Manchester Memoirs, Vol. xlii. ( 1 898), No. 0. 1 1 



TJie development of the thermal measurements. 



The appliances were originally designed in 1887, for 

 the purpose of the study of the action of steam in the 

 engines and certain problems in hydraulics and dynam- 

 ometry, without any intention of their being used for the 

 purpose of determining the heat-equivalent of the work 

 absorbed. It was obvious that, as the measured work was 

 all spent in heating water, it was only necessary to 

 measure the change in temperature and quantity of the 

 water used to obtain an approximate estimate of the heat- 

 equivalent, but the recognitition of the extreme difficulty 

 of obtaining an}' first-hand assurance as to the accuracy 

 of scales of thermometers, and the fear of creating 

 erroneous impressions as to the value of the equivalent, 

 prevented the making of any provision for the introduction 

 of thermometers in the first instance. 



But after the engines and brake had been in use for 

 two years, and had been found to possess attributes in 

 steadiness of running and delicacy of adjustment and 

 balance beyond expectation, and particularly to be able to 

 work with an almost absolutely steady current of water 

 through the brake, doing steady work whatever the speed 

 and load, the author recognised that, by working two trials 

 with the same thermometers, on the same parts of the scales, 

 and with the same loads and the same temperatures of 

 water, but at different speeds, since the relative balance of 

 the brakes would be the same, the difference of the results of 

 the two trials, made under the same surrounding tempera- 

 tures, would afford the means of determining the loss of heat 

 by radiation, and, this being known, the differences of two 

 trials, both made at the same water temperatures as the pre- 

 vious, and both at the same speed but with different loads, 

 would afford data for determining the error of balance, 

 without introducing the value of the equivalent, or the 

 scales of the thermometers except to identify equal 



