CONDITIONS AFFECTING PRECIPITATION. 195 



per cent by weight ) of certain snbstances, which remain behind in a solid form 

 when the water is evaporated to entire dryness. The nnmber of the elements 

 which have been detected in the ocean is very large (somewhat over fifty 

 have been already proved to thus e.xist); but much the larger portion of the 

 material in solution in the ocean-water is made up of a few combinations 

 which are abundantly distributed through the earth's crust, and at the same 

 time extremely soluble. Of these common salt is by far the largest in 

 amount and the most important. These soluble combinations are taken up 

 by the water everywhere circulating through the superficial layers of the 

 earth, go with this water wherever it finds its way, and finally reach the 

 ocean, where they remain, as in a permanent reservoir, for an indefinite 

 period. It is only when peculiar conditions isolate a portion of the ocean 

 surfiice from the rest, and this portion becomes evaporated, that the saline 

 substances resume the solid form. This operation has taken place on a grand 

 scale in former geological ages, as is testified to by the existence, in various 

 countries, of deposits of salt several thousand feet in thickness. The fact 

 that the percentage and nature of the substances held in solution in the 

 ocean is ever3Mvhere very nearly the same, is certain proof that the mixing 

 process has been going on for an immense length of time. Of course this 

 uniformity of composition refers to water taken at some distance from the 

 land, where not influenced by the immediate presence of river-water. 



The amount of surface on the earth occupied by water flowing in rivers, 

 or standing as lakes, is by no means inconsiderable as compared with the 

 area of the land, but almost insignificant Avhen contrasted with the vast 

 expanse of the combined oceans. The ocean, then, is the great reservoir 

 from which the moisture to be precipitated upon the earth in the form of 

 snow or rain is necessarily drawn. The larger the area occupied by the 

 ocean, therefore, other conditions remainino; the same, the lara;er on the 

 whole will be the j)recipitation. The sun's heat becoming more intense, 

 and the .area of the ocean being at the same time enlarged, we have a double 

 reason for an increase of the rain-fall ; with a reversal of these conditions, 

 on the other hand, we are warranted in expecting results of an opposite 

 character; while an increase of temperature accompanied by a diminution 

 of ocean surface would be a combination of conditions more or less neutral- 

 izing eacli other, the part contributed by each to the general result being 

 perhaps extremely difficult to make out. 



That the relative area of land and ocean surface is an element of the 



