4fj VELOCITY OF PROJECTILES. 



A^iitiorrjl ma- To prevent any milfake from want of attention in the perfons 



chm^ry tor employed, Col. Grobert has added certain pieces of mechanifm 

 counting tune, - ' > 



& c . to his apparatus, by means of which the weight, when it has 



defcended to a certain point, puts in motion a fecond pendu- 

 lum to count the time, and a fyftem of wheels and pinions 

 connected with the wheel and axle to indicate the number of 

 turns made by it. By fimilar contrivances it difcharges the gun, 

 and flops the pendulum and the counter of the turns at the 

 proper time. Thefe may occafionally be of ufe, but compli- 

 cated machinery is always liable to get out of order, and it 

 may be difpenfed with here, if the obfervers be ever fo little 

 expert and attentive. 

 The motion of It might be fufpected, that the motion of the fir ft difk would 

 the diflcs does cau ( e feme deflection of the ball from its true path before it 

 asihof die bail, reached the fecond. To afcertain this, three fcreens were fixed 

 at equal diflances, the fecond and third being placed before the 

 fix fl: and fecond difks refpectively. Now it is obvious, that the 

 hole in the third fcreen would not be in the fame vertical plane 

 with thofe made in the firft and fecond, if any deviation took 

 place. 

 Experimental A bail being fired through the apparatus thus arranged, a 



P rcot * plumb line was fufpended before the centre of the hole in the 



firft fcreen, and the mod accurate obfervation could not dif- 

 cover any deviation, but that the fame line cut the centres of 

 all the three holes. This experiment was feveral times re- 

 peated with the fame event. 

 This owing to The fact no doubt is, that the extreme fhortnefs of the time, 

 the velocity. (for the femidiameter of the ball is not the forty thousandth part 

 of a fecond palling the difk) does not allow the difk fenfibly to 

 affect the path of the ball ; much lefs can the ball hava any 

 effect on the motion of the difk. 



It may not be amifs to obferve, that the difiance of the far- 

 theft fcreen being about twelve yards only, the inflexions ob- 

 ierved by Robins in diflances of a hundred yards were not likely 

 to take place. 



Fad. 



