386 PROFESSOR TAIT ON IMPACT, II. 



Finally, to finish as I began, with an application to golf, although, from the nature of 

 the case, the experimental data are not very directly applicable : — we see that, as the 

 coefficient of restitution from wood is about 0'66, and the mass of the ball about O'l lb., 

 the club must be moving at some 300 feet per second to produce an initial speed of 

 500 feet per second : — and the time-average of the force during collision must be 

 reckoned in tons' weight. The experiments on hammered, and on unhammered balls, 

 all made at the same time and of the same material, show clearly how very small is the 

 gain in coefficient of restitution, and therefore in initial speed, which is due to the 

 hammering : — and thus force us to look in another direction for an explanation of the 

 unquestionable superiority of hammered over unhammered balls. 



[It is very curious that the law of force in terms of the distortion (as given above) is 

 the same as that which results from Hertz' investigations. For, what is called D above 

 is the diminution, in length, of the ivhole cylinder operated on ; while, in Hertz' work, 

 the quantity which he calls a, and to whose 3/2th power the force is proportional, is the 

 advance towards one another (since the first contact) made by points chosen in the two 

 bodies, whose distance from the (infinitesimal) surface of contact is finite, yet very 

 small in comparison with the dimensions of the bodies themselves. In my experiments 

 the vertical shortening extends throughout the whole of at least the protruding part of the 

 cylinder, and in extreme cases the distortion is so great that the diameter at the middle 

 becomes more than double that at the ends ; in Hertz' investigations it is assumed to be 

 mainly confined to the immediate neighbourhood of the surface of contact. 



It is even more curious to find that the same law holds, at least in a closely approxi- 

 mate manner, for the very, large and unsymmetrical distortions produced by the ridged 

 base, as shown by the data of 7/11/91. 



Some additional details connected with this investigation, including a sketch of the 

 apparatus and of the trace of 13/7/92, will be found in an article Sur la Duree du Choc, 

 which appeared in the Revue des Sciences pures et appliquees, 30/11/92. ] 



