If, how- 
 ever, we employ the true law of the distribution of heat 
 9 
 
118 THE CAUSE OF AN ICE AGE. 
 
 between the seasons we can correct the inaccuracy, and 
 write the passage as, of course, Herschel would have 
 done had his attention been sufficiently given to the 
 matter. Every mathematician who will study the Ap- 
 pendix to this book will agree with me that the latter 
 part of Herschel's paragraph should have read as fol- 
 lows: 
 
 " In the Northern we should have a short but very 
 mild winter with a long but very cool summer, i. e., 
 an approach to perpetual spring; while ths Southern 
 Hemisphere would be inconvenienced and might be 
 rendered uninhabitable by the fierce extremes caused by 
 concentrating sixty -three per cent, of the annual supply 
 of heat into a summer of very short duration, and 
 spreading the remaining thirty-seven per cent, over a 
 long and dreary winter sharpened to an intolerable in- 
 tensity of frost when at its climax by the much greater 
 remoteness of the sun." 
 
 It thus appears that the accurate law of the dis- 
 tribution of heat had escaped the attention of Herschel ; 
 it seemed, therefore, not impossible that it might also 
 have been missed by other writers. Indeed, on such a 
 matter it was not unnatural that the words of Herschel 
 should have been adopted without any very critical 
 scrutiny. I examined Croll's books carefully on this 
 point. I need hardly repeat here the extent of my in- 
 debtedness to Croll, and how cordially I add my testi- 
 mony to the value of his work. I must, however, say 
 that after close study of his writings I have been obliged 
 
WHY THIS BOOK HAS BEEN WRITTEN. H9 
 
 to come to the conclusion that he also has not made use 
 of the true law of distribution of heat between the sea- 
 sons. So far as I can gather from the various allusions 
 to the astronomical elements of the problem in his writ- 
 ings, it would seem as if he entertained the same view 
 as that which is incorrectly set down in The Outlines of 
 Astronomy. I have at all events failed to find in his 
 most famous book, Climate and Time, or in his later 
 work, Climate and Cosmology, any use made of the law 
 that under all circumstances and on either hemisphere 
 63 per cent, of the total heat of the year is received dur- 
 ing summer, so that only the remaining 37 per cent, is 
 left for winter. 
 
 And yet this law is essential to the right under- 
 standing of the subject. It is impossible to form a just 
 appreciation of the connection between the eccentricity 
 of the earth's orbit and the climatic phenomena of the 
 globe unless under the guidance of the principle which 
 underlies the law we have so often mentioned ; it must 
 be constantly borne in mind, and regarded as an essen- 
 tial element which astronomy contributes to the prob- 
 lem. 
 
 It always seems to me that Dr. Croll would have been 
 enabled to do his theory much more justice if he had 
 been in possession of this law. It would surely have 
 made a material difference in his reasoning if he had 
 been aware that the total heat received in the course of 
 a summer, whether long or short, was nearly twice as 
 much as the total heat received in any winter, whether 
 
120 THE CAUSE OF AN ICE AGE. 
 
 short or long. In fact, Dr. Croll's task would have 
 been greatly facilitated, for the agencies at his disposal 
 would have been of greatly increased efficiency. I felt 
 desirous to see what the Astronomical theory of the Ice 
 Age would look like when the true law of the distri- 
 bution of heat between the seasons was employed. I 
 found that the evidence of the origin of the Ice Age 
 from astronomical causes was greatly strengthened, 
 and I have written this little book in order to have an 
 opportunity of expounding the matter. 
 
 As I have been obliged to point out the necessity for 
 a correction in a passage in the Outlines of Astronomy, 
 I will take the opportunity of suggesting how it prob- 
 ably happened that a statement which has been the 
 source of so much misconception came to be made. It 
 is certainly true that during the summer in one hemi- 
 sphere the heat received on the whole earth is equal to 
 the heat received on the whole earth during the ensuing 
 winter on the same hemisphere It seems likely that 
 the assertion in the Outlines of Astronomy was some- 
 what hastily set down in the belief that it was merely 
 another way of expressing the remarkable proposition 
 just given. A little closer reflection will, however, show 
 that the two statements are not at all equivalent. 
 
 The doctrine at which we have arrived is so impor- 
 tant to the theory of the Ice Age, that I feel tempted to 
 suggest it as a very ready test to see whether any one 
 who professes to discuss the subject has correctly per- 
 ceived the astronomical bearing of the case. The es- 
 
WHY THIS BOOK HAS BEEN WRITTEN. 121 
 
 sence of the subject is mainly comprised in two fixed 
 quantities, represented by the numbers 63 and 37 re- 
 spectively. No one can understand the Astronomical 
 theory of the Ice Age without at once recognizing their 
 significance. They denote the percentages of the total 
 heat of the year received on one hemisphere during 
 summer and winter respectively ; they derive their im- 
 portance from their constancy. They would remain 
 the same however the dimensions of the orbit be altered, 
 however its eccentricity be altered, or in whatever direc- 
 tion the plane of the earth's equator may intersect the 
 plane of the earth's revolution around the sun. These 
 numbers are both functions of but a single element, 
 which is the obliquity of the ecliptic. As this fluctuates 
 but little, at least within the periods that are required 
 for recent Ice Ages, the numbers we have given are re- 
 garded as sensibly constant throughout every phase 
 through which the earth's orbit has passed within gla- 
 cial times. The effect of eccentricity, though powerless 
 so far as the quantities expressed by these numbers are 
 concerned, is shown by the alterations in the lengths of 
 the seasons. We may have a short summer and a long 
 winter, or a short winter and a long summer. Be the 
 summer, however, long or short on the Northern Hemi- 
 sphere, that hemisphere will invariably receive 63 per 
 cent, of its total sun heat for the year during the sum- 
 mer, and 37 per cent, during the winter. Obviously the 
 most beneficial arrangement will be that of a long sum- 
 mer and a short winter, for the extremes of climate will 
 
122 THE CAUSE OF AN ICE AGE, 
 
 be thus moderated. The greatest difference in the 
 lengths of the seasons will be when one is 199 days and 
 the other 166 days ; one of these is to be the summer 
 and the other to be the winter. 
 
 Let us suppose that a gentleman whom we shall call 
 A has a salary of 100 a year, which is paid in two in- 
 stallments of 63 a -:d 37 respectively. One of these 
 sums is understood to be payment for the summer por- 
 tion of the year, leaving the other for the winter. Let 
 us now make a calculation to see how he will fare under 
 varied circumstances. We may first suppose that there 
 is the longest possible summer and the shortest possible 
 winter. The summer lasts for 199 days, during which 
 a sum of 63 furnishes to him the means of support. 
 His daily allowance during this season would of course 
 be found by dividing 199 into 63, by which we ob- 
 tain 6s. 4:d. In the following winter he receives only 
 37; however, this has not to last more than 166 days, 
 so that his average receipt per diem would be 4s. 5-j-d. 
 The difference between the mean daily summer income 
 and the mean daily winter income is, therefore, not 
 more than Is. 10-Je?. Under these circumstances A en- 
 joys a fairly beneficent arrangement. To take another 
 case, let us suppose that the summer is the short season, 
 leaving the winter to be the long one. A is then lavish- 
 ly provided for during the brief summer of 166 days, 
 in which he has to spend 63. His daily average would 
 then be 7s. Id. This extravagance will leave him but a 
 very scanty allowance during the winter. He has in 
 
WHY THIS BOOK HAS BEEN WRITTEN. 123 
 
 that long and dreary season to eke out 37 for 199 
 days, during which his daily allowance will be but 3s. 
 8$d. His condition is a very unhappy one ; he is in a 
 glacial epoch. Let us now write our account of this 
 gentleman's affairs in a tabular form : 
 
 Summer Winter 
 
 A's daily income . . 6s. 4d. 4s. 5%d. genial 
 " " . . 78. ?d. 3s. 8%d. glacial 
 
 It needs only a glance at these figures to show that 
 there will be a considerable difference in the comforts 
 enjoyed in the two cases. The most appropriate method 
 for ministering to A's daily wants would obviously be 
 that by which the income may be equally distributed 
 over the year ; and if perfect uniformity can not be ob- 
 tained, then the nearer approximation to equality the 
 better. Under the condition which we have here 
 marked as genial, no doubt A enjoys more in summer 
 than he has in winter, but the difference between the 
 daily allowance in the two seasons is far greater in the 
 second case than in the first. In fact, he can only de- 
 vote 3s. 8%d. to his malntainence per diem during the 
 long and severe winter, which is less than half that 
 which he receives during the summer 7s. Id. Now, 
 to a certain extent this will illustrate the condition of 
 our earth as regards the daily receipt of heat from the 
 sun. When the eccentricity is at its greatest, and when 
 the inequality of the seasons is at a maximum, then the 
 Northern and the Southern Hemispheres will each be 
 represented by one of the two cases just referred to. In 
 
124: THE CAUSE OF AN ICE AGE. 
 
 the Northern Hemisphere, for instance, the daily receipt 
 of heat during summer may not be half as much again as 
 the daily heat of winter, and then the genial conditions 
 prevail. In the Southern Hemisphere, at the same 
 time, the average daily receipt of heat in summer will 
 be more than double that in winter, and the glacial con- 
 ditions will prevail. Perhaps the illustration of an in- 
 come of 100 a year hardly conveys an adequate con- 
 ception of the disparity between the glacial and genial 
 conditions. By taking a somewhat smaller sum we may 
 easily conceive a case in which, while the same propor- 
 tions were observed, the lowest sum on which life could 
 be supported should lie between the winter allowance in 
 genial and glacial times respectively. Under such cir- 
 cumstances, while the income would be sufficient for 
 mere maintenance so long as the genial division of sea- 
 sons was observed, the same income would prove insuffi- 
 cient for the maintenance of life when the seasons were 
 divided, as in a glacial epoch. A prudent person would, 
 however, have saved up a little of the abundance of 
 summer to help him over the winter. Something of 
 this kind does happen on our globe in the case we are 
 trying to illustrate ; the waters on the earth store up a 
 portion of the redundant supplies of heat in summer, 
 and employ it to mitigate the rigors of the ensuing 
 winter. 
 
