Sept. 9, 1875] 



NATURE 



421 



radiation ; and that the surface cooled and became a solid crust, 

 leaving a central nucleus of much higher temperature within. 

 The earth's surface appears now to have reached a temperature 

 which is virtually fixed, and on which the gain of heat from the 

 sun is, on the whole, just compensated by the loss by radiation 

 into surrounding space. 



Such a conception of the earliest stage of the earth's existence 

 is commonly accepted, as in accordance with observed facts. It 

 leads to the conclusion that the hollows on the surface of the 

 globe occupied by the ocean, and the great areas of dry land, 

 were original irregularities of form caused by unequal contrac- 

 tion ; and that the mountains were corrugations, often ac- 

 companied by ruptures, caused by the strains developed in the 

 external ci-ust by the force of central attraction exerted during 

 cooling, and were not due to forces directly acting upwards 

 generated in the interior by gases or otherwise. It has recently 

 been very ably argued by Mr. Mallet that the phenomena of 

 volcanic heat are likewise consequences of extreme pressures in 

 the external crust, set up in a similar manner, and are not 

 derived from the central heated nucleus. 



There may be some difficulty in conceiving how forces can have 

 been thus developed sufficient to have produced the gigantic 

 changes which have occurred in the distribution of land and 

 water over immense areas, and in the elevation of the bottoms 

 of former seas so that they now form the summits of the highest 

 mountains, and to have effected such changes within the very 

 latest geological epoch. These difficulties in great measure 

 arise from not employing correct standards of space and time 

 in relation to the phenomena. Vast though the greatest heights 

 of our mountains and depths of our seas may be, and enormous 

 though the masses which have been put into motion, when 

 viewed according to a human standard, they are insignificant in 

 relation to the globe as a whole. Such heights and depths 

 (about six miles) on a sphere of ten feet in diameter would be 

 represented on a true scale by elevations and depressions of less 

 than the tenth part of an inch, and the average elevation of the 

 whole of the dry land (about 1,000 feet) above the mean level of 

 the surface would hardly amount to the thickness of an ordinary 

 sheet of paper. The forces developed by the changes of the 

 temperature of the earth as a whole must be proportionate to its 

 dimensions ; and the results of their action on the surface in 

 causing elevations, contortions, or disruptions of the strata, 

 cannot be commensurable with those produced by forces having 

 the intensities, or by strains in bodies of the dimensions, with 

 which oar ordinary experience is conversant. 



The difficulty in respect to the vast extent of past time is 

 perhaps less great, the conception being one with which most 

 persons are now more or less familiar. But I would remind you, 

 that great though the changes in human affairs have been since 

 the most remote epochs of which we have records in monuments 

 or history, there is nothing to indicate that within this period 

 has occurred any appreciable modification of the main outlines 

 of land and sea, or of the condition of climate, or of the 

 general characters of living creatures ; and that the distance 

 that separates us from those days is as nothing when compared 

 with the remoteness of past geological ages. No useful approach 

 has yet been made to a numerical estimate of the duration even 

 of that portion of geological time which is nearest to us ; and 

 we can say little more than that the earth's past history extends 

 over hundreds of thousands or millions of years. 



The solid nucleus of the earth with its atmosphere, as we now 

 find them, may thus be regarded as exhibiting the residual 

 phenomena which have resulted on its attaining a condition of 

 practical equilibrium, the more active process of aggregation 

 having ceased, and the combination of its elements into the 

 various solid, liquid, or gaseous matters found on or near the 

 surface having been completed. During its passage to its 

 present state many wonderful changes must have taken place, in- 

 cluding the condensation of the ocean, which must have long con- 

 tinued in a state of ebulhtion, or bordering on it, surrounded by 

 an atmosphere densely charged v/ith watery vapour. Apart 

 from the movements in its solid crust caused by the general 

 cooling and contraction of the earth, the higher temperature due 

 to its earlier condition hardly enters directly into any of the 

 considerations tlrat arise in connection with its present climate, 

 or with the changes during past time which are of most interest 

 to us ; for the conditions of climate and temperature at present, 

 as well as in the period during which the existence of life is 

 indicated by the presence of fossil remains, and which have 

 affected the production and distribution of organised beings, are 



dependent on other causes, to a consideration of which I now 

 proceed. 



