DR. BUTTON'S THEORY OF RAIN. 445 



Dr. Hutton. He supposes two currents of air of different 

 temperatures, both nearly saturated with vapor, to be min- 

 gled together, and that a precipitation of course takes place, 

 in accordance with the known fact that at their mean tem- 

 perature all their vapor cannot be retained, and therefore 

 the surplus will be precipitated. This theory is defective 

 -in two respects : first, it does not show how two currents of 

 air could be mingled to any considerable extent; and second, 

 it does not show by calculation, that rain, to any considera- 

 ble amount, would be produced, even if large masses of air 

 at very different temperatures should be mingled together, 

 which it would be easy to show never can happen, espe- 

 cially in the torrid zone. It may fairly be presumed that 

 no advocate of the Huttonian theory would suppose that 

 more than five hundred feet of a stratum of cold air could 

 be mingled with a stratum of warm air five hundred feet of 

 perpendicular height. Now, it will be found by calculation, 

 that if one of these strata is at 60, and the other at 40, 

 and both saturated previous to their mixture, the whole 

 amount of precipitation, provided they took the mean tem- 

 perature of 50, would be less than a grain and one half on 

 each square inch of surface. But as the latent caloric 

 evolved in the condensation of the vapor would not suffer 

 the mean temperature of the two strata, when mixed, to be 

 acquired, but some temperature above 50, therefore a less 

 quantity than that mentioned would be precipitated. Such 

 a quantity, in most cases, would be entirely evaporated in 

 passing down through the air below, and never reach the 

 -earth. 



It was mentioned before, that 5.1 inches of rain fell in 

 Wilmington, on the 29th of July, 1834, in two and a half 

 hours ; let us see whether such a rain could be produced at 

 all, on the Huttonian principles, making the most extrava- 

 gant allowance for the quantity of air mingled, and also for 

 the difference of temperature of the two strata. 