 There is, however, another way in which the horrors 
 of an Ice Age are to some extent lessened. At the time 
 when one ^hemisphere is glaciated the opposite hemi- 
 
WHY THIS BOOK HAS BEEN WRITTEN. 125 
 
 sphere is in a genial condition, and by the agency of 
 winds and of ocean currents the abundant warmth of 
 the fortunate hemisphere is conveyed to the succor of 
 the glaciated districts. Even the very snowfalls them- 
 selves, in so far as they arise from vapor conveyed from 
 the genial hemisphere, are vehicles for the conveyance 
 of vast quantities of latent heat, which is liberated on 
 the congelation of the vapor. There can not be a 
 doubt, if the two hemispheres were completely isolated 
 by an impassable barrier around the Equator, so that no 
 winds could cross from one hemisphere to the other, or 
 no waters traverse the dividing line, that then the dis- 
 tinction between a glacial epoch and a genial epoch 
 would be much more strongly accentuated. Suppose 
 that a trough is divided into two parts by a movable 
 partition, if one of the parts be filled with hot water 
 and the other with cold water it is obvious that if the 
 partition be withdrawn the two waters will speedily 
 blend, the lighter hot water will flow over the adjacent 
 cold water, and the cold water will penetrate beneath 
 the hot water, and an equilibrium of temperature will 
 be more speedily attained. Probably a similar inter- 
 change of heated water for cooled water takes place on 
 an enormous scale at the Equator during the time when 
 one hemisphere is glaciated and the other is in a genial 
 state. During the glacial winter the daily receipt of 
 heat is only -68, while in the genial summer enjoyed 
 simultaneously on the opposite hemisphere the daily 
 receipt of heat is 1-16. The difference between them 
 
126 THE CAUSE OF AN ICE AGE. 
 
 is -48, and we must always remember that the unit in 
 which these quantities are expressed is a very large one, 
 sufficiently large, indeed, to express the daily supply of 
 heat that must be communicated to the earth to pre- 
 vent it from sinking down over 300 degrees in tempera- 
 ture. A tremendous export of heat must take place all 
 through the summer from the genial hemisphere, thus 
 alleviating in some degree the wants of the long winter 
 in the glaciated hemisphere. 
 
 To return to our illustration, let us suppose that the 
 gentleman A has a brother B in the Southern Hemi- 
 sphere who receives the same salary, and whose emolu- 
 ments are payable to him in the same manner, viz., a 
 sum of 63 for the summer, and of 37 during the 
 winter. At a time when the difference between the 
 lengths of the seasons is a maximum, one of the hemi- 
 spheres is glacial and the other is genial. ISTo doubt 
 the most beneficial arrangement would be one by which 
 A would receive aid from B during the dreary glacial 
 winter, and B will be able to afford this because he has 
 6s. 4:d. per diem, while A will sadly want help, having 
 only 35. S^d. Thus, by contributions from the opposite 
 hemisphere the sufferings of glaciation may be some- 
 what lessened. Of course, when the next great trans- 
 formation has taken place, by which the glaciated hemi- 
 sphere and the genial hemisphere change places, it will 
 be A's turn to afford succor to B. 
 
 Thus we learn that there are two causes by which 
 the severity of a glaciation is somewhat modified. 
 
WHY THIS BOOK HAS BEEN WRITTEN. 127 
 
 There is, first, the actual storage of some of the copious 
 heat of summer in the glaciated hemisphere itself, to be 
 doled out again during winter ; there is, secondly, the 
 contributions of heat from the opposite hemisphere, 
 which may be conveyed via air or via water across the 
 Equator into the glaciated regions. 
 
 I now proceed to explain the way in which we are 
 able to form some conception of the actual quantitative 
 effect on climate which may be expected to arise from 
 the fluctuations in the eccentricity of the earth's orbit. 
 The subject presents many difficulties, and I naturally 
 feel some diffidence in discussing it. I think, however, 
 that the views which I shall here give seem hardly open 
 to much exception. At all events, I do not think they 
 can be said to err on the side of attributing too great a 
 significance to the climatic consequences of increased 
 eccentricity. 
 
 Let us begin by considering the present state of our 
 globe. We shall speak more particularly of the North- 
 ern Hemisphere, because the facts with reference to it 
 are, of course, more accurately known than are those of 
 the Southern Hemisphere. Under existing circum- 
 stances the lengths of the two seasons are respectively a 
 summer of 186 days and a winter of 179 days. At pres- 
 ent, as at all other times, 63 per cent, of the total 
 annual heat on the hemisphere is imparted during the 
 summer, and but 37 per cent, is left for the winter ; 
 there is, therefore, a wide difference between the daily 
 receipt of heat during the two seasons. If, as before, 
 
128 THE CAUSE OF AN ICE AGE. 
 
 we denote by unity the average amount per hemisphere, 
 then that during summer will be 1-24, and during win- 
 ter 0-75. 
 
 It is to this inequality that we are to attribute the 
 familiar difference between the mean temperature of 
 summer and that of winter, and our first impression 
 may well be one of surprise when we notice that the 
 contrast in warmth between the two seasons is not a 
 much larger one than our experience has shown it to be. 
 It will be observed that the difference between the fig- 
 ures we have just given amounts to 0-49, that is, almost 
 to half the unit of heat which we have chosen. Re- 
 membering what a considerable thermal magnitude we 
 have chosen as the unit of heat we might at first expect 
 that the difference between the temperature of summer 
 and the temperature of winter should be much larger 
 than it is. For the unit of heat is that which on the 
 average sustains the earth at the temperature which it 
 has above the temperature of space. Since, then, the 
 unit of heat is competent to sustain a temperature 
 which is to be measured by some hundreds of degrees, 
 it might not unreasonably be expected that, when half 
 a unit a day is received in summer above the daily 
 allowance for winter, the range of temperature between 
 a summer's day and a winter's day should also require 
 to be expressed by three figures. 
 
 Probably this would be the case were it not for the 
 equalizing tendency of the agents to which we have 
 already referred. These agents exercise their beneficent 
 
WHY THIS BOOK HAS BEEN WRITTEN. 129 
 
 influence with especial emphasis on the British Islands. 
 The consequence is that on our Western shores the 
 mean difference between the temperature of the sum- 
 mer and the temperature of winter is about 20. In 
 the interiors of the great continents the climatic effects 
 of the ocean are experienced less, and consequently the 
 range of temperature between summer and winter rises. 
 In the middle of North America, at the same latitudes 
 as the British Islands, the range between summer and 
 winter is about 60. In Eastern Asia, still in the same 
 latitudes, the range rises to 80, while a little to the 
 north, in Siberia, the range rises to 100. Thus we see 
 that, when the influences of mitigating agents are suffi- 
 ciently lessened, the range between summer and winter 
 attains a magnitude approaching to the theoretical re- 
 sult that might be expected from the fact that the daily 
 receipt of heat was almost half a unit less in winter 
 than in summer. 
 
 Mr. Scott, in his well-known Elementary Meteorology, 
 tells us that at Yakutsk (Siberia) the temperature in 
 the hottest month is 65-8, while it sinks to 44-9 below 
 zero in the coldest. Here the range is over 100 ; but 
 it will of course be observed that these figures refer, not 
 to the mean . temperature of summer and winter, but to 
 the maximum and minimum temperatures respectively. 
 
 Let me here specially ask the reader's attention to 
 the line of argument that is to be followed. I shall 
 take the case of the British Islands, and endeavor, so 
 far as may be, to form some estimate of the extent of 
 
130 THE CAUSE OF AN ICE AGE. 
 
 the climatic changes that could be there induced by 
 such a combination of astronomical conditions as gives 
 rise to an Ice Age. 
 
 At the present moment the difference between the 
 daily receipt of heat during the summer and during the 
 winter is on the average -49 of the unit. It is this 
 difference which is the cause of the disparity between 
 summer and winter temperatures. At present this dis- 
 parity is in the British Islands at so low a value as 20. 
 The mitigating causes act so vigorously in Great Brit- 
 ain, owing to the peculiar circumstances of its geo- 
 graphical position, that -49 of the unit only produces 
 the trifling effect of 20; nothing can, however, be 
 more certain than that these two quantities stand in 
 such a relation to each other that, if one of them be 
 altered, the other will also alter, or, as the mathema- 
 tician would say, the mean range of temperature must 
 be a function of the mean difference between the sum- 
 mer and winter daily receipts of heat. To avoid misap- 
 prehension, I should and that of course there are many 
 constant elements necessarily involved, which depend 
 on the locality. 
 
 At the time of high eccentricity, when the summer 
 only lasted for 166 days and the winter for 199, the 
 daily unit of heat received during the hot and brief 
 summer is 1*38, while the daily average heat received 
 through the long and dreary winter is only -68. The 
 difference between the two seasons is thus on the aver- 
 age -70 of a unit. By the method of treatment that we 
 
WHY THIS BOOK HAS BEEN WRITTEN. 131 
 
 are now adopting, we avoid any embarrassing considera- 
 tions as to the absolute temperatures. It is sufficient for 
 us to note that in each locality the excess of the mean 
 summer temperature over the mean winter temperature 
 may not unreasonably be taken to be proportional to 
 the excess of the daily heat received in summer above 
 the daily heat received in winter. If this excess be 
 altered in any ratio, the range of temperature will be 
 increased in a ratio which is about the same. We have 
 seen that certain ranges are appropriate to a heat differ- 
 ence of -49. If the heat difference become -70, the 
 range may be expected to be increased in a similar pro- 
 portion ; in other words, the localities which now show 
 a range of 20 will show a range of 28. In the same 
 case the countries which have at present a mean range 
 between summer and winter of 40 would acquire a 
 range of 57 in a glacial period, while in the bleak 
 regions which now have a range of 60 we should find a 
 range of 85 during the prevalence of glaciation. 
 
 These changes are quite large enough to imply pro- 
 found differences in the climatic condition. It is to be 
 observed that, generally speaking, the coldest places are 
 those of the greatest mean annual range. We are there- 
 fore entitled to infer that the effect of such a change in 
 the eccentricity as we have supposed, would be to in- 
 crease the range, lower the temperature of the hemi- 
 sphere, and thus induce the glacial period. 
 
 The contrast between the conditions of the genial 
 hemisphere and of the glaciated hemisphere can also be 
 
132 THE CAUSE OF AN ICE AGE. 
 
 estimated with some precision by the same line of rea- 
 soning. If at a time of maximum eccentricity the 
 summer has the greatest possible duration and the win- 
 ter the shortest, the daily average receipt of heat in the 
 summer is 1-16, and the daily average for winter is '81. 
 The difference between these quantities is -35. If there- 
 fore a difference in the daily receipt of heat amounting 
 to -49 corresponds to a range of temperature of 20, it 
 ought to follow that in the genial period the range 
 should be reduced in the ratio of -35 to -49 that is to 
 say, the range of 20 is reduced to 14. We are thus 
 entitled to infer that during a Genial Age the mean 
 range in the British Islands would be 14, while during 
 a Glacial Age the mean range would be 28. 
 
