Resistance of Air below One Thousand Feet a Second. 531 



rupture itself may vary in uniformity at the different screens ; 

 for the wire may stretch before breaking, touch the projectile 

 on one side, or remain in electrical contact after mechanical 

 severance. 



To ensure greater accuracy in this research some novel 

 features have been introduced. The screens are light-beams 

 one hundredth of an inch thick, which do not bend nor offer 

 resistance, and may be cut in one millionth of a second by a 

 projectile of moderate speed. The bullets are light wooden 

 spheres, some solid, others hollow, whose retardation is twenty 

 to forty times greater, and hence as many times more precisely 

 measurable, than that of solid steel ones. The measurements 

 are made indoors in homogeneous still air. 



The general plan of the apparatus is shown in fig. 1. The 



Fig. 1. 



gun is at Gr, and in front of it are eight smoke-screens S-S r 

 for stopping the blast when the gun is fired. The ball, 

 emerging from these screens, passes through still air, cutting 

 three sunbeams, and lodges in a box of cotton. The sun- 

 beams, springing from the mirrors, M M M, pass through the 

 slits, AAA, BBB, and are reflected from the right-angle 

 prisms R RR, to a camera, where they come to focus on a 

 photographic plate P. When the gun is fired the plate falls 

 and the sunbeams trace on it three fine straight lines close 

 together, each line being momentarily interrupted when its 

 tracing sunbeam is cut by the bullet. The positions of these 

 three interruptions serve to calculate the bullet's velocity 

 and resistance. 



The gun is a four-inch brass pipe 7 feet long, with a; 

 toy cannon screwed into its breech for an explosion chamber, 

 and using smokeless powder. The bullets thus far used have 



