268 THE RHYTHM OF MOTION. 



the resulting clouds are warmer than the air that precipi- 

 tates them, and much warmer than the high rocky surfaces 

 round which they fold themselves. Hence in the course of 

 the storm, these high rocky surfaces are raised in tempera- 

 ture, partly by radiation from the enwrapping cloud, partly 

 by contact of the falling rain-drops. Giving off more heat 

 than before, they no longer lower so greatly the temperature 

 of the air passing over them; and so cease to precipitate 

 its contained water. The clouds break; the sky begins to 

 clear; and a gleam of sunshine promises that the day is 

 going to be fine. Bflt the small supply of heat which the 

 cold mountain's sides have received, is soon lost: especially 

 when the dispersion of the clouds permits free radiation into 

 space. Very soon, therefore, these elevated surfaces, be- 

 coming as cold as at first, (or perhaps even colder in virtue 

 of the evaporation set up,) begin again to condense the va- 

 pour in the air above; and there comes another storm, fol- 

 lowed by the same effects as before. In lowland regions this 

 action and reaction is usually less conspicuous, because the 

 contrast of temperatures is less marked. Even here, how- 

 ever, it may be traced; and that not only on showery days, 

 but on days of continuous rain; for in these we do not see 

 uniformity : always there are fits of harder and gentler rain 

 that are probably caused as above explained. 



Of course these meteorologic rhythms involve something 

 corresponding to them in the changes wrought by wind and 

 water on the Earth's surface. Variations in the quantities 

 of sediment brought down by rivers that rise and fall with 

 the seasons, must cause variations in the resulting strata 

 alternations of colour or quality in the successive laminae. 

 Beds formed from the detritus of shores worn down and car- 

 ried away by the waves, must similarly show periodic differ- 

 ences answering to the periodic winds of the locality. In so 

 far as frost influences the rate of denudation, its recurrence 

 is a factor in the rhythm of sedimentary deposits. And the 

 geological changes produced by glaciers and icebergs must 