 Of course we may obtain similar results from other 
 ranges. Suppose, for example, a country which has at 
 present a range of 60 between its mean summer and 
 mean winter temperature. This is produced, of course, 
 by the fact that the daily average receipt of heat in 
 summer is -49 of a unit greater than in winter. This 
 fact modified by local circumstances causes in the region 
 under consideration a range of 60. But now suppose 
 that the quantity which is at present *49 were raised, as 
 it would be in a glacial period, to -70, and that the other 
 circumstances affecting the question (being mainly geo- 
 graphical) remain substantially the same : we should 
 then be entitled to assume that the range between the 
 mean temperature of summer and the mean tempera- 
 ture of winter would be raised in the same proportion 
 
WHY THIS BOOK HAS BEEN WRITTEN. 133 
 
 that is, from 60 to 85. On the other hand, during 
 a genial period, when the quantity which is now -49 
 drops to -35, there is a proportional fall in the range 
 from 60 to 43. 
 
 This calculation brings out the significant fact that 
 in a genial period the range in any locality would seem 
 to be merely one half of what it was when the glaciation 
 reigned in that hemisphere. Of course it will be under- 
 stood that the presence of an ice-sheet must largely 
 affect any such statements. If we remember that when 
 one hemisphere is glaciated the other is in a genial state, 
 the contrast between the two seasons is emphasized in a 
 very marked way. It may also be noticed that we gen- 
 erally find the mean annual temperature is high iii 
 places where the range of temperature between summer 
 and winter is small. A great range is generally a con- 
 comitant of a low mean annual temperature. 
 
 I have been unable to perceive the validity of the 
 arguments by which Dr. Croll strives to show that a 
 great ocean current like the Gulf Stream must be actu- 
 ally diverted from one hemisphere to the other, as a con- 
 sequence of the transference of the glaciation from one 
 hemisphere to the opposite. I can not think this is 
 likety, nor does it seem necessary for the explanation of 
 the Ice Age. Once the true significance of the astrono- 
 mical facts has been realized, it is apparent that, leaving 
 the other agents much as they are at present, the altera- 
 tion in the range of temperature will be amply sufficient 
 
 for even extreme changes in climate. 
 10 
 
134: THE CAUSE OF AN ICE AGE. 
 
 At the present time the range in the Southern Hem- 
 isphere between the mean summer temperature and the 
 mean winter temperature appears to be less than in the 
 Northern Hemisphere. It must be admitted that at the 
 first glance this is a little disappointing in view of the 
 theory now put forward. As the seasons are at this 
 time 179 days and 186 days, and as in the Southern 
 Hemisphere the winter lasts for the longer of these 
 periods and the summer for the shorter, it might seem 
 that at present there should be to a certain extent an 
 approach to a glacial period in the Southern Hemisphere. 
 We should accordingly have expected that the ranges 
 there were greater than the ranges in corresponding 
 latitudes in the Northern Hemisphere. The opposite, 
 howerer, is the case, as is pointed out in Scott's Element- 
 ary Meteorology to which I have already referred. No 
 doubt the explanation of the discrepancy is to be found 
 in the far greater extent of ocean-covered surface in the 
 Southern Hemisphere than in the Northern. The sun- 
 beams which fall on the sea are retained sufficiently 
 long to enable the redundant heat of summer to con- 
 tribute effectively to the amelioration of the winter. 
 
 I hope the position that I have here taken with re- 
 spect to Dr. CrolPs views may not be misunderstood. 
 Like him, I thoroughly believe in the Astronomical 
 theory, but it has a far greater claim on our belief than 
 that which previous writers seem to have imagined ; in 
 fact, Dr. Croll seems not to have been really aware of 
 the full strength of the argument from astronomy. He 
 
WHY THIS BOOK HAS BEEN WRITTEN. 135 
 
 appears, in common with many others, to have been in- 
 fluenced by Herschel's unfortunate inadvertence in the 
 statement quoted above. Even on the supposition the 
 erroneous supposition as we now know it to be, of equal 
 heats in the two seasons on the same hemisphere, it was 
 considered by the advocates of the Astronomical theory 
 that the inequality in duration of the two seasons would 
 be sufficient, directly or indirectly, to originate some 
 climatic changes. However, we now know that the 
 total heat in one of the seasons is nearly double that in 
 the other ; and this being so, see what a powerful lever 
 is afforded for the production of climatic effects. If the 
 long season be the summer, and the short season the 
 winter, then the double supply of heat is distributed 
 over the long period, and the single supply over the 
 short one. This has a beneficent effect on climate, and 
 a Genial Age is the consequence. If, on the other hand, 
 the double supply of heat be poured in like a torrent 
 during the short season, while the single supply of heat 
 is constrained to do duty over the long season, then an 
 intolerable climate is the result. The total quantity of 
 heat received on the hemisphere in the course of a year 
 is no doubt the same in each case, but its unsuitable 
 distribution bespeaks a climate of appalling severity 
 an Ice Age, in fact. 
 
 There is another point of view from which it will be 
 seen that Dr. Croll's theory is much stronger when ex- 
 hibited in the amended form rendered necessary by the 
 law of distribution of heat between the two seasons. 
 
136 THE CAUSE OF AN ICE AGE. 
 
 If the heat in summer and the heat in winter were 
 equal, then there would be only the inequality in the 
 lengths of the seasons to be taken into account in esti- 
 mating the agencies of glaciation. On the supposition 
 that the receipts of heat during the two seasons were 
 equal, we find that at a time of glaciation the daily in- 
 flux of heat during the long and cold winter would be 
 92, while the daily receipt of heat during the shorter 
 glacial summer would be about 1-10. In making these 
 calculations we have assumed that the heat was uni- 
 formly received throughout the summer, and uniformly 
 received throughout the winter. According to this 
 view the climate is glacial when the summer is short 
 and the winter is long, and genial when the summer is 
 long and the winter short. In any case, however, the 
 greatest contrast between the mean daily influx of heat 
 in summer and the mean daily influx of heat in winter 
 are to be represented by the figures TIO and -92 respect- 
 ively. The difference between these two is *18 of our 
 heat unit. This shows in a striking manner what a 
 wholly inadequate presentation the Astronomical theory 
 received, when expounded, under the belief that the 
 total heat of summer was equal to the total heat of win- 
 ter. Were this the case, the utmost difference between 
 an average summer day's heat and an average winter 
 day's heat could be only -18 of a unit, while, as we have 
 already seen, when accurate data are employed, the 
 maximum difference between an average summer day's 
 heat and an average winter day's heat may amount 
 
WHY THIS BOOK HAS BEEN WRITTEN. 137 
 
 to -70. The difference between these two numbers 
 amounts to more than half our unit of heat, and when 
 we remember what that unit of heat implies, it shows 
 how necessary it is that the facts should be properly 
 stated. I venture to commend the points raised in this 
 chapter to the careful consideration of the numerous 
 workers at the fascinating subject of the monuments 
 left by ancient ice. 
 
CHAPTER VII. 
 
 HOW THE ASTRONOMICAL THEORY IS CORROBORATED. 
 
 IT is of the essence of the Astronomical theory that 
 a glacial epoch in one hemisphere shall be accompanied 
 by a genial epoch in the other, and that after certain 
 thousands of years the climatic conditions of the two 
 hemispheres shall become interchanged that the ice 
 shall leave the hemisphere desolated, and fly to the 
 other, while the regions it has abandoned shall become 
 clothed with luxuriant vegetation characteristic of a 
 genial epoch. Such fluctuations seem to have occurred 
 again and again ; in fact, that they do so is a necessary 
 consequence of the Astronomical theory of the Ice Age. 
 The oscillations of glaciation from one hemisphere of 
 the earth to the other is a phenomenon of the most 
 singular interest, and it is natural that we should de- 
 sire to study it from every point of view. The Astro- 
 nomical theory does not give us many of the details 
 which we should like to know. It tells us, however, 
 with reasonable approximation the periodic time of 
 these changes. From the time of the climax of an Ice 
 Age in one hemisphere to the time when, having left 
 that hemisphere, it has become established in the oppo- 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 139 
 
 site one, has abandoned it, has returned to its original 
 home and has again regained its climax, the interval is 
 about 21,000 years. This period is not affected with 
 the uncertainty that attaches to certain other attempts 
 at the chronology of the Ice Ages. It depends on two 
 astronomical constants, and of these the first is the pe- 
 riod of the precession of the equinoxes. If the earth's 
 orbit retained a constant position in its plane, then we 
 could assert, with every confidence, that the interval 
 from one phase of an Ice Age to the return of the same 
 phase in the same hemisphere would be the period of 
 25,694 years. However, the earth's orbit does not al- 
 ways remain in the same situation, for the attractions 
 of the planets cause the elements of the orbit of the 
 earth to be more or less variable. The position of the 
 axis of the earth's orbit forms no exception to this 
 law ; it has a slow motion in the plane of the orbit, 
 which can be determined with sufficient accuracy. Its 
 effects become blended with those of the precession of 
 the equinoxes, thus producing a net movement of the 
 axis relatively to the equinoxes of 1 42-6' every century. 
 Thus we obtain for the period of the changes a term of 
 about 21,000 years. It should be distinctly understood 
 that neither in this place nor in any other do I make 
 any attempt to estimate either the date of the last gla- 
 cial epoch that desolated the earth, or the date at which 
 the next may be expected. It is true that such esti- 
 mates have been formed, but they depend upon formulae 
 which can hardly be relied on for such an extreme ap- 
 
140 THE CAUSE OF AN ICE AGE. 
 
 plication as is demanded of them. What I now desire 
 to convey is, that when, after the lapse of gigantic pe- 
 riods, a series of conditions suitable to glaciation have 
 supervened, then, for so long as those conditions remain 
 fulfilled, the ice-cap oscillates between one hemisphere 
 and the other with the interval of 21,000 years that we 
 have just determined. 
 
 To Dr. Croll must be ascribed the credit of having 
 pointed out that the remarkable succession of Ice Ages 
 which we have here indicated was a necessary conse- 
 quence of the Astronomical theory of their origin. At 
 first it was regarded as a little startling that a demand 
 should be made not for one Ice Age nor for two, but for 
 such a series, or rather for such a series of clusters of 
 associated Ice Ages as those which the Astronomical 
 theory seemed to require. It was also regarded as some- 
 what startling that the marvelous oscillation of glacia- 
 tion from hemisphere to hemisphere should have taken 
 place, as it most assuredly must have done, from the 
 consequences of the relations of the earth to other bodies 
 in the solar system. It was therefore natural that an 
 appeal should be made to the facts of geology to see 
 whether they could offer any confirmation of phenomena 
 so extraordinary. Geology affords us distinct traces of 
 the repetition of glacial phenomena, but it could hardly 
 be expected that the facts of that science could throw 
 much direct light on chronological matters as to the 
 order of succession of Ice Ages in one hemisphere and 
 the other. We might, no doubt, find indeed we actu- 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 
 
 ally do find traces of an Ice Age in the Southern 
 Hemisphere just as we find them in the Northern ; but 
 though the rocks have been engraved with unmistak- 
 able characters by glaciers and ice-sheets, yet these in- 
 scriptions do not tell quite so much as we should like to 
 know. They assure us, no doubt, that Ice Ages have 
 occurred in both hemispheres, but they leave us unin- 
 formed as to whether these Ice Ages were consecutive, 
 or as to whether they were concurrent. Now this is not 
 a mere matter of ordinary significance, as it involves an 
 absolutely vital point in the Astronomical theory of the 
 Ice Age. So much so is this the case, that if it could be 
 shown that the Ice Ages in the two hemispheres were 
 concurrent, the Astronomical doctrine would have to be 
 forthwith abandoned. Of course, there is no chance 
 of such a contingency arising, and I merely enunci- 
 ate it to show the significance of the doctrine, that the 
 Ice Ages in the opposite hemispheres were not concur- 
 rent but were consecutive ; in fact, we may feel con- 
 vinced on astronomical grounds alone that this must 
 have been the actual state of things. 
 
