ON THE MECHANICAL EQUIVALENT OF HEAT. 563 



Two very large thermometers about a yard long were specially made, 

 having twenty-five inches to fifty degrees, or half an inch to a degree, 

 and these were used throughout for taking the temperature of the cold 

 inflowing water and the hot outflowing water, whilst other thermometers 

 were used throughout the outside tank to enable it to be kept to the 

 same temperature as the outflowing water. The temperature of the out- 

 flowing water was of course taken immediately as it flowed out from the 

 Fronde's dynamometer, not at the waste. The waste water was carefully 

 taken at short given intervals and weighed (not measured). Several 

 careful observers took observations continually : one took the revolutions 

 of the engine per minute and the total revolutions by a counter that was 

 always going, and registered every revolution throughout the day ; 

 another observer took the weight lifted by the dynamometer ; another 

 the temperature of the inflowing water ; another that of the outflowing 

 water ; and another the general temperature of the tank ; whilst one in 

 command watched the whole, and saw that everyone kept his register 

 closely. 



Before entering on the calculations and results obtained, it will prob- 

 ably be more interesting if the apparatus is first described, and it is to 

 be understood that the object aimed at was to employ continuously a laro-e 

 amount of power (viz., about 5 horse-power) and heat a very considerable 

 quantity of water per minute (viz., about" a gallon a minute) to a con- 

 siderable extent (viz., about 20° Fahr.), whilst all efiects of radiation and 

 conduction were neutralised as far as possible. The ' Froude Dynamo- 

 meter ' is shown in elevation and end view, and the lever connected with 

 it, with its rod and scale, for the reception of the weights to be lifted. 

 B is a tank surrounding the 'dynamometer ' ; C is an outer tank surround- 

 ing the inner tank : this is well clothed outside with three thicknesses of 

 hair-felt. 



D is a small steam-pipe to keep the outer tank up to the temperature 

 of the inner tank and dynamometer. 



The water to be heated is passed into the dynamometer through the 

 india-rubber inlet pipe I, which is itself jacketed with water of the same 

 temperature as the inflowing water ; the pipe O is the outlet pipe where 

 the hot water flows out from the dynamometer. The power for driving 

 the dynamometer is communicated through the shaft S, and a piece of 

 wood is introduced between the flanges of the coupling in order to pre- 

 vent the communication of heat either way, though the temperature of 

 this shaft is kept up by the water in the outer tank. 



Thermometers were placed throughout the apparatus to enable it to 

 be kept at an even temperature. 



It will at once be seen how completely loss or gain of heat was pre- 

 vented, as the temperature of the inner tank was the same as the outflow- 

 ing hot water from the outlet pipe O, and the hot water from it flowed 

 into the outer tank, which had a very small quantity of steam to keep it 

 to the temperature of the hot water from the outlet pipe O. 



Thus the outer tank was, so to speak, ' down stream,' and, even if its 

 temperature varied a little, it is impossible to conceive that it could prac- 

 tically affect the temperature of the hot water coming out of the dynamo- 

 meter, especially as the quantity passing continually was very great, and 

 had thus full command over the temperature of the inner tank. 



This it was that enabled the apparatus to be kept in a normal state 

 for many hours together, and from which results might be obtained for 



