198 THE POPULAR SCIENCE MONTHLY. 



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 equi- 

 librium, the more active process of aggregation having ceased, and 

 the combination of its elements into the various solid, liquid, or gas- 

 eous matters found on or near the surface having been completed. 

 During its passage to its present state many wonderful changes must 

 have taken place, including the condensation of the ocean, which must 

 have long continued in a state of ebullition, or bordering on it, sur- 

 rounded by an atmosphere densely charged with watery vapor. 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 connection with its present climate, or with the changes dur- 

 ing 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 re- 

 mains, and which have affected the production and distribution of 

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

 of which I now proceed. 



The natural phenomena relating to the atmosphere are often ex- 

 tremely complicated and difficult of explanation ; and meteorology 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, "\aried 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, eleva- 

 tion above the sea-level, and condition of the surface, and by the 

 great mobility of the atmosphere and the ocean. 



The intimate connection between climate and local geographical 

 conditions is everywhere apparent ; nothing is more striking than the 

 great differences between neighboring places where the effective local 

 conditions are not alike, which often far surpass the contrasts attend- 

 ing 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 gives rise to periodical winds the trades and monsoons 

 which maintain their general characteristics over wide areas, but pre- 

 sent almost infinite local modifications, whether of season, direction, 

 or force. The direction of the coasts and their greater or less conti- 

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



