24 PHILOSOPHY OF STORMS. 



This law holds good for a wet bulb as high as 75, and as 

 low as 20, as I have verified by many hundred experi- 

 ments. Near the freezing point, however, great care is ne- 

 cessary that the water should either be all frozen or none. 

 And what is not a little remarkable, I have frequently ob- 

 served when the temperature of the air is only a little above 

 the freezing point, and the dew point much below, that two 

 thermometers would exactly agree at all temperatures be- 

 tween 32 and 27, one being covered with ice, and the 

 other with a wet rag. Lower than 27 the wet rag would 

 always freeze. I may add here, that it is essential to swing 

 or blow the thermometers, for the wet and dry bulbs will 

 always indicate a greater difference when they are blown 

 than when they are in still air. As these experiments are 

 both contrary to the doctrines of Leslie, now considered 

 correct in Europe, I have been at the greatest pains to put 

 them to the strictest scrutiny. 



54. I have now mentioned only a few of the many facts 

 which I have been able to collect in favor of a theory 

 which explains, with the simplicity of the law of gravita- 

 tion, many phenomena which have heretofore baffled all 

 attempts at explanation, and some which have not ever 

 been attempted. Enough, however, have been adduced to 

 establish, beyond the possibility of doubt, the leading fact 

 in the theory, the upward motion of the- air in the region 

 of a cloud at the time of its formation ; and as the explana- 

 tion of this upward motion is founded on facts established 

 by the most careful experiments made by such men as 

 Black, Dalton, Gay-Lussac, Ure, Berard and Delaroche, 

 and Clement and Desormes, and Petit and Dulong, the 

 theory may claim for itself not merely plausibility, but 

 absolute certainty. When the ancients were amusing them- 

 selves by demonstrating the properties of the sections of the 

 cone, they little thought what aid their investigations would 

 afford to the future astronomer. So the chemists, who dis- 



