p^ehcity of Projectiles tJirown from Caruion. 221 



The question then is, to measure the velocity of the 

 bullet. 



1st, To impress an angular, uniform, and known velo- 

 city to the system of the axis and the two disks. 



sd. To measure the arc comprehended between the two 

 planes passing throuph the axis, and each of the holes or 

 passages which the bullet has opened through the disks. 



In the experiments which were made, t!ie motion became 

 sensibiv vmiform when the weight had arrived nearly at the 

 half of the vertical space which it traversed : this was ascer- 

 tained by measuring at two periods the iuv,c elapsed during 

 the third and fourth quarters of the fall, and then comparing 

 these times with the corresponding spaces passed over. For 

 these measures we employed two excellent time-pieces that 

 beat seconds; one by Louis Berthoud, and the other by 

 Breguet. 



In almost the whole of the experiments we substituted 

 for the measure of the vertical space passed over by the 

 weight, that of the number of turns and fractions of turns 

 made bv the arbor of the wheel and axle during a givea 

 number of seconds, which in every respect was nmch more 

 precise and convenient. 



Then, to measure the arc passed over by the disks while 

 the ball v.'ent from the one to the other, we placed before 

 each of these disks a screen or fixed piece of pasteboard, 

 which was at a very small distance from it ; so that the ball 

 during its passage first traversed the first screen, then the 

 first disk^ then the second screen, and afte»wards the se- 

 cond disk. When the piece was discharged the hole ef 

 the first disk was brought opposite to that of the first screen, 

 and theae two holes were in the same straigiiL line with that 

 made in the second screen ; a wire, directed horizontally 

 through the centre of the latter hole, pierced the second 

 disk ; and the arc, having its centre in the axis of rotation 

 comprehended between the extremity of that wire and the 

 centre of the hole made by the ball in the second disk, gave 

 the measure of the angle described by the systcju of the 

 two disks, while the ball had passed ever the length of the 

 axis. 



It may be readily seen that the fixed screcnf;, which give 

 the absolute direction of the ball in space, furnish the means 

 of making an allowance for the want of parallelism, if there 

 be any between that direction and the axis of rotation of 

 the disks. 



The cannon employed for throwing the projectile was 

 placed horizon.ally and parallel to the arbor of the dibks, at 



a sufficient 



