MECHANICS AND USEFUL ARTS. 93 



and one of the pencils stops marking, and so on with the remaining 

 targets ; and by means of a pendulum the records are reduced to fig- 

 ures, which give the initial velocity of the shot, at various parts of its 

 flight, which is of the utmost importance, as it includes the resist- 

 ance of the air, and affords practical data for the most correct calcula- 

 tions through larger nights. The resistance of the air has always been 

 estimated from certain known laws, but now it may be determined by 

 practical experiments. It is not expected that the instrument will 

 register correctly beyond seven hundred or one thousand yards. 



Illustration of the Movement of Projectiles in Vacuo. The fol- 

 lowing is a description of an invention by Col. Fox, of the British 

 army, intended to demonstrate the parabolic theory of projectiles in 

 vacuo. The initial velocity being taken at 107 yards per second, a 

 bar is provided with movable wires and beads at the extremities, the 

 length of which increases as the square of the times ; on placing the 

 bar, which represents the plane of the direction of the shot, at any 

 required angle, a beautiful parabola is produced. The instrument 

 shows at any elevation the range of the projectile ; for example, at 

 20 elevation the range is 700 yards, at 30 920 yards, 45 greatest, 

 when it registers 1050 yards. If the angle is still further increased, 

 the range diminishes in proportion, showing that forty-five degrees is 

 the maximum elevation for the greatest range. The apparatus is 

 also capable of showing the range of a shot when fired down from 

 an eminence with depression ; and a parallelogram arrangement is 

 adapted to the parabolic curves, to prove that they are the result 

 of a compound force. 



The Nyctoscope. Sir W. Armstrong has described to the Lon- 

 don Institution of Civil Engineers the principle of the Nyctoscope, 

 an ingenious instrument designed by him for enabling the gunners 

 to maintain a fire upon any given object after nightfall. The prin- 

 ciple of the instrument is to render a false object in the rear, or at 

 one side, visible upon a vertical line in a mirror, when the gun is laid 

 upon the true object. A lamp attached at night to the false object 

 becomes visible upon the same mark in the mirror, when the gun is 

 in line with the true object. The vertical adjustment for elevation 

 is effected by a spirit-level clinometer, forming part of the instru- 

 ment. 



RODMAN'S EXPERIMENTS IN GUNNERY. 



It is perhaps well known to many of our readers that there has 

 been in progress for several years a series of costly experiments (in- 

 stituted by the U. S. War Department) to determine the best form 

 and material for cannon, and the qualities desirable in gunpowder. 

 These experiments have been conducted, for the most part, under 

 the direction of Capt. T. J. Rodman, of the ordnance department, 

 U. S. A., and by him have been recently published in an illustrated 

 volume. 



Some of the most interesting facts developed by these experiments, 

 and set forth at length in the volume above referred to, are in rela- 

 tion to the pressure in a cannon, at the time of its discharge, exerted 

 by the gases resulting from the combustion of the powder. To meas- 



