496 



If the space be nearer to the station, or if the difference in den- 

 sity be more than 1-1 00th part, these deflections must be multiplied 

 by a corresponding quantity. 



7. The paper is concluded by a revision of some of the calculations 

 in the former communication. The mass of the mountain region 

 above the level of the plains is shown to be somewhat more than 

 four-millionths of the mass of the earth. 



JIL " On the Thermal Effects of Compressing Fluids." By J. P. 

 JOULE, LL.D., F.R.S. &c. Received October 9, 1858. 



(Abstract.) 

 The author in this paper gives an experimental demonstration of 



the correctness of Professor Thomson's formula, 



^, 

 JK. 



where is 



the thermal effect, T the temperature from absolute zero, e the ex- 

 pansibility by heat, p the pressure, J the mechanical equivalent of 

 the thermal unit, and K the capacity for heat. The fluids experi- 

 mented on were water and oil, with the results tabulated below : 



IV. " Note on Archdeacon PRATT'S paper on the Effect of Local 

 Attraction on the English Arc." By Captain CLARKE, R.E. 

 Communicated by Lieut. -Colonel JAMES, ft.E. Received 

 June 30, 1858. 



The following letter of Colonel James will explain the nature of 

 this communication ; the numerical statements, being not susceptible 



