382 PROFESSOR TAIT ON IMPACT, IT. 



The experiments, whose results are tabulated at the end of the paper, were (with the 

 exception of the first, presently to be noticed) made with a new set of specimens of 

 various elastic substances, considerably larger in all their dimensions than those pre- 

 viously employed. They were, as before, cylinders very slightly rounded at their upper 

 ends ; but their lengths, as well as their diameters, were 56 mm instead of 32 mm as 

 formerly. As I could not procure a piece of good cork of the requisite dimensions, the 

 cylinder of that substance employed was built up of two semi-cylinders, gently kept 

 together by two india-rubber bands. The glass cylinder turned out to be somewhat 

 difficult of manufacture, and the experiments with it are altogether defective. But, after 

 the third, and most considerable, impact to which it was subjected it presented a very 

 interesting appearance. There was formed inside it a fissure somewhat in the shape of a 

 portion of a bell ; meeting the upper surface in a nearly circular boundary 12 mm in 

 diameter. This fissure showed the colours of thin plates in a magnificent manner. It 

 gave the impression that the portion of the glass contained within it had, by the shock, 

 been forced downwards relatively to the rest. Its lower, and wider, extremity did not 

 come within 4 mm of the sides of the cylinder, and this was at a depth of about 6 mm below 

 the upper surface. 



One result of the new experiments is obvious at the first glance. The duration of 

 impact is notably longer than before ; in consequence of the increased dimensions of the 

 elastic bodies operated on. But the coefficient of restitution is only slightly affected. 



As the old block had been split during some experiments in which it was allowed to 

 fall on vulcanite from heights of 3 m and upwards, a new one (also of plane tree) was 

 obtained. The mass of this new block was 3*75 lbs., and (except where it is otherwise 

 specially noted in the tables of experimental results) had its lower end shod with a flat 

 plate of hard steel 6 mm in thickness, and 1 lb. mass. The main object of this was to 

 prevent the " wriggles " formerly noticed. Another plate of the same material, with a 

 blunted wedge-shaped ridge projecting from its lower surface, was occasionally substi- 

 tuted for this (as noted) in some of the experiments on vulcanized india-rubber. It was 

 tried on cork also, but the result was disastrous. 



The object of this ridge was to test the effect, on the coefficient of restitution and on 

 the duration of impact, produced by applying a given momentum of the falling body in 

 a more concentrated form, by restricting the surface-region of its application to the elastic 

 solid. The results obtained by this process, though unfortunately limited to one elastic 

 substance, are very interesting. The duration of impact is notably increased, in spite of 

 the increased distortion ; but the coefficient of restitution is practically unaltered. 



The first set of experiments given below (7/4/91) were made with the old cylinder of 

 Vulcanized India Rubber. They were designed to form a link between the present 

 experiments (with the steel plate) and the former set (in which the impinging surface was 

 hard wood). 



Mr Shand has again made the measurements of the traces, and reduced the observa- 

 tions, precisely in the same manner as before : — and it will be seen, from the numbers in 



