Mr Vincent, Experiments on Impact. 339 



mm. across. If however the lead be cooled slowly by leaving it in 

 the iron mould in which it has been melted, the crystals are as 

 much as a cm. in diameter. 



In these experiments with lead the one surface, planed as 

 above and rubbed with a clean cloth, was used throughout any 

 one set of experiments. Results shown by different curves refer 

 to different blocks of lead or to a newly planed surface. 



The character of a leaden surface varies in different parts and 

 thus the averaging of many results is indicated ; but this method 

 is inapplicable, as one would use up the whole surface in deter- 

 mining a single point on a curve. Where indentations occurred 

 near the edges of the surface or near other dents they were not 

 measured. 



The depression which the ball makes in the lead is exactly a 

 cast of the surface of the ball. This was proved by the fact that 

 although the ball would stand without shake in the dent which it 

 had made, yet if a very small grain of dust were introduced into 

 the dent, the ball would not fit exactly but would rock on the 

 speck of dust. 



The curves in Fig. 6 are drawn by plotting the squares of the 

 diameters of the dent in cms. against the velocities of approach. 

 By the diameter of the dent is meant the diameter of its circular 

 edge. This was measured easily in reflected light by a traversing 

 microscope whose vernier read to "01 cms. 



From these curves we see that 



u = bd 2 , 



where d = diam. of dent — 2r, 



u = velocity of approach, 



and b = a constant. 



The volume of a spherical cup is 



rf{a-g, 



where I is the length of the radius R of the sphere, cut off by the 

 plane of section. If the cup is small we may put the volume as 



ttPR, 



r 4 



but P is approximately equal to ^- 2 and thus the volume is 



approximately 



7rr 4 