 No matter how confident we may feel as regards an 
 important result arrived at by one line of reasoning, it 
 must necessarily be of interest to note the confirmation 
 of that fact by reasoning of a wholly different character ; 
 especially would this be the case when the evidence in 
 the second case was elicited from a distinct branch of 
 science. There is no more interesting illustration of 
 the principle just mentioned than the fact that botany 
 
142 THE CAUSE OF AN ICE AGE. 
 
 affords a striking confirmation of the Astronomical the- 
 ory of the Ice Age. 
 
 When the tourist climbs the Alps in early summer 
 he finds that a charming flora claims a share of that 
 admiration which he so freely bestows on the glorious 
 objects around him. He is struck by the contrast be- 
 tween the flowers on the mountains and the flowers 
 that bloom in the valleys beneath. Nor is it only in 
 the Alps that the peculiar characteristics of a mount- 
 ain flora will arrest the attention of the tourist. The 
 flora on the summits of the Pyrenees and on all other 
 great mountains are of special types, differing in im- 
 portant respects from the plants in the adjacent low- 
 lands. 
 
 In our own islands the summits of lofty mountains 
 offer the conditions congenial to remarkable plants that 
 find no habitat on the plains. In the New World, also, 
 the summits of mountains nourish plants differing 
 widely from those that grow at their feet. Indeed, a 
 similar statement may be made about every continent. 
 The labors of botanists have demonstrated that the 
 vegetation on the summits of lofty mountains has 
 often certain features in common, even though those 
 mountains may be situated in different continents and 
 separated by thousands of miles of intervening ocean. 
 In fact, it is usual to employ the common term Alpine 
 to indicate the vegetation characteristic of mountain 
 summits. There are some interesting pages on this 
 subject in the Origin of Species. I shall here endeavor 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 143 
 
 to summarize, from what Darwin says, the facts which 
 bear on the subject of this little volume. 
 
 It is surely remarkable to find that the summits 
 both of the Alps and of the Pyrenees are the abode of 
 plants which also thrive in the bleak and dreary ex- 
 treme North of Europe. In the intervening low coun- 
 tries these plants do not occur. How does it come to 
 pass, that where the conditions congenial to these plants 
 are found, there those plants grow, notwithstanding 
 that they are separated by hundreds of miles from the 
 similar flora in other regions? No one will believe that 
 they were independently created in many widely sepa- 
 rated localities; and therefore some other explanation 
 must be sought. 
 
 There are kindred facts of a still more striking 
 character. The plants which thrive on the summits of 
 the White Mountains in the United States are the same 
 as those in Labrador, and we have the authority of Asa 
 Gray for the assertion that such plants are also closely 
 related to others on the loftiest mountains of Europe. 
 
 This remarkable phenomenon in the distribution of 
 plants can be satisfactorily accounted for only as a con- 
 sequence of the existence of the Ice Age. To illustrate 
 the argument, it will be convenient to follow the 
 method of Darwin, and trace some of the consequences 
 of the advent of an Ice Age, and then its gradual disap- 
 pearance. 
 
 In the normal condition of the Northern Hemi- 
 sphere, the characteristic but scanty Arctic flora lends 
 
144 THE CAUSE OF AN ICE AGE. 
 
 what verdure it may to the solitudes in the neighbor- 
 hood of the Pole. The severity of the climate, which 
 would prove fatal to the plants adapted to an abode in 
 Southern and more genial climes, is necessary to the 
 well-being of these Arctic plants. At present the tem- 
 perate lowlands of the earth are uninhabitable by such 
 organisms; they would perish there just as the plants 
 of our temperate regions would at the North Pole. 
 With the advent of a glacial period the temperate re- 
 gions of the globe undergo such a mutation of climate, 
 that the inhabitants, both animal and vegetable, which 
 now thrive there would gradually be driven to southern 
 regions in search of the warmth which was indispensable 
 to their existence. The same climatic changes would, 
 however, offer an inducement to the Arctic plants to 
 visit those regions at present temperate, but which, by 
 the change of climate, became adapted to a cold-loving 
 flora. The vegetation which, under normal circum- 
 stances, grew on the summits of mountains in the tem- 
 perate regions would, with the approach of a glacial 
 period, descend the mountain sides and commingle in 
 the plains with the productions of the Arctic regions 
 which had migrated southward. It is also to be noticed 
 that the present Arctic flora appears to be of a similar 
 character all round the Pole. Consequently, as the 
 Arctic plants traveled southward in every direction 
 during the centuries of a glacial period, they would 
 convey to both the Old World and to the New a vege- 
 tation of the same character. Thus we should find, 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 
 
 during a glacial epoch, the lowlands of the temperate 
 regions all round the hemisphere inhabited by plants 
 possessing the characteristics of the Arctic flora at the 
 present day. 
 
 Let us now suppose that the long period of glaciation 
 is drawing to a close, and that more genial conditions 
 begin to prevail. The Arctic flora becomes uncomfort- 
 able, and at length has to fly from a climate too kindly 
 to suit it. But whither are the plants to go ? Natu- 
 rally the main line of retreat will lie toward the Pole, 
 where the conditions they desire will be afforded them. 
 But there will also be another means of escape. From 
 the warmth of the lowlands the plants can retreat up 
 the slopes of the mountains until at the summits they 
 find those conditions to which they are adapted. The 
 lowlands thus deserted by the Arctic flora, and with a 
 hospitable climate restored, will again welcome back the 
 ordinary temperate plants which had fled to the South 
 at the beginning of the glacial epoch. These plants re- 
 sume their old homes, and the earth regains its normal 
 appearance, while on the summits of the lofty mount- 
 ains vestiges of the Arctic flora continue to flourish. 
 
 It is thus shown that the phenomena of distribution 
 of Alpine plants have corroborated the general argu- 
 ments by which the former existence of a glacial period 
 has been established. It is true that we scarcely wanted 
 corroboration as to this particular, the records of the 
 rocks and the presence of the bowlder clay and erratic 
 bowlders supplied evidence too indisputable to need ex- 
 
146 THE CAUSE OF AN "iCE AGE. 
 
 traneous support. It is, however, interesting to note that 
 delicate plants tell the same story as mighty bowlders 
 weighing hundreds of tons. I have, however, introduced 
 the subject of botanical distribution, not alone because 
 it throws light on the elementary fact that Ice Ages 
 have existed, but for a far more important reason. 
 
 We have already mentioned as an important conse- 
 quence of the Astronomical theory that the glaciation 
 oscillated between one hemisphere and the other ; it is 
 especially as to this point that the botanical evidence 
 is instructive. We have the high authority of Sir J. 
 Hooker for the remarkable fact that a great number of 
 the flowering plants in Patagonia are either identical 
 with, or are closely allied to, plants in temperate North 
 America and Europe. To realize the significance of 
 this fact, consider not so much that Patagonia and 
 Northern Europe are separated by thousands of miles 
 of land and sea, as that between them lies the torrid 
 zone, in which these plants adapted to temperate re- 
 gions could not live. There is no continuity between 
 the flora of Patagonia and that of North America, for 
 equatorial America is a barrier through which such or- 
 ganisms could not pass. How, then, are we to explain 
 the community of botanical forms in two regions so 
 remote ? It is impossible to believe that these separate 
 floras can have sprung independently into being, for all 
 the analogies of nature demonstrate that they must have 
 had some common source. The glacial theory is at hand 
 to render an explanation of the facts. 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 147 
 
 Let us once again suppose that a glacial period is 
 approaching in the Northern Hemisphere. We have 
 already seen how the plants which normally reside in 
 the temperate regions would be gradually compelled to 
 travel southward, so that a large number of temperate 
 forms would spread through equatorial regions. In all 
 such arguments we must remember the long duration 
 of the glacial periods, so that during the lapse of thou- 
 sands of years certain species of plants may have become 
 dispersed over wide ranges when the conditions offered 
 are suitable. At such a time, according to CrolPs theory, 
 the Southern Hemisphere would be in that mild genial 
 condition which alternates with the glacial epoch. Let 
 us suppose that the glaciation of the Northern Hemi- 
 sphere has run its course, and that a genial condition 
 of things has returned. Normal heat is restored to the 
 torrid regions; the temperate forms to which such 
 warmth is ungrateful must retreat; some of them fly 
 back to the temperate zones of the Northern Hemi- 
 sphere, others escape to the summits of the lofty mount- 
 ains, which even in the tropics, under normal condi- 
 tions, will afford the climate they enjoy. But now a 
 glaciation begins to grip the Southern Hemisphere: 
 the temperate forms fly southward to the equatorial 
 regions ; the plants on the summits of the mountains 
 begin to steal down the sides and to spread over the 
 lowlands, which now attract them with a suitable cli- 
 mate, there they commingle with the temperate forms 
 which have retreated before the glaciation in the South- 
 
148 THE CAUSE OF AN ICE AGE. 
 
 ern Hemisphere. Again, after a few thousands of years 
 the glaciation is about to release its hold, the temperate 
 forms are driven away before the increasing heat of the 
 Equator, and are inclined to return to the zones which 
 they had originally left. Thither they accordingly go, 
 bearing with them some of the forms which came from 
 the summits of tropical mountains; and thus the North- 
 ern forms at first isolated on tropical mountain tops are 
 thence carried into the Southern Hemisphere. By this 
 means we are able to explain how the plants in Pata- 
 gonia have an unmistakable affinity with the plants in 
 temperate regions of the Northern Hemisphere. 
 
 There is another circumstance in the distribution of 
 plants which receives its explanation at the same time. 
 On the summits of lofty mountains in the tropics cer- 
 tain plants find a habitat which the adjoining lowlands 
 would not afford. It is remarkable that the flora of the 
 summits of these tropical mountains all round the world 
 have certain common characteristics, and this can like- 
 wise be explained. 
 
