Page:The American Cyclopædia (1879) Volume VIII.djvu/341

 GUNNERY metal. But the intensity of the force of gunpowder is highly variable at different por- tions of the path along the bore, being very great near the seat of the shot, and rapidly de- clining toward the muzzle ; hence equal velo- cities will be imparted only when, at different points in the path along the bore, the respec- tive intensities are proportional to the columns of metal. A complete analysis of the relations existing between the force of gunpowder and the motion of the shot in the gun, in terms of time, space, and mass, has never been attempt- ed ; it is a very formidable problem, and its chief difficulty is our ignorance of the rate at which the gases are developed and the quantity of heat evolved. But the greater the resistance opposed to the expansion of the gases of gun- >wder, the more rapidly will the powder burn develop gases, and the higher will be their iperature. Such an increased resistance is offered by an increased column of metal ; and ice the conclusion that a longer column of 3tal carries with it the power of developing we force from a given quantity of powder a shorter one. On the other hand, the >rter column of metal will still have the jher velocity, though the longer will have greater energy (mass multiplied into square velocity) ; the difference in energy in favor the latter being due to its greater mass, rhich more than compensates for its lower velocity. But if the quantity of powder is proportional to the column of metal, a larger charge will develop at every moment more gas than a smaller charge, and give a more intense force. But a larger charge occupies more space in the bore, and robs the projectile of a part of its travel, and hence of a part of the time in which it can receive acceleration. Increasing the charge will increase velocity up to a certain point, but beyond that point will diminish it. In small cannon the maximum pressure is probably reached before the shot has travelled three inches, and in large guns before it has travelled a foot. The time occu- ied by the shot in traversing the bore prob- ly ranges from -^ to TWG of a second, and spends mainly upon the length of the bore and 10 quantity of powder. A bold attempt was le by Rodman in 1858 to measure the dis- ribution of the forces of gunpowder, by pla- ig pressure gauges along the bore to register pressure at different points ; and to mea- ire the time of passing over different parts " the bore, by a series of ruptures of electric suits. (For a description of the pressure and the electric velocimeter, see GUN- HVDER, and VELOCIMETER.) It is obvious lat an increase either in the column of metal in the charge involves an increase in the itensity of the pressure of the gases, and 3nce an increased strain upon the gun. As e strength of a gun is limited, both the col- m of metal and the charge must be regula- 1 accordingly. It is the maximum pressure rliich is danerous. In large guns this diffi- GUNPOWDER 327 culty is serious. Not only is a higher pressure produced by the longer column of metal, but the pressure is distributed over a larger area of bore, and the bursting tendency is in the ratio of the product of these two quantities. The greater thickness of walls gives increased resistance, but this increase is in a lower ratio than that of the bursting tendency, and hence large guns are relatively weaker than small ones. To compensate for this difficulty, con- structors have resorted to metals of great- er strength, and especially have modified the action of the powder, so that the maximum pressures have been reduced, and the subse- quent lower pressures have been increased. Thus the total effort of the powder upon the shot is undiminished. (See GUNPOWDER.) The column of metal of a spherical shot is two thirds its calibre ; that of an elongated shot is usually between one and three fourths and twice its calibre. The latter limit has been found to be about as great as the strength of the gun will permit in large calibres. It is sometimes exceeded with very little advantage in the smaller and intermediate calibres. The charge of powder varies from one fourth to one tenth the weight of the projectile. With round shot it is sometimes as high as one third ; but it is found that the velocity is not much increased when the charge is greater than one fourth. The velocities imparted to round shot vary from 1,400 to 1,750 ft. per second, and those of elongated shot from 1,150 to 1,500 ft. For a good introduction to the science of gunnery, see " Ordnance and Gun- nery,"' by Major J. G. Benton, TJ. S. A., and "Treatise on Artillery," by Lt. Col. C. H. Owen, R. A. GUMY, a coarse cloth made in India of the fibres of two species of corchorus, and used for the sacks in which saltpetre, pepper, and other articles are packed for exportation. The bag- ging itself is also exported. The export of gunny bags and cloth from Calcutta is chiefly to the United States, and they are mainly used at the south for cotton bagging. For the year ending June 30, 1872, the imports of gunny cloth and bags were as follows: PORTS. Pounds. Value. 641,262 $44,207 Boston and Charlestown 3,340,723 81,148 133,859 4,996 New York 7,850,394 810,829 21 8 068 11,240 8,765 277 Other ports 2,244 58 Total 12,137,603 $505,566 There were reexported 654,139 Ibs. of gunny cloth and bags, valued at $34,929, chiefly to England and Turkey. (See JUTE.) GUNPOWDER, a compound of nitre, charcoal, and sulphur, employed as an explosive. Its composition is described in the article EXPLO- SIVES. The date and the author of the inven-