210

THEORY AND PRACTICE

TRACT 37.

Again, in prop. 58, pa. 219, are delivered the theory andcalculations of a beautiful military experiment, invented byMr. Robins, for determining the true degree of velocity withwhich balls are discharged from guns, with any charges ofpowder. The idea of this experiment, is simply, that theball is discharged into a very large but moveable block ofwood, whose small velocity, in consequence of that blow,can be easily observed and accurately measured. Then, fromthis small velocity, thus obtained, the large one of the ball isimmediately derived by this simple proportion, viz, as theweight of the ball, is to the sum of the weights of the balland the block, so is the observed velocity of the last, to a 4thproportional, which is the velocity of the ball sought.—It isevident that this simple mode of experiment will be thesource of numerous useful principles, as results derived fromthe experiments thus made, with all lengths and sizes ofguns, with all kinds and sizes of balls and other shot, andwith all the various sorts and quantities of gunpowder; inshort, the experiment will supply answers to all enquiries inprojectiles, excepting the extent of their ranges; for it willeven determine the resistance of the air, by causing the ballto strike the block of wood at different distances from thegun, thus showing the velocity lost by passing through thosedifferent spaces of air; all which circumstances are fullyshown in the foregoing 34th Tract.

3. Lastly,in prob. 17 on Forces , near the end of that volume,some results of the same kind of experiment are successfullyapplied, to determine the curious circumstances of the firstforce or elasticity of the air resulting from the fired gun-powder, and tlie velocity with which it expands itself. Theseare circumstances which have never before been determinedwith any precision. Mr. Robins, and other authors, it maybe said, have only guessed at, rather than determined them.That ingenious philosopher, by a simple experiment, trulyshowed that, by the firing of a parcel of gunpowder, a quan-tity of elastic air was disengaged, which, when confined inthe space only occupied by the powder before it was fired,