400 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



273. From this cquatiou, it follows that at the surface of the eartb, or 

 p = 7G0 millimeterSj a cliauge of temperature of 0O.104 C, corresponds 

 to a change of pressure of 1 millimeter, so that a barometric fall of 20 

 millimeters corresponds to a cooling of 2^.1 C. But since such a fall in 

 pressure requires with us, under the most favorable case, an interval of 

 twenty-four hours, therefore are the changes of temperature resulting 

 from this cause completely hidden by greater changes due toother causes, 

 and I do not think it will be possible to demonstrate them. AYhen, 

 therefore, Sir John Herschel considers the rapid fall in pressure as a 

 very frequent inducement to the formation of precipitations, which latter 

 he calls barometric fogs, and minutely characterizes them at page 93 of 

 his Meteorology, he therein evidently overestimates the fall of temper- 

 ature due to this source. Ou the other hand, in the tropical cyclones, 

 in which the air flows into a space wherein the pressure has been dimin- 

 ished by from 40 to GO millimeters, sensible precipitations due to this 

 cause can occur. The fundamental equation on page 23 can be written 



thus: 



dp _c J (U 



y ~ IF ■ T • 



The integration of this equation gives, if by^>, T, and by^', T, we 

 designate the values of pressure and absolute temperature that belong 

 together, 



log ' '--' 



or 



> p' K * \T 



p' " \T'J ' 



since we have, as before shown, 



_R 



then is 



c Jc 



where 



or the ratio of the two specific heats of air ; therefore, 



or 



p'~\'i'j 



h k 



i>' ~ V2730 -\-t>J ~ VI + a t'J 



But this is the well-known equation of tensions deduced in an entirely 

 different manner by Poisson. The value of 



-r^ = 3.444, or A- = 1.41, 



A.' — L 



