About the beginning of the eighteenth century most countries had reduced their powders to two or three, which were of the same composition, and differed only in grain. In 1742 Benjamin Robins, by his “New Principles of Gunnery,” placed gunnery upon a strictly scientific basis, and by his epoch-making invention of the ballistic pendulums[483] enabled gunners for the first time to measure the muzzle-velocity of projectiles with considerable accuracy. It may have been owing to the lessons taught by this instrument that, between 1742 and 1781, we changed the proportions of the ingredients of our powder from 75—12½—12½ to 75—15—10. Profiting by the rapid progress of electricity during the first half of the nineteenth century, Sir Charles Wheatstone proposed in 1840 his electro-magnetic chronoscope,[484] which registered to the 1/730 part of a second, to replace Robins’ ponderous pendulum. Wheatstone’s instrument was not adopted by our Government, but his idea was followed up and improved upon by Captain Navez, of the Belgian Artillery, who in 1847 brought forward his electro-ballistic pendulum.[485] Only one instrument was now wanting to enable the mechanical effect of the explosion to be directly and completely observed—an instrument to measure the pressure upon the bore of the gun; and this want was supplied in 1861 when Captain T. J. Rodman, Ordnance Department, United States Army, produced his Indenting Apparatus and his Internal Pressure Gauge.[486] The following Table gives the results of some experiments with the new instruments:— TABLE VI. _Showing the connection between the Size of the Grain, Muzzle Velocity, and Pressure on Bore._ +-------------+---------+-----------+-----------+------------------+ | Diameter of | Charge. | Weight of | Muzzle | Pressure on | | Grains. | Lbs. | Shot. | Velocity. | Bottom of Bore. | | Ins. | | Lbs. | F.s. | Tons per Sq. In. | +-------------+---------+-----------+-----------+------------------+ | .1 | 8 | 43 | 1261 | 21.5 | | .15 | ” | ” | 1235 | 21.0 | | .2 | ” | ” | 1199 | 18.8 | | .25 | ” | ” | 1151 | 17.1 | | .3 | ” | ” | 1146 | 15.3 | | .4 | ” | ” | 1187 | 14.2 | +-------------+---------+-----------+-----------+------------------+