 Darwin mentions a group of facts which seem to 
 make it certain that a mild, temperate climate must 
 have occasionally encircled the earth at the Equator ; for 
 how otherwise could we account for the circumstance 
 that the plants in the high lands across equatorial 
 Africa, India, Ceylon, the Malay Archipelago, and in 
 some degree across tropical South America, possess 
 common features resembling those of a temperate flora, 
 consisting of plants that could not live on the interven- 
 
CORROBORATION OF THE ASTRONOMICAL THEORY. 149 
 
 ing lowlands. Here we have a demonstration of that 
 critical doctrine in the Astronomical theory of the Ice 
 Age which asserts that the Equator must have been oc- 
 casionally visited by a temperate climate. 
 
 It would render the utmost assistance to the investi- 
 gation of this subject to obtain whatever evidence may 
 be forthcoming as to the existence of recognizable in- 
 dications of glaciation through the various geological 
 formations. That there are such indications to be had 
 seems admitted, but here is one of those cases in which 
 the imperfection of the geological record has most fre- 
 quently to be regretted. I can only say that a strict 
 search for glacial indications among all deposits, pri- 
 mary, secondary, and tertiary, would be one of the most 
 valuable pieces of scientific work possible at the present 
 time. 
 
 11 
 
CHAPTER VIII. 
 
 CONCLUDING CHAPTER. 
 
 So long as a glaciated stone lies softly surrounded by 
 the bowlder clay, its peculiar tracery is preserved. Let 
 the protective covering be once removed, let the ice- 
 engraved stone be rolled in the shingle on the sea beach, 
 and presently its inscriptions are obliterated, and the 
 record of its glacial origin is destroyed. The sheets of 
 bowlder clay themselves are of a perishable nature, and 
 liable to be washed away. The marks on glacier-eroded 
 valleys have no greater permanence than the surfaces 
 of the rocks on which they are written ; wind and 
 weather will at last crumble the mightiest erratics into 
 dust. The advent of one ice-sheet plows away the traces 
 of preceding ice-sheets, while the greater transforma- 
 tions that have been experienced during geological times 
 are generally adequate to efface indications of such 
 merely transient phenomena as the coming and the 
 going of Ice Ages. It is not surprising, therefore, that 
 the records of the rocks should be so imperfect as we 
 find them when we come to investigate the glacial 
 phenomena of former times. That there have been Ice 
 Ages and Ice Ages hardly admits of a question; but 
 
CONCLUDING CHAPTER. 151 
 
 to prove their sequence through geological time is diffi- 
 cult in the extreme, while to determine their number by 
 the observation of geological phenomena is hopelessly 
 impossible. It is one advantage of the Astronomical 
 theory of Ice Ages that it exhibits the sequence of the 
 phenomena in such a way as to suggest the kind of geo- 
 logical facts that seem likely to bear on the subject. 
 
 We have seen that, according to the Astronomical 
 theory, the succession of Ice Ages takes place in this 
 wise. The earth's orbit must first assume a high degree 
 of eccentricity : so long as the eccentricity of the orbit 
 is sufficiently high an Ice Age will be possible, provided 
 the other condition is fulfilled. This other condition is, 
 of course, that the position of the line of equinoxes shall 
 be perpendicular to the major axis of the earth's orbit, 
 so as to make the inequality between the duration of the 
 two seasons as great as is compatible with the eccentric- 
 ity at the time. There are thus two conditions con- 
 tributory to the formation of an Ice Age, one lying in 
 the state of the eccentricity, and the other in the posi- 
 tion of the line of equinoxes. 
 
 There is a somewhat more simple method of investi- 
 gating the conditions of the question. The really essen- 
 tial point necessary for the production of an Ice Age in 
 one hemisphere, and the simultaneous production of a 
 Genial Age in the opposite hemisphere, can be entirely 
 expressed by the difference in the lengths of the sea- 
 sons. The essence of the Astronomical theory resides 
 in the fact that the total heat from the sun received 
 
152 THE CAUSE OF AN ICE AGE. 
 
 during the summer in one hemisphere is a constant, 
 and that the total heat received during the winter is 
 also a constant. The highest degree of contrast be- 
 tween the climatic conditions of the two seasons is, 
 therefore, obtained by making the difference in their 
 durations as great as possible. The critical magnitude 
 which decides whether a certain year is a glacial year 
 or not simply depends, according to the Astronomical 
 theory, on the difference between the lengths of the 
 seasons in that year. Viewing the question in this way 
 greatly simplifies the matter. The differences between 
 the lengths of the seasons is, as a mathematician would 
 say, a function of -two other independent quantities; it 
 partly depends upon the eccentricity of the earth's orbit, 
 and partly on the longitude of the perihelion, that is to 
 say, on the position of the line of equinoxes with respect 
 to the longer axis of the earth's orbit ; if there were no 
 eccentricity there could be no difference in the lengths 
 of the seasons, no matter where the line of equinoxes 
 may lie. On the other hand, no matter what the eccen- 
 tricity may be, there would be no difference in the 
 lengths of the seasons if the line of equinoxes passed 
 through perihelion. Taking the eccentricity and the 
 longitude of the perihelion as they are found at any 
 moment, the number of days in the difference of the 
 seasons is a determined quantity. It fortunately hap- 
 pens for our present investigation that the only element 
 we require is the difference in the duration of the sea- 
 sons ; we need, therefore, only consider the joint effect 
 
CONCLUDING CHAPTER. 153 
 
 of eccentricity and position of perihelion as estimated in 
 the single element of the difference in the lengths of 
 the seasons arising from their joint influence. 
 
 We have already seen that the number 33 expresses 
 the maximum number of days that by a combination of 
 favorable circumstances expresses the difference in the 
 duration of summer and winter respectively. From 
 this maximum the difference may descend step by step 
 to its present value of seven days, or to zero, in which 
 the two seasons are of equal duration. The greater the 
 difference the more intense becomes the contrast be- 
 tween the climatic conditions of the two hemispheres. 
 That hemisphere which has the long summer and the 
 short winter will be in a genial condition, while that 
 with the short summer and the long winter will be gla- 
 ciated. Other things being equal, we may certainly 
 affirm that the greater the number of days in this dif- 
 ference the more intense will be the severity of the cor- 
 responding Ice Age. 
 
 So long as the eccentricity of the earth's orbit re- 
 mains at its present value the difference between the 
 lengths of the seasons will fluctuate between the extreme 
 values of a winter seven days longer than a summer, and 
 a summer seven days longer than a winter. But be- 
 tween these limits there can be no Ice Age; if there 
 were a possibility of such, our Southern Hemisphere 
 should be at this moment in a glacial state, and, of 
 course, the Northern Hemisphere in a correspondingly 
 genial state. 
 
154: THE CAUSE OF AN ICE AGE. 
 
 It is only, however, when the difference between the 
 lengths of the seasons becomes much more considerable 
 that an Ice Age proper may be the result. We have no 
 means of knowing what relation may exist between the 
 latitudes at which ice-sheets remain permanent from 
 year to year, and the corresponding difference in the 
 duration of the seasons. A perfect theory of Ice Ages 
 should, of course, provide some information on this 
 point ; for the present, however, it will be sufficient to 
 remark that when the difference, which is at present 
 only seven days, makes some considerable approach 
 toward its maximum value, which is nearly five weeks, 
 then our globe is placed in that interesting condition in 
 which Ice Ages and Genial Ages oscillate between one 
 hemisphere and the other; the interval between the 
 time of the maximum of glaciation and the maximum 
 of the opposite condition in either hemisphere being 
 10,500 years. 
 
 I have spoken chiefly of those recent Ice Ages which 
 have engraved their marks on the mountain flanks, and 
 which have deposited sheets of bowlder clay over so 
 large an area of the temperate regions. It is, however, 
 an interesting consequence of the Astronomical theory 
 that the causes which produce them may arise again. 
 We have good grounds for believing that the general 
 arrangement of our solar system has been substantially 
 the same as it is at present, throughout the range of 
 time indicated by the ordinary chronology of geology. 
 No one imagines, for instance, that the planet Jupiter, 
 
CONCLUDING CHAPTER. 155 
 
 as we know him to-day, revolved in an orbit, during the 
 Silurian period, which differed to any considerable ex- 
 tent from the orbit which he now follows through the 
 skies. Venus and Saturn then, doubtless, described 
 orbits very much like those which they follow at pres- 
 ent. On the other hand, no one supposes that the con- 
 figurations of our system are absolutely eternal. La- 
 grange's theorems on the stability of the solar system 
 go not so far as that. We are entitled to believe that, 
 vast as are the eras contemplated by geology, they are 
 yet small in comparison with the periods that would be 
 required, before the evolution through which our solar 
 system is progressing shall have succeeded in effecting 
 any fundamental dislocation of the planetary orbits. 
 We can not, therefore, doubt that the perturbations of 
 the earth's orbit were produced in early geological times 
 much as they are at present. There is no reason to 
 think that the length of the year has ever been appre- 
 ciably different within geological times from the length 
 of the year as it now glides by ; indeed, one of the most 
 beautiful of Lagrange's discoveries was that which proved 
 the inability of planetary perturbations to affect, perma- 
 nently, the length of the major axis of the earth's orbit 
 at least, if the bodies be regarded as solid. Kepler's 
 law asserts that the periodic times of planets and 
 the lengths of their major axes are so connected that 
 one could not alter if the other remained unchanged. 
 As the major axis of a planetary orbit is unaltered by 
 perturbation, it follows that the period of that planet's 
 
156 THE CAUSE OF AN ICE AGE. 
 
 revolution must be also free from interference by the 
 other planets. In early geological times, though the 
 year may have been what it is at present, yet the sea- 
 sons then, as now, changed their relative durations as 
 an immediate consequence of perturbation. We have 
 seen how the last Ice Age might be immediately attrib- 
 uted to a disparity in the lengths of the seasons, com- 
 bined with the law of the distribution of heat between 
 the two seasons. Similar influences were at work dur- 
 ing remote geological epochs, hence we are entitled to 
 infer, as a necessary consequence of the Astronomical 
 theory, that throughout the whole period spoken of as 
 geological time, Ice Ages have from time to time re- 
 curred according to a law of succession prescribed by 
 astronomical conditions. 
 
 It is not, however, to be presumed that Ice Ages 
 occur in any steady sequence at definite intervals of 
 time, nor that when they do occur they are all of the 
 same intensity. The intervals between their recurrence 
 may, it is true, be not unfrequently 21,000 years, but 
 the period will often be far greater. I may for the 
 moment illustrate this point by referring to the laws 
 of recurrence of the transits of Venus across the sun. 
 There was one in 1874 and another in 1882 at an in- 
 terval of eight years. There will not, however, be an- 
 other until 2004, and then there will be another in 
 2012. In other words, the transits of Venus usually, 
 but not always, occur in pairs, the two being separated 
 by eight years, and then there is a long and variable 
 
CONCLUDING CHAPTER. 157 
 
 interval of more than a century between one pair and 
 the next. All this is well understood by the astrono- 
 mer. There is a certain regularity in the recurrence of 
 the transits of Venus, for we are able to calculate those 
 that will happen for thousands of years to come. There 
 is, however, no definite numerical relation between the 
 successive recurrences that can be readily stated. When 
 the transits of Venus do occur they present every variety 
 as to duration and as to the regions on our globe from 
 which they may be seen. 
 
