836 Bain. 



as rain. This explanation is insufficient. It would, indeed, 

 be just as reasonable to reverse the argument thus : a wind 

 blowing towards the equator must bring rain ; for as it brings 

 cold air into warm regions, if the air in these regions is nearly 

 saturated, the introduction of cold air must lead to the precipi- 

 tation of a part of the moisture, and vice versa, a wind blowing 

 towards the poles must be a dry, because it is a heat-bearing 

 wind. The simple explanation of the law is, that winds 

 blowing towards the equator are dry, because they are blowing 

 from regions over which moisture is less, to regions over which 

 moisture is more abundant, and vice versa. Of course we must 

 superadd to this the facts mentioned above, because a moist 

 wind blowing towards a heated region would not bring rain 

 with it, and a comparatively dry wind, blowing towards a cold 

 region, might bring rain. But it must not be forgotten that 

 the main question to be considered is the relative moistness of 

 the transported masses of air. 



We meet with corresponding laws affecting the rain-pro- 

 ducing powers of winds travelling over continents and oceans. 

 A wind blowing over an ocean towards a continent brings rain 

 to the continent, unless the heat over the latter exceeds slightly, 

 or at the least, does not fall short of the heat over the neigh- 

 bouring ocean. Such a wind is certain to bring rain to an 

 elevated continental region not protected by a mountain barrier 

 yet more elevated. On the other hand, a wind blowing over a 

 continent towards the ocean in general brings no rain. 



Lakes, marshes, and rivers, act in a small way a similar 

 part towards the adjoining lands as oceans towards neighbouring 

 continents. 



There are circumstances also to be considered as affecting 

 the rainfall in a different manner, viz., not by supplying a 

 greater or less amount of moisture to the atmosphere, but by 

 affecting the power of the atmosphere to keep the moisture it 

 supports in the vaporous state. Snch are the contour and 

 elevation of a country, the nature of its soil, the quantity of 

 forest land, or, wanting this, the relative abundance or paucity 

 of trees, and so on. 



A moist and warm current of air impinging on a mountain 

 range, or even on any well-defined rising slope, so as to be 

 carried with sufficient suddenness to colder and rarer regions, 

 is compelled to part with a large portion of its moisture in the 

 form of rain ; and conversely a wind which has passed over a 

 mountain range or an elevated plateau, and descends to a 

 lower region, appears as a dry wind, unless that region is one 

 over which a continual process of evaporation sufficient to 

 maintain the air nearly in a state of saturation is going on. In 

 this latter case the effects of the descending wind will vary 



