S30 Velocity of Projectiles thrown from Ccumon. 



It may be readily conceived that in the hypothesis of devia- 

 tion the hole made by the ball in the third fixed screen 

 ought not to be in the same vertical plane with those made 

 in the first and second screens ; and thus this deviation be- 

 comes easy to be ascertained and measured. 



Several balls were fired, the apparatus bcine arranged as 

 here described. Each tiitie a plumb-line was placed before 

 the centre of the hole made in the first screen ; and taking 

 in a line this thread and the centre of the hole made in the 

 gecond screen, it was very easy to see whether the centre ot 

 the third hole was in the vertical plane containing the other 

 two centres. These observations were made with care and 

 precision, and yel it was not possible to perceive tbo devia- 

 tion estimable in the direction of the line passing through 

 the centres of the three holes : thus the motion of the ball 

 through the moveable disks is sensibly the same as if these 

 disks were at rest. We are of opinion, however, that it 

 livill be useful to employ alwavs three screens, disposed as 

 above, in the experiments made in future on this subject: 

 by these means we mav either ascertain whether there be 

 any deviation, or discover whether it be sensible ; and thei'e 

 can be no doubt that it will be so, either in the easic when 

 small velocities are communicated to the projectile, or, in 

 general, in those where the ratio between thai velocity and 

 the resistance which the bullet experiences in traversing the 

 disks shall exceed certain limits. 



It is proper to add, that the distance of the cannon from 

 the furthest distant screen was about twelve metres, and 

 that there was no reason to apprehend the inflections ob- 

 served by Robins in distances of about a huntlred metres, 

 which according to him render rigorously the trajectory a 

 curve of a double curvature. 



What we have said on the insensible ef}eet which the 

 action of the disk has upon the ball to make it deviate, 

 proves that the re-action of the ball on the disk cannot di- 

 minish its velocity by a sensible quantity : this conclusion 

 might be deduced, besides, Irom other facts, relative both to 

 the mechanism of the apparatus and to the data furnished 

 by the experiments, and particularly bv the nieastire of time 

 before and after the discharge of the piece :-— but it is need- 

 less to enter into further detail.-!. 



To conclude : we are of opinion that the means to mea- 

 sure the. initial velocity of projectiles thrown by fire-arms 

 in directions both horizontal and inclined, proposed by co- 

 lonel Grobert, and confbrmal)Ie to the summary description 

 tniud in the above rcpprt, merit the approbation of the 



class. 



