THE RHYTHM OF MOTION. 267 



obvious examples are furnished by the movements of the 

 ocean and the atmosphere. Marine currents from the equa- 

 tor to the poles above, and from the poles to the equator be- 

 neath, show us an unceasing backward and forward motion 

 throughout this vast mass of water a motion varying in 

 amount according to the seasons, and compounded with 

 smaller like motions of local origin. The similarly-caused 

 general currents in the air, have similar annual variations 

 similarly modified. Irregular as they are in detail, we still 

 see in the monsoons and other tropical atmospheric disturb- 

 ances, or even in our own equinoctial gales and spring east 

 winds, a periodicity sufficiently decided. Again, 



we have an alternation of times during which evaporation 

 predominates with times during which condensation pre- 

 dominates: shown in the tropics by strongly marked rainy 

 seasons and seasons of drought, and in the temperate zones 

 by corresponding changes of which the periodicity, though 

 less definite, is still traceable. The diffusion and precipita- 

 tion of water, besides the slow alternations answering to dif- 

 ferent parts of the year, furnish us with examples of rhythm 

 of a more rapid kind. During wet weather, lasting, let us 

 say, over some weeks, the tendency to condense, though 

 greater than the tendency to evaporate, does not show itself 

 in continuous rain ; but the period is made up of rainy days 

 and days that are wholly or partially fair. Nor is it in this 

 rude alternation only that the law is manifested. During 

 any day throughout this wet weather a minor rhythm is 

 traceable; and especially so when the tendencies to evapo- 

 rate and to condense are nearly balanced. Among moun- 

 tains this minor rhythm and its causes may be studied to 

 great advantage. Moist winds, which do not precipitate 

 their contained water in passing over the comparatively 

 warm lowlands, lose so much heat when they reach the cold 

 mountain peaks, that condensation rapidly takes place. 

 Water, however, in passing from the gaseous to the fluid 

 state, gives out a considerable amount of heat; and hence 



