294 Messrs. Balfour Stewart and Tait on the Heating [Dec. 6, 



served in rotation cannot be due to heat conducted from the bearings 

 through the ebonite washer, since a temperature difference between the 

 bearings and the disk ten times greater than that produced by rotation 

 causes a heating effect at least six times less than that caused by rotation 

 in a somewhat less time. 



(2) The ebonite washer used to prevent the heat of the bearings from 

 reaching the aluminium disk, is a cylindrical disk, its thickness being two- 

 tenths of an inch, and the area of one of its faces 3*15 square inches. It 

 is shielded behind by a brass disk, of similar size, which brass disk being 

 near the bearings, and metallically connected with them, we may suppose 

 to have the same temperature as the bearings. Thus one face of the 

 ebonite washer is in contact with brass, having the temperature of the 

 bearings, while the other is in contact with the aluminium disk. Sup- 

 posing that this washer, used to protect the aluminium disk from the heat 

 of the bearings, was of iron instead of being of ebonite, we can calculate 

 approximately from Principal Forbes's determinations of the absolute 

 conductivity of this metal, how much heat would be conducted across the 

 washer during the experiment. According to these observations, if a cube 

 of iron whose side is one foot, have one of its faces kept permanently at 

 a temperature 1° C. higher than the opposite face, the quantity of heat 

 conducted across in one minute will be 'Oil unit nearly, a unit denoting 

 the amount of heat required to raise a cubic foot of water 1° C. Since 

 in these observations both the temperature difference and the unit are 

 expressed in the same thermometric degrees, we may, if we choose, sub- 

 stitute degrees Fahr. for degrees Centigrade in the above expression for 

 conductivity. Now, if we assume as an approximation that during the 

 whole experiment of rotation the heat conducted across such a washer 

 is the same as if for one minute the temperature difference between both 

 sides of the washer were kept at 2° Fahr., and if we make allowance for 

 the surface and for the thickness of the washer, we obtain the following 

 expression as approximately representing the heat conducted across the 

 v^rasher during the experiment, 



Heat=-011 X 2 X— X— =-028 unit nearly, 

 144 -2 



where the first factor is on account of the double temperature difference, 

 the second on account of the surface, and the third on account of the 

 thickness. 



But a unit of heat in the above expression denotes the amount necessary 

 to raise a cubic foot of water (or nearly 1000 ounces) 1° Fahr. Now the 

 weight of the disk is 10*5 ounces, and its specific heat is 0*22. Hence the 

 above amount of heat will raise the disk 



•028 X X 1 2° Fahr. in temperature. 



Hence we see that if the material of the washer had been of the metal 

 bismuth, of which the conductivity is 7 tim.es less than that of iron, and if 