 In these respects we may find a resemblance between 
 the recurrence of the Ice Ages and the recurrence of 
 the transits of Venus. Transits of Venus do not occur 
 every eight years, and the planet does not always pass 
 centrally across the sun, nor is it always visible from 
 Greenwich. So it is with regard to Ice Ages; they 
 do not recur every 21,000 years, they are not always 
 equally severe, nor are their geographical effects always 
 the same. The only general fact that can be stated 
 with regard to the recurrence of transits of Venus is 
 that they generally appear in pairs. The corresponding 
 fact with regard to Ice Ages is that they generally 
 appear in groups of two or more, the intervals between 
 the consecutive members of the group being 21,000 
 years. More than this as to the epochs of recurrence 
 of Ice Ages I have not asserted. I can not say when 
 the last took place nor when the next may be expected. 
 No one who is competent to deal with mathematical 
 formulae would venture on such a prediction in the 
 
158 THE CAUSE OF AN ICE AGE. 
 
 present state of our knowledge. All we know is that 
 the interval between two groups of Ice Ages seems to 
 be generally enormously long as compared with the 
 21,000-year interval at which they follow each other in 
 the group. And then as to the group of Ice Ages 
 when it does come. At first, perhaps, the glaciation 
 may be slight, and will increase with each recurrent 
 21,000 years until it reaches the maximum for that 
 group. But the maximum of the group may fall far 
 short of the maximum that may be possible in some 
 other group by a different combination of circum- 
 stances , 
 
 It is so important to realize the variety of possible 
 forms of Ice Age that the Astronomical theory offers, 
 that I propose to go a little into detail. In order to 
 illustrate the argument it has generally seemed best to 
 take the most extreme case possible, in which the differ- 
 ence between the length of the seasons has its greatest 
 possible value of thirty-three days'; but the whole phe- 
 nomena are graduated from this maximum to the pres- 
 ent condition of the earth, in which the difference in 
 the length of the seasons is but seven days. This does 
 not seem enough to produce anything worthy of the 
 name of an Ice Age. So far as it goes, however, it 
 would indicate that the Southern Hemisphere had a 
 slight tendency toward an Ice Age at present, and the 
 Northern Hemisphere a tendency toward a Genial Age. 
 In 10,500 years from the present time the winter in the 
 Northern Hemisphere will be about seven days longer 
 
CONCLUDING CHAPTER. 159 
 
 than the summer ; then the tendency toward a glacial 
 period will be met with in the Northern Hemisphere, 
 and the Southern will tend toward the genial period. 
 The possible difference in the length of the seasons 
 in some other phase of the earth's orbit may be, let 
 us say, twenty days. This will continue long enough to 
 admit of two or more oscillations, so that the glacial 
 and genial garbs shall be interchanged between the 
 two hemispheres. What the severity of the glaciation 
 might be under such circumstances it would be hard to 
 say, but it would clearly be of a milder type than when 
 the greatest possible difference between the seasons was 
 obtained, other things remaining the same. 
 
 The Astronomical theory offers to us a vista of Ice 
 Ages at irregular intervals and of varied severity. We 
 are to expect neither uniformity in recurrence nor 
 uniformity in intensity. This must be carefully kept 
 in view when we come to seek records in the rocks as 
 to the existence of past Ice Ages. It must also be 
 borne in mind that the astronomical forces will neces- 
 sarily blend with the other agents which have been 
 invoked for the explanation of Ice Ages in past times. 
 The effects produced by glaciation will, of course, be 
 largely dependent on the disposition of land and water 
 which may happen to constitute the geography of the 
 hemisphere at the time of the glacial invasion ; while, 
 of course, the position of the high-lying lands will also 
 be of much importance. The ocean currents, it need 
 hardly be added, will be among the agencies which af- 
 
160 THE CAUSE OF AN ICE AGE. 
 
 feet the distribution of the ice, and these are primarily 
 influenced by the contour of the coasts. The presence 
 of forests as a climatic influence contributes a further 
 element of variety to the recurring Ice Ages during geo- 
 logical time. 
 
 From the consideration both of astronomy and of 
 geology alike, it is thus evident that we can lay down 
 no hard and fast lines as to when Ice Ages have oc- 
 curred, nor as to their severity when they did occur. 
 We can lay down no hard and fast lines as to what the 
 glaciation may have been able to do and what tasks lay 
 beyond its accomplishment. In seeking for traces of 
 former Ice Ages we are beset with many difficulties, 
 and there are not a few pitfalls in the path of him who 
 would rashly approach this subject. Special instances 
 seem full of peril. To say that this is the product of 
 a Genial Age, and that of a Glacial Age, is sometimes 
 tempting, but generally dangerous. The modifying cir- 
 cumstances are but partly known, or more often wholly 
 unknown. That things are not what they seem is, in 
 this case at all events, a maxim that should be carefully 
 respected. 
 
 It is only in the most general and the broadest sense 
 that I propose to summon the facts of geology to bear 
 testimony to the doctrine expounded in this book. If 
 there be one fact that seems beyond dispute, it is that 
 the earth has undergone great changes in climate from 
 time to time during its past history. The testimony of 
 the rocks is here at least certain. That what we call 
 
CONCLUDING CHAPTER. 
 
 climate is a highly complex system of conditions is, of 
 course, admitted ; but we are, I think, entitled to suggest 
 that the thermal irregularities of which astronomy dem- 
 onstrates the existence can not be disconnected from 
 those changes in climate which geologists universally 
 recognize. 
 
 Let us consider one of the most patent of all geo- 
 logical facts, which seems not improbably connected 
 with glacial phenomena during the times that the sedi- 
 mentary rocks of various ages were in process of for- 
 mation. I will introduce the subject by a brief investi- 
 gation of the kind of geological phenomena which such 
 rocks ought to present if the Astronomical theory of an 
 Ice Age be really the true one. 
 
 I shall, therefore, suppose that a time has arrived 
 when the eccentricity of the earth's orbit has attained 
 a value in or about the greatest that it can possibly 
 reach. For a period of time, which ft doubtless to be 
 reckoned by many tens of thousands of years, this ec- 
 centricity remains sensibly constant, or at all events 
 great enough to satisfy the condition requisite for glaci- 
 ation. Let us suppose that it is the Northern Hemi- 
 sphere which is glaciated and the Southern Hemisphere 
 which is genial. In 10,500 years the condition has 
 been reversed, and the Southern Hemisphere is glaci- 
 ated and the Northern Hemisphere is genial; yet 
 another 10,500 years, and the interchange has once again 
 taken place. "We shall suppose that this grand oscil- 
 lation has been repeated two, three, or more times be- 
 
162 THE CAUSE OF AN ICE AGE. 
 
 fore the decline in the eccentricity of the earth's orbit 
 shall have deprived us of a necessary factor in the pro- 
 duction of a considerable Ice Age ; it is surely reason- 
 able to investigate the relation which these coming and 
 going Ice Ages bear to that chiefest of geological opera- 
 tions, the manufacture of stratified rocks. 
 
 Let us briefly recall the process employed in their 
 production. Rain washes the soil into the rivulets 
 which combine to form a river. The waters enter the 
 sea, and there deposit along the floor of the ocean the 
 materials that they have brought down. Other agents 
 contribute to the efficiency of the process. Frosts loosen 
 and crumble the solid materials, and floods largely aug- 
 ment the quantities transported by the river. The more 
 potent the agents by which the materials of the earth's 
 crust are pulverized, the more rapidly will the rivers be 
 able to supply the material which is gradually consoli- 
 dating into stone at the bottom of the sea. In an 
 earlier part of this book I had occasion to describe the 
 force exerted by a glacier creeping down an Alpine 
 valley, and I had also to point out how the same engine 
 for grinding rocks into mud would be exhibited on a 
 much gpander scale in a country covered by an ice-sheet. 
 The creeping of an ice-sheet over Greenland, for in- 
 stance, must daily produce an enormous bulk of ma- 
 terial ground and prepared for the formation of strati- 
 fied rock, if only the necessary vehicle for conveying it 
 down to the seas can be obtained. As the ordinary 
 agents in the process of comminuting rock must neces- 
 
CONCLUDING CHAPTER. 163 
 
 sarily be affected by the presence of ice, it is difficult to 
 believe that the manufacture of stratified rocks proceeds 
 at a rate which is independent of glacial phenomena. 
 If our hemisphere were alternately in a glacial state and 
 in a genial state, at periods separated by intervals of 
 10,500 years, it is only reasonable to expect that there 
 shall be corresponding variations in the rate of produc- 
 tion of stratified rocks. No doubt the questions in- 
 volved are by no means of a simple nature ; it seems 
 probable that more material would be ground up during 
 a glacial period than during a genial one ; but there is 
 also the problem as to the means by which this material 
 is transported to the ocean. It may well be, that the 
 time when the grinding machinery was at its utmost 
 efficiency was not at all the same time as that at which 
 the agents for transportation were best able to cope 
 with their work. I do not, therefore, assert that it was 
 actually during the maximum of a Glacial Period that 
 the maximum amount of stratified rock was formed, as 
 this is really unnecessary for our argument. The point 
 which I especially desire to note is, that there must have 
 been a species of rhythm in the manufacture of the 
 stratified rocks, corresponding to the rhythm with 
 which glacial periods and genial periods succeed each 
 other. 
 
 The subject may be illustrated by what happens in 
 the growth of a tree. During winter the vital oper- 
 ations in the tree seem quiescent, but in the spring 
 there is the putting forth of the leaf, and during the 
 
164: THE CAUSE OF AN ICE AGE. 
 
 summer and the autumn every leaf is a laboratory in 
 which woody tissue is being prepared to add to the 
 growth of the tree. The work of the leaf done, it 
 withers and falls, and a second period of inaction is 
 again entered upon. When the tree is cut down the 
 series of rings which its timber shows are an indication 
 of the various seasons through which the tree has 
 passed. Were the growth of the tree continuous, then 
 such rings would not be found. The greater the con- 
 trast between the seasons the more strikingly is this 
 subdivision manifested. 
 
