INTRODUCTION. 



XV11 



is sometimes very dry, and as much of this air goes in below the 

 base of the cloud and up with the ascending column, large por- 

 tions of the air in the cloud may thus not be saturated with vapor, 

 and, of course, rain in this case will not be produced. Professor 

 Stevelly, of Belfast, told the author, that he knew that clouds are 

 sometimes not saturated. These are some of the means contrived 

 by nature to prevent up-moving columns from increasing until rain 

 would follow. Without some such contrivances, it is probable that 

 every up-moving column which should begin to form cloud when 

 the dew point is favorable, would produce rain, for as soon as 

 cloud forms, the up-moving power is rapidly increased by the evo- 

 lution of the caloric of elasticity. 



On the leeward side of very lofty mountains, there cannot be 

 rain ; for as the air on the windward side rises up the sides of the 

 mountain, it will condense all the vapor which can be condensed 

 by the cold of diminished pressure, before it reaches to the top, 

 and even if cloud passes over the top to the other side, it would 

 soon disappear, because in passing down the slope it will come 

 under greater pressure, and thus be dissolved by the heat produc- 

 ed. These are some of the causes which prevent rains at particu- 

 lar times and in particular localities. If, however, the air is very 

 hot below, with a high dew point, and no cross currents of air 

 above to a great height, then, when an upmoving current is once 

 formed, it will go on and increase in violence as it acquires per- 

 pendicular elevation, especially after the cloud begins to form. 

 At first the base of the cloud will be flat; but after the cloud be- 

 comes of great perpendicular diameter, and the barometer begins 

 to fall considerably, as it will do from the specific levity of the 

 air in the cloud, then the air will not have to rise so far as it did 

 at the moment when the cloud began to form, before it reaches 

 hi^h enough to form cloud from the cold of diminished pressure. 

 The cloud will now be convex below, assuming 'successively the 



