EXPERIMENTS ON IMPACT 89 



contained a complete record of the whole motion of the impinging block ; and 

 from this record all the numerical data of the experiment could be obtained, 

 such as the successive heights of rebound, the time of duration of impact, and 

 the amount of compression of the substance on which the block fell. 



To be able to measure time intervals it was necessary to have a definite 

 time record impressed on the revolving plate. This was simply effected by 

 means of a tuning fork delicately adjusted so that a tracing point attached to 

 one end traced out a sinuous curve on the plate concentric with the curves 

 traced out by the impinging block. 



Although originally undertaken with the aim of determining the resilience 

 of rubber and guttapercha, the experiments were not confined to these golf 

 ball materials. The first series of experiments dealt with the impact proper- 

 ties of plane tree, cork, vulcanite, and vulcanised indiarubber ; and in the 

 second series lead, steel, glass, new native indiarubber, and various kinds of 

 golf balls were added to the list of substances experimented with. 



The results embodied in these papers are of the highest physical interest. 

 Among the practical applications we may mention Tait's estimates of the 

 duration of impact of a hammer and nail (croooa sec.) and of the time-average 

 force (300 lb.-wt.). As regards the golf ball problem which suggested the 

 experiments, the very much smaller speeds of approach attainable in the 

 experiments render the data not very directly applicable. But it was estimated 

 that the time-average of the force during the collision (which may have lasted 

 about O'OO5 sec.) of the golf club and ball must be reckoned in tons' weight. 



Closely connected with the golf ball enquiry were the ballistic pendulum 

 experiments, described in the second paper on the path of a spherical projectile 

 (1896, Set. Pap. Vol. ii, p. 371). The final type of pendulum used was a 

 bifilar suspension with a bob formed by a long horizontal bar oscillating in 

 the plane of the bifilar. The one end of the bar was faced with clay, and 

 into this soft material a golf ball was driven from a "tee" a few feet away. 

 The momentum of the ball at impact was transferred to the pendulum and 

 ball together, and could be easily estimated in terms of the distance through 

 which the bob was driven to the extremity of its range. This was observed 

 directly by an observer who was protected from being hit by the ball (should 

 that by any chance miss the clay) by the half-closed door past which the 

 pendulum swung. Tait's son Freddie and other powerful players visited the 

 laboratory and experimented with this form of apparatus. The general con- 

 clusion was that under ordinary conditions a well driven golf ball left the " tee " 



T. 12 



