398 On Heating of a Disk by rapid Rotation in vacuo. [Dec. 6, 



•which there is a strictly temporary effect, either in the direction of heat or 

 cold, at the surface of the disk, owing probably to condensation or evapora- 

 tion of small quantities of aqueous vapour ; but this effect disappears the 

 moment the motion is stopped, leaving behind the permanent effect ap- 

 parently unaltered. 



(2) This temporary effect disappears when the disk has been left for 

 some hours in the vacuum. 



Next, with regard to vapour of oil, we caimot suppose its effect to be so 

 large or so different in character and constancy from that of aqueous vapour 

 as to account for the effect observed. Add to this that the effect takes 

 place with an uncoated metallic disk probably to the same extent as with 

 a coated one. The same remark may be made with regard to vapour of 

 mercury. The effect would therefore appear to be independent of the che- 

 mical nature of the residual gas and vapour around the disk. 



In order to prove that this effect is also independent of the pressure of 

 the residual gas, it is only necessary to refer to the whole body of experi- 

 ments which have been described, to see that between 4 inches and 0*25 

 inch there is no perceptible variation in the effect observed. 



24. The following generalization may now be made : — 



(1) If a perfectly true aluminium disk (without vibrations) be made to 

 rotate in a vertical plane at the earth's surface, after the manner herein 

 described, there will be an increase of the temperature of the disk, which 

 is not due to communication of heat from the bearings or machinery, nor 

 to the earth's magnetic force. 



(2) This heating effect is independent of the density and chemical con- 

 stitution of the residual air and vapour which surround the disk. 



(3) It is probable that the quantity of heat developed in disks of similar 

 extent of surface and similar circumstances of motion is the same. For, 

 in the first place, the quantity of heat developed in three aluminium disks, 

 '05, '0375, '025 of an inch in thickness respectively, would appear to be 

 the same, the relative thermometric effect for these disks varying inversely 

 as their thickness, and being in the following proportions, 30, 43, 60, as 

 determined by one complete set of experiments. 



Again, the quantity of heat developed in the thick aluminium disk, with 

 its surfaces both uncoated, was probably the same as when one surface was 

 coated and one left bare, or as when both surfaces were coated. 



25. The authors will not attempt here a further generalization, but 

 they would desire to make one remark. In absence of definite know- 

 ledge of the nature of that medium which transmits radiant light and 

 heat, it might be supposed possible that when a radiant body is in rapid 

 motion, the intensity of its radiation is somewhat increased. But if we 

 bear in mind that in these experiments the effect was observed after 

 bringing the disk to rest, and that the temporary effect during rotation 

 sometimes observed can probably be otherwise accounted for, we are 

 forced to conclude that, as far as we may judge from these experiments 



