406 Dr. J. Shields on a Mechanical Device for 



characteristic period r for conduction of heat in a given gas, 

 and let t m denote the period relating, for the same gas, to 

 internal friction. The coefficient of conductivity in Fourier's 

 equation, as usually written, is ^c v k, k denoting the same 

 quantity as in § 19. Now in the Kinetic Theory of Gases it is 

 shown that this coefficient is equal to 



Tfo- 1 V» W 



if 7 be written for the ratio c Jc v of specific heats, and fi for 

 the coefficient of viscosity [for example, see Prof. Boltzmann's 

 Vorlesungen iiber Gastheorie, equations (238), (54), and (57)]. 

 Hence 



T E = I(7-1>M 0r =! T M> ' • • ( 2 ) 



since, strictly speaking, our calculation requires the gas to 

 be monatomic. In a similar manner may all the periods of 

 relaxation, corresponding to the various powers of coercion 

 of a given body, be mutually connected ; and every such 

 simple equation, if it holds, is equivalent to a definite physical 

 law. 



XLII. A Mechanical Device for Performing the Tempera- 

 ture Corrections of Barometers. By John Shields, 

 D.Sc, Ph.D.* 



THE height of the barometer is generally reduced to 0° C. 

 by means of the formula 



Vi --R 1+ ^ 



where B is the reduced height, B^ the observed height at the 

 temperature t, and ft and 7 the coefficients of expansion of 

 the mercury and scale respectively ; or, since fi and 7 are in 

 general both very small, we may write 



B„=B,[1- (5-7)0- 



In order to facilitate the reduction, tables containing the 

 corrections corresponding to definite temperatures and observed 

 heights have been compiled, and in the laboratory it is only 

 necessary to consult such a table, and if necessary perforin a 

 simple interpolation in order to find the correction which 



* Communicated by the Author. 



