1 54 Royal Society : — 



hotter than that of England, instead of one actually below the 

 freezing-point. 



Now, during the glacial epoch, when Europe was almost 

 covered with snow and ice, the summers could not possibly have 

 been much warmer than they are at present in Arctic and Ant- 

 arctic regions. In other words, during the glacial epoch the 

 mean summer temperature would be very little above the freez- 

 ing-point. 



[To be continued.] 



XX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. (>S.] 



May '2S, 18GS. — Lieut. -General Sabine, President, in the Chair. 



ri^IIE following communication was read : — 



X "On the Impact of Compressible Bodies, considered with refer- 

 ence to the Theory of Pressure." By 11. Moon, M.A., Honorary 

 Fellow of Queen's College, Cambridge. 



Suppose that we have two rigid cylinders of equal dimensions, which 

 have their axes in the same straight line ; suppose, also, that one of 

 the cylinders is at rest while the other moves towards the first with 

 the velocity V in a direction parallel to both the axes ; the conse- 

 quence of the collision which under such circumstances must take 

 place, will manifestly be that half the momentum of the moving cy- 

 linder will be withdrawn from it, and will be transferred to the cy- 

 linder which originally was at rest. 



The mode in which velocity or momentum will thus be collected 

 from the different parts of the one cylinder, and distributed amongst 

 those of the other, is obvious. Exactly the same amount will be 

 withdrawn from the velocity of each particle of the impinging cylinder, 

 and exactly the same amount of velocity will be impressed on each 

 particle of the cylinder struck. 



And the reason of this is equally obvious, since, if such were 

 not the ease, the particles of each cylinder would contract — a suppo- 

 sition which is forbidden by the very definition of rigidity. 



But if, instead of being perfectly rigid, each cylinder is in the 

 slightest degree compressible, a variation in the effect will occur. 



As before, momentum of finite amount will be transferred from 

 the one cylinder to the other, but the mode of collection of the velo- 

 city withdrawn from the one, and the mode of distribution of that 

 injected into the other, will no longer be the same as before. 



In order that the moving cylinder may not be reduced to absolute 

 rest by the collision, it is obvious that the cylinder originally at 

 rest, or a portion of it, must be moved out of the way, so as to 

 allow of the continuance, even in a modified degree, of the other's 



