On the Mechanical Equivalent of Heat. 



293 



has been shown to prove that no higher degree of tractive force is to 

 be expected from the double-pole method of determining the mag- 

 netisation limit of magnetic substances than has been obtained hy 

 the single-pole method. 



May 20, 1897. 



The LORD LISTER, RR.C.S., D.C.L., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The Bakerian Lecture was delivered as follows : — 



BAKERIAN Lecture. — " On the Mechanical Equivalent of 

 Heat." By Osborne Reynolds, M.A., F.R.S., Professor of 

 Engineering, Owens College, and W. H. Moorby. Re- 

 ceived March 10, 1897. 



(Abstract.) 



The purpose of this research differs essentially from that of any 

 previous research on the mechanical equivalent of heat. In order 

 to diminish the loss of heat by radiation, as well as to obtain the 

 equivalent for water in the neighbourhood of ordinary temperatures, 

 the ranges of temperature over which the previous dynamical 

 measurements have been made are greatly less than the standard 

 interval between the physically fixed points of temperature to which 

 all thermal measures are referred, and so have of necessity involved 

 the use of scales, the intervals of which depend on the constancy of 

 the relative expansions of such substances as glass, mercury, and 

 air. On the other hand, in this research the object has been to deter- 

 mine the mechanical equivalent of the total heat necessary to raise 

 the temperature of water over the standard interval of temperature, 

 and thus to obtain directly the equivalent of the mean specific heat 

 between the freezing and boiling points. 



This undertaking is the result of the occurrence of circumstances 

 which afforded an opportunity such as might not again occur. This 

 consisted in the facilities offered by the appliances which formed 

 the original equipment of the Whitworth Engineering Laboratory, in 

 1888, the more essential of these being an engine of 100 H.P., 

 working one of Professor Reynolds' hydraulic brakes. This brake 

 maintains any constant moment of resistance on the engine shaft, 

 independent of the speed, all the work done being converted into 

 heat, which appears in the rise of temperature of a steady stream of 



