228 Royal Society : — Messrs. Balfour Stewart aud Tait on 



conclusion that this effect is due to rotation under the earth's mag- 

 netic force. 



It would therefore appear to be proved that in these experiments 

 the increased temperature of the aluminium disk is not due to rota- 

 tion under the earth's magnetic force. 



1 9. It might perhaps be said that the heating of the disk may be 

 due to heat conducted from the bearings into the disk and then 

 distributed outwards ; and this conjecture will require to be examined, 

 since the bearings are, no doubt, heated by friction during the mo- 

 tion. This heating effect on the bearings was measured by means 

 of a very delicate thermometer, which was inserted into a small hole 

 in the bush through which oil is supplied to the spindle, and made 

 to be in metallic contact with the sides of this hole ; the mean of 

 three observations made the heating effect at the spindle due to ro- 

 tation to be 4° Fahr. 



In the next place, the aluminium disk, separated from its metallic 

 spindle by the ebonite washer, and in every respect the same as 

 when made to rotate, had its spindle heated artificially by a mercury 

 bath, generally at least 40° Fahr. above the previous temperature. 

 After the lapse of two minutes the effect upon the pile was hardly 

 perceptible — not more than five divisions. 



(1) It would appear from this experiment that the heating effect 

 observed 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 pro- 

 duced 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. Supposing 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 *01 1 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, substitute 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 



