Intelligence and Miscellaneous Articles, 483 



plates separate partially by bulging from each other, in opposite 

 directions, leaving a hollow between them like a double convex 

 lens, the centre of which corresponds to the hole formed by the ball ; 

 the circular aperture due to the passage of the projectile attains 

 more than 10 centims. diameter. 



With thick clay plates the aperture has the shape of a truncated 

 cone, but, contrary to what happens in the case of indefinite media, 

 the base of the cone is on the side at which the ball emerges. 



In large blocks an ellipsoidal or double conical shape is met with, 

 and at a small depth the figure is met with deduced from the able 

 analysis and the celebrated experiments of MM. Didon, Probert, and 

 Morin. 



The general shape and the greater or less degree of acuteness of 

 the cone appear to depend on the density of the projectile. I have 

 even produced cavities resembling a semiellipsoid of revolution, when, 

 for instance, wooden balls were used, which burst. 



I am indebted to the kindness of M. Regnault for having been 

 able to repeat the experiments, originally made on common clay, 

 with a mass of Sevres porcelain prepared for hardening in the fire. 

 I may be allowed to express my obligation to the able director of the 

 Imperial Manufactory of Sevres. 



I have lately examined the shape of the aperture produced by 

 lightning in a glass pane 4 millims. in thickness, and have resumed 

 on this subject experiments which I made long ago. The results 

 of these experiments are completely opposed to what is assumed with 

 regard to the phenomena of the communication of motion*. 



Hundreds of experiments, in fact, without exception, give the 

 following results : — 



(1) When a pressure is exerted on the centre of a square gradu- 

 ally increasing until it is broken, a certain number of slits are made 

 which radiate around the centre of pressure ; these slits are gene- 

 rally in almost straight lines. 



(2) A cork bullet fired by a strong charge of powder against a 

 pane coated on one side with gummed paper produces slits analogous 

 to the preceding; these are sometimes intersected by slits, which in- 

 dicate that the motion commenced at the point struck. 



(3) If from the same pistol and with a small charge of powder a 

 leaden bullet be fired against a pane suspended by metal wires, the 

 glass is perforated and broken as in experiment No. 1. 



(4) Other things being equal, if the charge of powder be increased 

 to about 0*2 grm., the ball traverses the square, producing a greater 

 or less aperture surrounded by short and almost rectilinear slits ; 



* The following- extract is taken from M. Delaunay's excellent Traite 

 de Mecanique Elementaire (vol. i. p. 156, 3rd edition): — "A lead bullet 

 gently projected against a window-pane would be repelled without produ- 

 cing fracture. If thrown strougly by the hand, it would pass through the 

 square, producing a large number of slits which radiate round the hole 

 through which it has passed. But if the bullet is fired from a gun, it will 

 merely make a round hole in the pane ; the rest of the pane will be un- 

 changed." 



