200 sm. T. HOPKINS ON THE 



vapour is equal to half an inch of mercury, the wind at the 

 time blowing up a valley and sloping sides of a mountain. 

 The lowest stratum of air being 59", the temperature and 

 dew-point would be reduced at a height of 



300 feet to SS" 

 600 „ 67" 

 900 „ 56° 



1,200 feet to 55° 

 1,500 „ 54° 

 1,800 „ 53° 



2,100 feet to 52" 

 2,400 „ 51° 

 2,700 „ 50° 



And all the vapour that existed in the air between the 

 dew-points of 59° and 50° would be successively condensed 

 by the time that the air and vapour reached the height of 

 2,700 feet, and rain, the product of that amount of conden- 

 sation, would be produced at the various heights as the 

 cooling proceeded. 



There is, however, a second process going on under such 

 circumstances as those just described, which, as it modifies 

 the first, it is necessary to notice. When condensation of 

 vapour takes place heat is liberated, and the temperature of 

 the locality is raised. The gases in the part are then warmed, 

 and they expand and ascend to a greater height, where they 

 are further cooled, and where they condense more vapour. 

 So that the vapour is condensed in the first place by the 

 atmospheric mass being forced up the inclined plane of the 

 land, mechanically — as a wind — and, secondly, by the as- 

 cent produced by the heating power of condensing vapour ; 

 and whilst the mass of air and vapour is carried up from 

 both these causes, it is moving forward horizontal as a 

 wind. In the locality, then, the wind moves mechanically 

 towards the upper part of the valley, whilst, from the heating 

 effects of condensation, it is ascending above that part; the 

 condensing vapour will therefore be liable to be carried above 

 the highest part of the land, and the greatest quantity of rain 

 may fall beyond that part. And, further — after the vapour 

 has been condensed, and the rain formed at a certain con- 

 siderable height in the atmosphere, it has to descend from 

 that height, and will be liable, while so descending, to be 