 At a time of maximum eccentricity the earth is sub- 
 jected to marked changes in seasons. A perennial sum- 
 mer of 10,500 years in one hemisphere is gradually 
 replaced by a winter of equal duration, to be again fol- 
 lowed by another summer, and by subsequent repetitions 
 for so long as the requisite conditions shall prevail. We 
 might naturally expect that in the growth of stratified 
 rocks some indications at least may be found of a rhyth- 
 mical movement not wholly unlike that in the growth 
 of a tree. Provided with this conception, let us now 
 approach the actual facts of geology and see what they 
 have to show us. 
 
 It would appear that one of the most familiar of all 
 geological facts confirms the conclusion to which our 
 reasoning has led. The quarry or the railway cutting 
 presents us with a fact of obvious bearing on the ques- 
 tion at issue. Were the conditions by which stratified 
 rocks are manufactured uniform in their action, then 
 
CONCLUDING CHAPTER. 
 
 we might reasonably anticipate that the masses of rock 
 would be a continuous mass of a thickness correspond- 
 ing to the length of time during which the uniform 
 conditions prevailed. No doubt we do frequently see 
 beds of stratified rock of great thickness, but surely 
 every one who has devoted even casual attention to such 
 matters will have noticed that the rocks in a quarry or 
 the rocks in a railway cutting are usually arranged in a 
 series of layers. From inspection of such sections it is 
 obvious that a certain system of conditions prevailed 
 while the first of these beds was being deposited, that 
 then some considerable changes took place by which the 
 deposition was interrupted. The original condition, 
 however, recurred, and the second bed of rock was pro- 
 duced. Another interruption took place, and again the 
 original conditions were restored. By subsequent repe- 
 titions one bed after another was produced until the 
 whole system was formed. Of course it is not pretended 
 that there is an absolute identity between the products 
 of one period of active rock-making and that by which 
 it was followed or by which it was preceded ; for that 
 matter the signs of growth in an ordinary tree show in- 
 dividual differences corresponding to the varieties of the 
 seasons. We must similarly expect that there will be 
 corresponding differences between the successive beds 
 of rock, which, however, still preserve a generic resem- 
 blance. 
 
 The same phenomenon is generally to be seen wher- 
 ever a section of aqueous rocks is exposed. We observe 
 12 
 
166 THE CAUSE OF AN ICE AGE. 
 
 one bed of rock laid upon another often separated by 
 intervening beds of different materials. We often find 
 a repetition of a rock clearly similar or identical occur- 
 ring in other beds higher up or lower down. This sub- 
 division of the beds demands some special explanation. 
 Why is it, for instance, that after a bed of one particu- 
 lar description has been laid down, the process has been 
 interrupted, and that then it has been repeated with 
 materials identical with or different from those in the 
 first instance ? What causes these interruptions ? They 
 must correspond to some disorganization of the agents 
 by which aqueous rocks are manufactured. Now the 
 doctrine of frequent Ice Ages variable in intensity and 
 in the periods of their recurrence can hardly be over- 
 looked in this connection. I certainly make no attempt 
 to harmonize the successive beds with the recurrent Ice 
 Ages. Such an attempt would be precarious in the ex- 
 treme when we remember the numerous factors that 
 must enter into the question; but that the recurrent 
 Ice Ages are in some manner connected with this pri- 
 mary geological fact it seems hardly possible to doubt. 
 
 In bringing this work to a conclusion it is necessary 
 to add that the agencies by which Ice Ages have been 
 produced are still in operation. At present the differ- 
 ence between the length of the seasons is, as we have 
 already stated, seven days. It so happens that the pres- 
 ent position of the perihelion of the earth's orbit is such 
 that the seasonal difference is near its maximum for the 
 present eccentricity of the orbit. The precession of the 
 
CONCLUDING CHAPTER. 167 
 
 equinoxes will gradually alter the relative lengths of the 
 seasons. From being unequal, as they are at present, 
 they will approach to equality, and then at the other 
 end of the swing the summer in the Northern Hemi- 
 sphere will be seven days shorter than the winter in the 
 same hemisphere, instead of being as much longer. 
 Again, the oscillation will carry the seasons back to 
 what they are at present, so that in 21,000 years or so 
 the summer will just exceed the winter by seven days, 
 as it does now. Ice Ages are, however, not to be ex- 
 pected from vicissitudes such as those which are at pres- 
 ent possible. This 21,000-year period which swings to 
 and fro will, however, prevail independently of the 
 changes in the eccentricity. The perturbations by 
 which those changes can be produced will, doubtless, 
 exist in the future as they have done in the past, if the 
 present order of nature be preserved. It will therefore 
 follow that, in the distant future, the eccentricity will 
 attain so high a value that the difference between the 
 duration of the seasons will rise to the point at which 
 the phenomena of glaciation are produced. Nor is there 
 any reason for thinking that there will only be a single 
 occasion on which the eccentricity shall rise to a suffi- 
 ciently high point. Just as there have been several max- 
 ima in the course of ages past, so we may infer from 
 mathematical laws that there will be several maxima of 
 eccentricity in time to come. Calculation leaves no 
 doubt on this point, although it does not, however, in- 
 form us, with any degree of certainty, as to the im- 
 
THE CAUSE OF AN ICE AGE. 
 
 mense periods that must elapse before the first of these 
 phases of high eccentricity will return It is, however, 
 a consequence of the Astronomical theory of Ice Ages 
 that they must return in the future. There is, there- 
 fore, the best of reasons for believing that as these tem- 
 perate regions have been submerged on various occa- 
 sions, in past ages, beneath ice-sheets a thousand feet or 
 more in thickness, so in future periods the ice-sheets 
 will again return and desolate those regions which now 
 contain the most civilized nations of the earth. 
 
 The ancients consulted the stars for the purpose of 
 reading in their movements the vicissitudes of human 
 affairs. Astrologers cast their horoscopes, and showed 
 how the career of the man was predicted in the config- 
 uration of the planets at his birth. Statesmen sought 
 the stars for guidance in the affairs of the empire ; while 
 a decision on all matters of moment could only be safely 
 taken after consultation with the heavens. We now 
 smile at the credulity of those who put faith in the 
 astrologers. We certainly do not now believe that the 
 positions of the planets are to be taken as prognostications 
 which convey a direction from heaven for the benefit of 
 those who seek it. We do not any longer believe that 
 Mercury and Venus, Mars, Jupiter, and Saturn, were 
 placed in the heavens for the sole purpose of prophesy- 
 ing weal or threatening woe to mankind. In the fif- 
 teenth century it happened that three planets came into 
 a remarkable relation, according to the received notions 
 at the time. Such an occurrence must portend some- 
 
CONCLUDING CHAPTER. 169 
 
 tiling ; what that was, the astrologers discovered by no- 
 ticing that the conjunction took place in a watery sign. 
 The interpretation was now obvious. Floods of water 
 were of course to drown Europe beneath their dismal 
 expanse. The consternation was wide-spread at the 
 prospect of this second deluge, and rafts and life-buoys 
 were got ready everywhere. Voltaire tells us of a doc- 
 tor of Toulouse who, having bought a boat for himself, 
 fortunately remembered that if his patients were all 
 drowned his medical practice would experience a corre- 
 sponding limitation ; the prudent physician accordingly 
 built a gigantic ark for the reception of his patrons and 
 for the security of his professional income. Happily, 
 the doctor's fears were not realized ; the great flood did 
 not come off. Long practice in many similar emergen- 
 cies had rendered the astrologers quite equal to the oc- 
 casion ; they explained how the deluge had been arrested 
 by a fortunate dispensation, which I recount in their 
 own words : 
 
 " Saturn disposes of the sun, who is posited in the 
 terms of Venus, and Venus and Saturn and the Moon 
 are all disposed of by the benevolent planet Jupiter, 
 who is himself disposed of by Mars, the principal signi- 
 ficator." 
 
 It is, therefore, obvious that the reason why the 
 deluge should have come and the reason why it failed 
 to come are both accounted for on equally satisfactory 
 principles. Yet, strange to say, all this nonsense about 
 the planets and their influence on terrestrial affairs 
 
170 THE CAUSE OF AN ICE AGE. 
 
 seems to possess a certain affinity with a great scientific 
 truth. No one now believes that any particular config- 
 uration in which the planets may happen to lie, portends 
 a pestilence, a war, or a deluge of water, but the truth 
 is much more striking than the wildest fiction of which 
 astrologers ever dreamed. Those who have followed 
 these pages will have learned that, according to the 
 Astronomical theory herein set forth, the influence of 
 the planets has occasionally visited some of the fairest 
 regions of our globe with a scourge more deadly than 
 the most malignant pestilence, more destructive than 
 the most protracted of wars, and more desolating than 
 the mightiest of floods. 
 
 Slumbering in the Arctic regions lies at this moment 
 the agent of the most dire of calamities. That agent 
 has once, and more than once, been aroused into activ- 
 ity. Time after time it has happened that the planets 
 have by their influence on the earth's orbit brought 
 down on our temperate regions the devastations of the 
 great ice-sheet. From its normal home at the Poles 
 the great glaciation has spread southward ; a sheet of 
 ice and snow hundreds or thousands of feet thick has 
 crept from the highlands of Norway and Sweden, has 
 invaded Central Europe as far as Saxony, while the 
 greater part of Great Britain was also submerged by an 
 icy covering. Nor did the New World afford a refuge 
 from the frightful inclemency. Wide-spread sheets of ice 
 of enormous thickness submerged Canada, and buried 
 much of that tract which now forms the Eastern States. 
 
CONCLUDING CHAPTER. 171 
 
 Did the imaginations of the old astrologers ever attrib- 
 ute to the operations of the planets any calamity at once 
 so vast and so fatal ? All that the astrologers have ever 
 claimed for the influence of the planets seems trivial in 
 comparison with the grim severity of the ice-sheet. The 
 Astronomical theory of the Ice Age, however, assures 
 us that it was the planets which drew down this icy in- 
 vasion, and that it was the planets which bade the ice 
 to withdraw. The influence of the planets followed the 
 retreating ice within its natural confines in the Arctic 
 circle, chased it entirely from the hemisphere, and per- 
 mitted the horrors of the Ice Age to be forgotten in the 
 joys of the summer by which it was succeeded. The 
 planets indeed have been potent agents in human affairs. 
 The Astronomical theory of the Ice Age presents us 
 with interesting thoughts as to the connection between 
 our earth and the other planets. Accepting this doc- 
 trine, it is hardly possible to overestimate the influence 
 which the planets have had in shaping our globe to the 
 form in which we now find it. It seems hardly fanciful 
 to say that we can recognize the handiwork of the 
 planets everywhere around us. I commenced this little 
 book with a reference to the Cloghvorra Stone on the 
 Kenmare River, a picture of which forms a frontispiece 
 to our volume. That this great bowlder was left where 
 we now find it by the action of ice is a fact as to which 
 doubt is impossible ; it is one of the indications of the 
 last severe Ice Age from which our country has suffered. 
 It has been my object to set before you the reasons for 
 
172 THE CAUSE OF AN ICE AGE. 
 
 thinking that the planets, and especially Jupiter and 
 Venus, have been the primary agents in the formation 
 of Ice Ages ; we have substantial grounds for attributing 
 to the agency of the planets the familiar indications of 
 glaciation. When we gaze on the Cloghvorra Stone a 
 fanciful person might almost think that it was the 
 thews of mighty Jupiter himself which ripped that 
 stone from its original bed, some miles away, and sport- 
 ively cast it down on the mountain side. When, also, 
 we examine the inscription on a glaciated stone, we need 
 not dissociate the engraving from the planetary agents 
 by whose operations it was ultimately produced. May 
 we not imagine the characters on the stone as having 
 been traced by the fair hand of Venus herself ? 
 
