Rain. 335 



Some of these general laws I now proceed to consider. 



In the first place, it may be accepted as generally true that 

 the amount of moisture present in the atmosphere is greatest near 

 the equator, and diminishes towards the poles. With the sun's 

 change of declination the zone of greatest moisture passes to the 

 north or to the south of the equator, following the sun. The 

 mean region, it is to be noted, is not absolutely coincident with 

 the equator, but some four or five degrees north of that circle. 

 It is easily intelligible that the hottest regions shouldbe, cceteris 

 paribus, those over which the amount of moisture present in 

 the atmosphere is greatest, since the heat vapourises the water 

 over these regions. It may not seem, at first sight, quite so 

 obvious that the same regions of greatest heat should also be 

 those in which the rainfall should be in general heaviest. For 

 it might appear that the same heat which produced the eva- 

 poration should maintain the water in the state of vapour. The 

 fact, however, that aqueous vapour is lighter than air, operates 

 to produce ascending currents over the region of evaporation, 

 currents strengthened by the expansive effects of the heat. 

 Accordingly, the vapour rises rapidly, and when it has thus 

 risen, many circumstances operate to produce precipitation. 

 First, the upper regions are rarer ; secondly, they are colder ; 

 thirdly, radiation of heat takes place rapidly from the upper 

 surface of clouds, brought here, as Tyndall expresses it, into 

 the presence of pure space {dry air having scarcely any appre- 

 ciable effect in checking radiation) . The result is, that the up- 

 lifting of clouds under the sun's influence is followed regularly 

 over the equatorial regions by the precipitation of heavy rain- 

 showers. And cceteris paribus, the fall of rain decreases with 

 distance from the equator of heat, though not so regularly as 

 the amount of moisture decreases. 



The next great law which presents itself to our considera- 

 tion is this, that winds blowing towards the equator are, in 

 general, dry winds, and winds blowing from the equator rainy. 

 This law is the direct consequence of the former, but it is 

 necessary, for several reasons, to present it as a separate law. 

 There is an erroneous method of accounting for this law which 

 is very commonly met with in works on meteorology. It is 

 argued that as winds blowing towards the equator are carrying 

 masses of air from colder to warmer regions, they are neces- 

 sarily dry winds, since, if the air is saturated, or nearly so, at 

 starting, it cannot be saturated when it has become warmer. 

 And vice versa, winds blowing towards the poles are carrying 

 masses of air to colder regions. The air accordingly grows 

 colder, and if not far from being saturated at starting, cannot 

 fail to become unable to keep its whole burden of moisture in 

 the state of vapour, and must accordingly precipitate a portion 



