184 REPORT — 1875. 



place, including the condensation of tlie ocean, which must have long continued in 

 a state of ebullition, or bordering on it, surrounded by an atmosphere densely 

 charged with 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 

 that arise in connexion 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 distri- 

 bution of organized beings, are dependent on other causes, to a consideration of 

 which I now proceed. 



The natm-al phenomena relating to the atmosphere are often exti-emely 

 complicated and difficult of explanation; and meteorology is the least ad- 

 vanced of the branches of physical science. But sufficient is known to indicate, 

 without possible doubt, that the primary causes of the great series of pheno- 

 mena, 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 revolution 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 connexion between climate and local geographical 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 con- 

 tinents give rise to periodical winds — the trades and monsoons — which main- 

 tain 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 tlie.se, with the periodical winds, tend on the one hand to equalize the 

 temperature of the whole sm-face of the earth, aud on the other to cause surprising 

 variations withiu a limited area. Ranges of mountains, and their position in 

 relation to the periodical or rain-bearing winds, are of primary importance in con- 

 trolling the movements of the lower strata of the atmosphere, in which, owing to 

 the laws of elastic gases, the great mass of the air and watery vapour are concen- 

 trated. 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 then- unfavom-able 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 of 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 dis- 

 position 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 earth, for which there is no justification in fact, in a manner 

 similar to that in which violent departures from the observed com-se of natm-e 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 temperatm-es 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 comparatively few years to which 

 the afiau'S of men are limited, become of great importance iu 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 natm-e to cause appreciable differences 



