Page:The American Cyclopædia (1879) Volume III.djvu/718

 708 CANNON ance with this idea, culverins wore made of great length, and were only shortened after repeated experiments showing that tho range increased at each reduction in length. The length of the hore has an important effect upon the velocity and range of the ball. This will be clearly seen by a consideration of the forces which accelerate and retard its movements. The accelerating force is due to the expansive effort of the burning powder, which is great- est when the grains are completely converted into gas, which in turn depends upon the size of the charge and the size and constitution of the grains. The retarding forces are the friction of the projectile against the sides of the bore, the shocks of the projectile striking against the sides of the bore, and the resist- ance offered by the column of air in front of the projectile. As the accelerating force of the charge increases up to a certain point, or till the combustion is completed, and rapidly diminishes as the space in rear of the projectile increases, and as the retarding forces are al- ways opposed to its motion, it follows that there is a point where these forces would be- come equal, and the prqiectile move with its greatest velocity ; it also follows that after the projectile passes this point its velocity decreases, until it is finally brought to a state of rest, which would be the case in a cannon of great length. Experiments made by Maj. Mordecoi show that the velocity increases with the length of bore up to 25 calibres, but that the gain be- yond 16 calibres gave an increase of only T ' 5 to the effect of a 4-lb. charge. Taking the cali- bre as the unit of measure, it has been found by experience that the length of bore is great- er for small arms which fire leaden bullets than for guns which fire iron shot, and great- er again for the latter than for howitzers and mortars which fire hollow projectiles. In the earlier days of artillery, when dust in- stead of grained powder was used, the weight of the charge was equal to that of the pro- jectile ; hut it is now admitted that a charge of powder equal to one fourth of the weight of the projectile, and a bore of 18 calibres long, are the most favorable combination that can be made in smooth-bored cannon, to obtain the greatest range with the least strain upon the piece and its carriage. The exterior form of a cannon depends upon the strength neces- sary to resist the varying pressure of the gas generated by the combustion of the gunpotf- der, and in general terms may he described as being largest at the seat of the charge, and gradually decreasing toward the muzzle, at or near which it is smallest. Various methods have been resorted to for determining the pres- sure of the gases throughout the bore, and deducing therefrom the proper exterior form for the different kinds of cannon. Tho most successful of these is a modification of a plan first used by Col. Bomford about 1841, and subsequently improved by the late Gen. Rod- man of the United States ordnance corps. It consists in boring a series of small holes through the sides of a cannon at right angles to its axis, at intervals of one calibre, and loading them with steel balls, which are projected by the force of the charge into a ballistic pendulum. The pressure at the various points is calculated from the velocity given to the balls. By this plan the form of the guns known as coliimbiads was determined. Gen. Rodman's modification consists in substituting for the bullets ,i steel punch which is pressed by the force of tho gases into a piece of soft copper. The weight necessary to make an equal indentation by the same punch in the same copper is then obtained by machinery for each hole in the side of the gun, and a curve constructed by plotting the results thus obtained, as in the following dia- gram. The ordinates of the curve A show the pressure on the bore at intervals of two cali- bres, commencing at the bottom of the bore, for grain powder; and those of the curve B show the same for cake pow- der. The latter produced only about one half the mean pressure on the length of the hore, and gave nearly the same velocity to the projec- tile. Cannon, in common with all other firearms, are subjected by usage to four 246 a 10 IZ FIG. 5. Pressure by Calibres. classes of strains: tangential strains, which act to split the piece open longitudinally; longitudinal strains, which act to pull the piece apart in the direction of its axis ; strains of compression, acting from the axis outward ; and transverse strains, acting transversely, as it were, upon the staves of which the piece may be supposed to consist. A formula em- bodying these strains, the pressure of the gas, and all other elements entering into the question, was deduced by Gen. Rodman from a series of original experiments. Its solution for particular cases gives a series of curved lines, a specimen of which is seen in the follow- ing figure of a Rodman gun. The exterior Fie. 6. Rodman Gun. of a cannon is divided into five principal parts: the breech, A ; the first reenforce, B ; the sec- ond reenforce, C ; the chase, D ; and the swell of the muzzle, E. The breech is the solid part of the piece in the prolongation of the axis; its length should be from one to one and a quarter