VELOCITY OF PROJECTILES. 4<J 



the difks : in the projectile not traverfing a cylindrical furface, The proje&ile is 

 but two vertical planes, the extent and diftance of which may bo&di&s J 

 be confiderable, and thus give very accurate meafures : and the rotation pre- 

 its being capable, which no other apparatus is, of meafuring J^ lts the fccond 

 the velocities of balls of different lizes projected at different oppofite that on 

 elevations. the firft P afte - 



... . i _.-,. . . . board. 



All that is neceffary in uhng this apparatus is to give a uni- Method of ufmg 

 form and known angular velocity to the. difks; and to meafure tlle inftrument. 

 thearccomprifed between two planes paffing through the axis 

 of the difk, and one of them through the hole in one difk, the 

 other through the hole in its oppofite. 



In the trials made, the motion became fenfibly uniform, Th e defcent of 

 when the weight arrived nearly in the middle of the vertical W eiStbecome« 

 fpace it had to traverfe, as was found by twice meafuring the uniform, 

 times of the third and fourth quarters of the defcent, and after- 

 wards comparing thefe times with the corresponding fpaces 

 pa (Ted through. An excellent ftop-watch by Lewis Berthoud, 

 and another by Breguet, were ufed for this purpofe. 



In moft of the experiments the vertical fpace paffed through The f p ace of 

 by the weight was meafured by the turns and parts of turns mea f ure d by 

 of the cord wound off in a given number of feconds, as in turns of the 

 all refpects moft accurate and commodious. 



To meafure the arc a fcreen, or pafteboard, was fixed before Method of mea- 

 each difk, a very little diftance from it, and the hole in the between hole 

 firft difk being brought oppofite to the hole in its corresponding and hole. 

 fcreen, a rod carried through the centre of thefe two holes and , 

 of the hole in the other fcreen which would be oppofite them, 

 mu ft pierce the feconddifkin the plane of the hole in the firft; 

 and the arc comprifed between this point and the centre of the 

 hole in the farther difk would meafure the angle defcribed by 

 the apparatus while the ball was traverfing the length of 

 the axis. 



It is obvious, that the fixed fcreens, which give the abfolute Two fixed 



direction of the path of the ball, afford the means of (hewing t ^ftew a a r ny U de- 



the defeft of parallelifm, if there be any, between this path and fed of paral- 



the axis on which the difks revolve. l f {m Jf**"" 



the path of the 



The gun-barrel was fixed horizontally, parallel to the axis ball and the 

 of the difks] and at fuch a diftance, that the concuffion of the axls * 

 air by the exploiion could not affect the motion of the neareft 

 difk. 



3 One 



