: density air at t 1 +0-003656*. The density of the vapour of 

 water is 0'622 of that of air. Hence, if t be the temperature of the 

 air in centesimal degrees, b its barometric pressure, v the pressure 

 of vapour, both in millimetres of mercury at Cent., the weight in 

 grammes of a litre of air at a place on the surface of the earth at a 

 height z above the mean level of the sea in lat. X, will be 



Regnault finds that in rooms not heated artificially, the pressure of 

 vapour is two-thirds of the maximum pressure corresponding to the 

 temperature (Memorie della Societa Italiana della Scienze in Modena, 

 t. xxv. p. 1). 



The weight of air used in reducing the weighings was calculated 

 from the above expression. 



The mean rate of expansion of brass, for 1 Cent., from Cent. 

 to 100 Cent., usually assumed 0-0000187 of its length at Cent., 

 is considerably larger than the rate of expansion at ordinary atmo- 

 spheric temperatures, according to the observations of Mr. Sheep- 

 shanks, who found that at about 17 Cent, the coefficient of the 

 linear expansion of brass =0*00001722 for 1 Cent. This value of 

 the expansion has been accordingly adopted. 



The linear expansion of platinum is assumed to be O'OOOOOQOO 

 for 1 Cent., as given by Schumacher in his first table (Phil. Trans. 

 1836). The expansion of water is calculated from a mean of the 

 experiments of Despretz, I. Pierre and Kopp, corrected for the error 

 of the assumed expansion of mercury by Regnault' s observations, 

 and assuming the temperature of maximum density to be 3 0- 945 

 Cent., in accordance with the result obtained by Messrs. Playfair 

 and Joule. The logarithms of the expansion to 7 places considered 

 as integers, are given with sufficient accuracy, between 4 Cent, and 

 25 Cent., by 32'72(*-3-945) 2 -0-215(*-3-945) 3 . 



Though it appears that only two of the nine weights with which 

 U was compared in 1826 and 1829 are in a state of unexceptionable 

 preservation, and that the number of trustworthy comparisons is 

 reduced from 669 to 440, these are amply sufficient for the purpose 

 of ascertaining the weight of U in air (*=65'66 Fahr., 6 = 2975 

 inches). But in order to find the absolute weight of U, or indeed 