 Finally, let me say, and, perhaps, in some degree 
 repeat, what I have urged with regard to the claims of 
 the Astronomical theory of the great Ice Age to our 
 acceptance. Geology having taught us by unmistak- 
 able evidence that Ice Ages have happened, it is ra- 
 tional to inquire into the causes by which phenomena 
 so startling have been brought about. We find that 
 the Astronomical theory offers at once an explanation 
 adequate to the effects which have been observed. We 
 find that this theory involves no gratuitous assumptions, 
 and that it does not require the co-operation of any 
 agents except those with which we are already familiar. 
 The theory is based on the movements of the heavenly 
 bodies as we now actually find them. The dynamical 
 relations of those movements are now so thoroughly 
 
CONCLUDING CHAPTER. 173 
 
 understood by mathematicians that there is here no 
 ground for doubt. We have shown that these necessary 
 movements must be followed by profound climatic 
 changes on this globe. We have even shown that the 
 astronomical conditions are so definite that astronomers 
 are entitled to direct that vigorous search be instituted 
 on this globe to discover the traces of those vast climatic 
 changes through which astronomy declares that our 
 earth must have passed. When the astronomer, who is 
 bent on this search, finds the geologist coming to seek 
 from him an explanation of the climatic changes of 
 which he has detected the evidence in the rocks, it 
 seems impossible to deny that the labors both of geolo- 
 gists and of astronomers have in this case both illus- 
 trated and supplemented each other. The geologist has 
 supplied the evidence which the astronomer wanted; 
 the astronomer has supplied the explanation which the 
 geologist wanted. 
 
 The Astronomical theory removes the glacial period 
 from the position of a " catastrophic " phenomenon. It 
 places the ice-sheet as an implement at the disposal of 
 the geological uniformitarian, to be used with the other 
 agents with whose powers he is already familiar. 
 
APPENDIX. 
 
 THE following is the calculation so often referred to in this 
 book, and in which for the first time, so far as I know, the 
 astronomical facts relating to Ice Ages have been correctly 
 given. I compute the total quantity of heat received by each 
 hemisphere of the earth during summer and winter respectively 
 as follows : 
 
 Let %Hja? be the quantity of sun heat falling perpendicu- 
 larly on an area equal to the section of the earth at the mean 
 distance a from the sun in the unit of time. 
 
 Let 8 be the sun's north declination. Then the share re- 
 ceived by the Northern Hemisphere will be 
 
 f (1 + sin 8), 
 and by the Southern 
 
 At the distance r, and in the time dt, the heat received in 
 the Northern Hemisphere will be 
 
 TTT 
 
 -(1 + sin 8) . dt; 
 
 but we have 
 
 r a de = Mt, 
 
 whence the expression becomes 
 
 rr 
 
 -(1 + sinS).^; 
 n 
 
 but we have 
 
 sin 8 = sin G . sin f, 
 
 where e is the obliquity. 
 
176 APPENDIX. 
 
 The total heat received by the Northern Hemisphere from 
 the vernal to the autumnal equinox is 
 
 r (1 + sin e sin ff) . dB = (IT + 2 sin e). 
 j on h 
 
 We have thus the following theorem, which constitutes the es- 
 sence of the Astronomical theory of an Ice Age, and to which I 
 invite the reader's particular attention : 
 
 Let 2E be the total sun heat received in a year over the 
 whole earth ; then this is divided into shares as follows : 
 
 ,TT + 2 sin e 
 
 Northern Hemisphere, summer, E 
 " " winter, E 
 
 27T 
 
 TT 2 sin 
 
 27T 
 
 with identical expressions for the summer and winter in the 
 Southern Hemisphere. 
 
 If we make e = 23 27' we find that the heat received dur- 
 ing the summer (equinox to equinox) of each hemisphere is 
 627 E, while the heat during the winter of each hemisphere 
 is -373 E. More briefly still : If each hemisphere receives in 
 the year a quantity of sun heat represented by 365 units, then 
 229 of these are during summer and 136 during winter. 
 These figures are independent of the eccentricity of the earth's 
 orbit. 
 
 The length of the summer is defined to be the interval 
 when the sun's center is above the equator. The length will, 
 of course, vary with the eccentricity and with the position of 
 the equinoxes on the orbit. "We need only take the extreme 
 case where the line of equinoxes is perpendicular to the major 
 axis of the orbit. The maximum difference between the length 
 of summer and of winter is thus 
 
 465 days x eccentricity. 
 
 I take the maximum eccentricity of the earth's orbit to be 
 0-0745, 
 
 this being the mean of the values by Leverrier, Lagrange, and 
 Stockwell, and, therefore, the greatest difference between 
 
APPENDIX. 177 
 
 summer and winter will be about 33 days i. e., one season is 
 199 days, and the other is 166 days. 
 
 The total quantity of heat received during the year on each 
 hemisphere is practically independent of the eccentricity ; but 
 the mode in which that heat is received at the different seasons 
 will vary, and thus give rise to the following extreme cases, 
 already referred to on page 102 : 
 
 GLACIAL 
 
 229 heat units spread over 166 days. 
 136 heat units spread over 199 days. 
 
 OR INTERGLACIAL 
 
 229 heat units spread over 199 days. 
 136 heat units spread over 166 days. 
 
 "We hence deduce the following, where unity represents the 
 mean daily heat for the whole year on one hemisphere : 
 
 GLACIAL 
 
 Mean daily sun heat in summer (short) . .1-38 
 Mean daily sun heat in winter (long) . . '68 
 
 INTERGLACIAL 
 
 Mean daily sun heat in summer (long) . . 1'16 
 Mean daily sun heat in winter (short) . . *81 
 
 PRESENT (NORTHERN HEMISPHERE) 
 
 Mean daily sun heat in summer (186 days) . 1*24 
 Mean daily sun heat in winter (179 days) . . -75 
 
 These figures exhibit a thermal force of great intensity. The 
 unit represents all the mean daily heat received from the sun 
 by which the earth is warmed up from the temperature of space. 
 The heat unit, in fact, maintains a temperature perhaps 300, 
 or even more, above what the earth would have without that 
 heat. Each tenth of a unit may thus roughly be said to cor- 
 respond to a rise or fall of mean temperature of 30 or more. 
 
178 APPENDIX. 
 
 The long winter of 199 days, when the average heat is only 
 two thirds of a unit, leads to the accumulation of ice and snow, 
 which forms the glacial epoch. The short winter of 166 days, 
 where the temperature is '06 of a unit above that of our pres- 
 ent winter, presents the condition necessary for the mild inter- 
 glacial epoch. 
 
INDEX. 
 
 Adhemar, 42. 
 
 Agassiz, x. 
 
 Alaskan ice, 23. 
 
 Algol, 50. 
 
 Alpine glaciers, 5. 
 
 Alternation of glacial and genial 
 
 ages, HO. 
 
 America, flora of, 146. 
 Antarctic ice, 13. 
 Asa Gray, 1 43. 
 Astrologers, 168. 
 
 Bowlder clay, 16. 
 Bowlders, 4. 
 
 Calculation of heat distribution, 
 
 Appendix, 176. 
 Canada, 29. 
 Clew Bay, 17. 
 Cloghvorra Stone, 1. 
 Constancy of sun heat in geological 
 
 time, 52. 
 Croll, 42. 
 erroneous notions about the 
 
 law of heat distribution, 119. 
 
 as to Gulf Stream, 133. 
 
 Croll's Climate and Time, 112. 
 
 Darwin, 143. 
 
 Eccentricity of orbit, 79. 
 Ecliptic, obliquity of, 87. 
 
 Elliptic orbit (fig. 2), 82. 
 Eudora, attraction of, 76. 
 Evidences of ice age, 23. 
 
 Geikie, Prof. James, x. 
 Genial epochs, 27. 
 Glacial state of globe, 102. 
 Glaciation effect on annual tem- 
 perature, 131. 
 Glaciated stone, 19. 
 Great Bear, 67. 
 Greenland, 11. 
 ice, 23. 
 
 Heat, law of distribution, 90. 
 Herschel, error with regard to ice- 
 age, 115. 
 Outlines of Astronomy, 116. 
 
 Ice age, Astronomical theory of, 
 
 99. 
 ages, modifying causes of, 
 
 160. 
 
 currents, 7. 
 
 sheet, extent of, 23. 
 
 sheets, simultaneity of, 29. 
 
 Interglacial beds, 34. 
 state of globe, 102. 
 
 Jupiter, attraction of, 74. 
 Kepler's laws, 60. 
 
180 
 
 INDEX. 
 
 Kenmare, 1. 
 Kerry, 1. 
 
 Lagrangc on stability of the sys- 
 tem, 67. 
 Ly ell's theory of ice age, 39. 
 
 Mira Ceti, 50. 
 Moraines, 7. 
 
 Newton, 61. 
 Orion, 57. 
 
 Patagonia, 146. 
 
 Planetary perturbations, 65. 
 
 Powalky on obliquity of ecliptic, 
 
 87. 
 
 Present state of globe, 102. 
 Problem stated, 38. 
 Pyramids, 2. 
 
 Royal Dublin Society, lectures at, 
 
 113. 
 
 Royal Institution, lectures at, 112. 
 Royal Irish Academy, paper at, 
 
 113. 
 
 Scandinavian ice sheet, 23. 
 Scott's Elementary Meteorology 
 
 quoted, 129. 
 Siberia, 129. 
 Stars can not account for ice age, 
 
 54. 
 Stockwell on obliquity of ecliptic, 
 
 88. 
 
 Stratiacation of rocks, 162. 
 Succession of ice ages, 32. 
 Swiss glaciers, 5. 
 
 Transits of Venus, 156. 
 Trichomancs radicam, 9. 
 
 Unit, thermal, 102. 
 
 Venus, attraction of, 73. 
 Voltaire, 169. 
 
 Wallace, 41. 
 
 Wright's Ice Age in North Ameri- 
 ca, 24. 
 
 Yakutsk, 129. 
 
 Zero of temperature adopted, 46. 
 
 THE END. 
 
 
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