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 ita 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 ean 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. 
_Af, 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 
