60 Prof. Potter on the Fourth Law of the Relations of the 



Taking a= -^—, as given by M. Rudberg's experiments, for 

 Fahrenheit's degrees of temperature, we have 



1 _ t - 1 _ 1±^ '° - < t0 ~ t,D ) 

 and 



Applying this formula to an experiment made on the 25th of 

 January, 1854, when the barometer stood at 30*30 inches and the 

 thermometer at 58 0, 5, we have as follows : — By a stroke of the 

 piston performed in one second with a large receiver and New- 

 man's air-pump, the mercury in the barometer-gauge rose, on the 

 average of seven experiments which differed only slightly, to 

 2*64 inches, and falling gradually during some minutes rested at 

 2*01 inches; then 



f = 58°-5-32° =26°-5 

 p =30-30— 2-01 = 28-29 inches of mercury 

 p' =30-30-2-64=27-66 „ „ 



and 



= 520 ' 5X 2-S> 

 = ll°-59. 



Now Poisson's assumption was that the temperature 116° 8 

 Centigrade, or 209° $ nearly Fahrenheit, would be lost by an 

 expansion B ; and in the above experiments we should have 



o.rn 



209 ° x H= 14 °- 85 > 



which differs a little from the experiment, in which we have 

 reason to expect a loss of heat. 



In other experiments with the same receiver, the air dried by 

 passing through a V-tube with pumice moistened with sulphuric 

 acid, the results gave 



* -/' = 13°-5, 



and Poisson's formula 14°-8 j which are closer than the former. 

 When smaller receivers were used and more rarefaction pro- 

 duced by the single stroke of the piston, the results did not so 

 well accord. In one set of experiments 



*°-*'°=29°'4 

 was found, whilst the formula gave 54°*3. 



