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X. Bakerian Lecture. — On the Mechanical Equivalent of Heat. 
Bj Professor Osborne Eeynolds, F.R.S., and W, H. Moorby, M.Se., late Fellow 
of Victoria University and 1851 Exhibition Scholar. 
Received March 10,—Read May 20, 1897. 
[Plates 3-8.] 
PART I. 
On the Method, Appliances and Limits op Error in the direct Determination 
OF THE Work expended in raising the Temperature of Ice-cold Water to 
that of Water boiling under a pressure of 29-899 inches of Ice-cold 
Mercury in Manchester.— By Osborne IIeynolds. 
The Standard of Temperature for the Mechanical Equivcdent. 
1. Ilie determination by Joule, in 1849, of the expenditure of mechanical effect 
(772-69 lbs. falling 1 foot) necessary to raise the temperature of 1 lb. of water, 
weighed in vacuo, 1° Fahr. between the temperatures of 50° and 60° Fahr. (at 
Manchester), together with the second, in 1878, 772-55 ft.-lbs., to raise the 
temperature of 1 lb. (weighed in cacuo) from 60° to 61° Fahr., at the latitude of 
Greenwich, established once for all the existence of a physically constant ratio 
between the work expended in producing heat and the heat produced ; while the 
extreme simplicity ol his methods, his marvellous skill as an experimenter, and the 
complete system ol checks he adojited, have led to the universal acceptance of the 
numbers he obtained as being within the limits he himself assigned (1 foot), of the 
true ratio of work expended in his experiments in producing heat and the heat 
produced as measured on the scale of the thermometer on which he sjDent so much 
time and care. 
The acceptance of J = 772, as the mechanical equivalent of heat, amounts to the 
acceptance of the scale between 50 and 60 on Joule’s thermometer 5 as the standard 
of temperature over this range. 
Joules thermometers are nowin the custody of the Manchester Literary and 
Philosophical Society (having been conhded to its care by Mr. A. Joule) ; so that 
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