HUTCHISON'S OBJECTIONS, WITH REPLIES. 455 



the atmosphere in the warmest latitudes of the earth, dur- 

 ing the rainy season, in order to supply upper currents then 

 diverging from these latitudes towards the north and south. 1 

 These causes are assisted, 1st. By the gradual intermix- 

 ture of different strata of air when the upper strata become 

 colder and superficially heavier than those underneath, 

 after making the necessary allowance for the reduction of 

 temperature, and diminution in the specific gravity of the 

 aerial strata, which results on ascending from the diminish- 

 ing incumbent pressure. 2 And it does not matter, whether 



1 If this was the case, the whole torrid zone would be covered with eternal 

 cloud. 



2 If cold air comes down from a height sufficient to double its density at the 

 surface of the earth, its temperature would be increased about ninety degrees, 

 and it would be capable of containing about eight times as much vapor as it 

 contained before it commenced its descent, even if it had been saturated ; 

 and the more it mingled with air in descending, the drier it would make it 

 all which is known by experiment. And if heated air goes up, it is also 

 known by experiment that it will condense more than one half its vapor by 

 the cold due to diminished pressure, before it reaches sixty hundred yards 

 high ; and that too without mingling with the air on the outside of the as- 

 cending column. It is also known by experiment, that the vapor thus con- 

 densed, if the dew point is at 70 Fahr. (about a mean in our summer at Phi- 

 ladelphia), would give out as much caloric of elasticity into the air, where 

 the cloud was formed, as would be given out by burning upwards of twelve 

 thousand tons of anthracite coal on each square mile over which the cloud 

 extends. And this would expand the air between six and seven thousand 

 cubic feet for every cubic foot of water generated in the cloudj after making 

 the allowance for the diminished volume due to the condensation of the vapor 

 itself. There fell in twenty-two hours, at Ardeche, in France, thirty-one 

 inches of rain, and it may easily be calculated that the caloric of elasticity 

 given out during this time was sufficient to heat the whole atmosphere over 

 the region where the rain fell to the above depth, 280 Fahr., provided no 

 allowance is made for .increased specific caloric of the air at great heights. 

 The explanation of this astonishing phenomenon is too plain, according to my 

 doctrine, to need any elucidation, except the simple statement, that a cold of 

 about 90 is generated in every portion of air which rises high enough to 

 become of half the density which it had at the surface of the earth. Nor is it 

 difficult to find a sufficient amount of vapor to produce the amazing quantity 

 of rain, which fell over the very limited region at Ardeche ; for though the 

 whole atmosphere, over the space where the rain fell, did not contain at any 



