PKOFESSOE. TAIT ON IMPACT. 229 



photograph. The changes of mass, just alluded to, were occasionally introduced by 

 firmly screwing on the top of the block a thick plate of lead of mass equal to 

 its own. 



3. A very troublesome difficulty was now and then met with, but chiefly when the 

 elastic substance employed was- a hard one, such as vulcanite or wood. For the block 

 was occasionally set in oscillation during the impact, and especially at the instant when it 

 was beginning to rebound. The trace then had a wriggling or wavy outline, altogether 

 unlike the usual smooth record. Sometimes the wriggle took place perpendicularly to 

 the disc, and the trace was then alternately broadened and all but evanescent. After 

 some trouble I found that the main cause was the slight dent (produced by repeated falls 

 on hard bodies) in the striking part of the block, which had originally been plane. The 

 wriggling always appeared when this dent did not fit exactly upon the (slightly convex) 

 upper end of the hard cylinder. To give free play at the moment of impact, the lower 

 parts of the guide-rails had been, by filing, set a very little further apart than the rest, 

 and thus small transverse oscillations of the block were possible. I hope to avoid this 

 difficulty in future, by fixing a hard steel plate on the striking part of the block, and 

 making all the remaining experiments with this. Of course a few of the former experi- 

 ments must be repeated in order to discover whether the circumstances are seriously, or 

 only slightly, modified by the altered nature of the striking surface. There can be no 

 doubt that the distortion, as tabulated, belongs in part to each of the impinging bodies ; 

 but it is not easy to assign their respective shares. 



The general nature of the whole trace of one experiment will be obvious from the 

 upper figure in the Plate, which is reduced to about 0'3 of the actual size. The lower 

 figures (drawn full size) show the nature of the trace during impact : — the first series, some 

 of which exhibit the " wriggles" above described, belonging to the pencil records of the 

 old apparatus ; the second series containing some of those obtained with the improved form 

 just described. 



In the earlier work, with the cartridge-paper, falls of 8 and even of 12 feet were 

 often recorded. The results of the later work have been, as yet, confined to falls of 4 feet 

 at most. But I intend to pursue the experiments much further, after fitting an automatic 

 catch on the apparatus ; such as will prevent the block from descending a second time 

 if it should happen to rebound so far that the needle-point leaves the glass disc. 



What precedes is of course designed to furnish only a general notion of the nature of 

 the apparatus ; the principle on which it works, and the results already obtained with it. 

 Some further remarks, on the physical principles involved, will be made after details of 

 dimensions, and of numerical data have been given. But it must be stated here that with 

 the later form of the apparatus it was found necessary to have a party of at least three, 

 engaged in each experiment ; one to attend to the driving-gear, a second to the falling- 

 block, and a third to the tuning-fork. My assistant, Mr Lindsay, took the first post ; I 

 usually took the second myself; and the fork was managed by Mr Shand, to whom I am 

 besides indebted for the greater part of the subsequent measurements and reductions. 