The natural phenomena relating to the atmosphere are often 

 extreniely complicated and difficult of explanation ; and meteor- 

 ology is the least advanced of the branches of physical science. 

 But sufficient is known to indicate, without possible doubt, that 

 the primary causes of the great series of phenomena, included 

 under the general term climate, are the action and reaction of 

 the mechanical and chemical forces set in operation by the sun's 

 heat, varied from time to time and from place to place, by the 

 influence of the position of the earth in its orbit, of its revolu- 

 tion on its axis, of geographical position, elevation above the 

 sea-level, and condition of the surface, and by the great mobility 

 of the atmosphere and the ocean. 



The intimate connection between cKmate and local geographi- 

 cal conditions is everywhere apparent ; nothing is more striking 

 than the great differences between neighbouring places where 

 the effective local conditions are not alike, which often far surpass 

 the contrasts attending the widest separation possible on the 

 globe. Three or four miles of vertical height produce effects 

 almost equal to those of transfer from the equator to the poles. 

 The distribution of the great seas and continents give rise to 

 periodical winds — the trades and monsoons — which maintain 

 their general characteristics over wide areas, but present almost 

 infinite local modifications, whether of season, direction, or force. 

 The direction of the coasts and their greater or less continuity 

 greatly influence the flow of the currents of the ocean ; and 

 these, with the periodical winds, tend on the one hand to 

 equalise the temperature of the whole surface of the earth, and 

 on the other to cause surprising variations within a limited area. 

 Ranges of mountains, and their position in relation to the 

 periodical or rain-bearing winds, are of primary importance in 

 controlling the movements of the lower strata of the atmosphere, 

 in which, owing to the laws of elastic gase?, the great mass of 

 the air and watery vapour are concentrated. By their presence 

 they may either constitute a barrier across which no rain can 

 pass, or determine the fall of torrents of rain around them. 

 Their absence or their unfavourable position, by removing the 

 causes of condensation, may lead to the neighbouring tracts 

 becoming rainless deserts. 



The difficulties that arise in accounting for the phenomena ot 

 climate on the earth as it now is, are naturally increased when 

 the attempt is made to explain what is shown by geological 

 evidence to have happened in past ages. The disposition has 

 not been wanting to get over these last difficulties by invoking 

 supposed changes in the sources of terrestrial heat, or in the 

 conditions under which heat has been received by the eardi, for 

 which there is no justification in fact, in a manner similar to that 

 in which violent departures from the observed course of nature 

 have been assumed to account for some of the analogous 

 mechanical difficulties. 



Among the most perplexing of such climatal problems are 

 those involved in the former extension of glacial action of 

 various sorts over areas which could hardly have been subject to 

 it under existing terrestrial and solar conditions ; and in the dis- 

 covery, conversely, of indications of far higher temperatures at 

 certain places than seems compatible with their high latitudes ; 

 and in the alternations of such extreme conditions. The true 

 solution of these questions has apparently been found in the 

 recognition of the disturbing effects of the varying eccentricity 

 of the earth's orbit, which, though inappreciable in the com- 

 paratively few years to which the affairs of men are limited, 

 become of great importance in the vastly increased period 

 brought into consideration when dealing with the history of the 

 earth. The changes of eccentricity of the orbit are not of a 

 nature to cause appreciable differences in the mean temperature 

 either of the earth generally or of the two hemispheres ; but 

 they may, when combined with changes of the direction of the 

 earth's axis caused by the precession of the equinoxes and nuta- 

 tion, lead to exaggeration of the extremes of heat and cold, or 

 to their diminution ; and this would appear to supply the means 

 of explaining the observed facts, though doubtless the detailed 

 application of the conception will long continue to give rise to 

 discussions. Mr. Croll, in his book entitled "Climate and 

 Time," has recently brought together with much research all 

 that can now be «aid on this subject ; and the general correct- 

 ness of that part of his conclusions which refers to the periodical 

 occurrence of epochs of greatly increased winter cold and 

 summer heat in one hemisphere, combined with a more equable 

 climate in the other, appears to me to be fully established. 



