126 Philosophy of Storms. 



of course no cumuli can be formed on that day, even though the 

 air may be saturated with vapor to such a degree as to condense 

 a portion of it on cold bodies at the surface of the earth. 



Neither can clouds form of any very great size, when there are 

 cross currents of air sufficiently strong to break in two an ascend- 

 ing current, for the ascensional power of the up-moving current 

 will thus be weakened and destroyed. This is one means con- 

 trived by nature to prevent up-moving columns from increasing 

 until rain would follow. Without some such contrivance 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 in- 

 creased by the evolution of the caloric of elasticity. 



If it should be found by observation that an upper current of 

 air is passing from the mountains of Abyssinia over Egypt to the 

 north, while the wind below is blowing from the north towards 

 the mountains of Abyssinia, this would manifestly be one rea- 

 son why it seldom rains in Egypt during the prevalence of this 

 wind, though it comes highly charged with vapor from the Medi- 

 terranean. Besides, it is known that during the prevalence of 

 this wind there are great rains in Abyssinia, and of course if the 

 upper current does flow over Egypt from the south, it would 

 bring in it a large portion of the caloric of elasticity, which it 

 received there, in the great condensation of the vapor as it rose 

 up the sides of the mountains at the head of the Nile ; of course 

 the columns of air rising over Egypt, when they entered that cur- 

 rent would cease to rise, for the temperature of that current would 

 be many degrees hotter than themselves, and therefore they could 

 not swim in it. 



Also, on the leeward side of very lofty mountains, there can- 

 not 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 a 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 produced. These are some of the causes which prevent 

 rains at particular 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 



