Seat of the Charge

—The form of the seat of the charge, or that part of the bore of a fire-arm which contains the powder, will have an effect on the force of the charge and the strength of the piece to resist it. The considerations most likely to affect the force of the powder are the form of the surface and its extent compared with the inclosed volume. To obtain the full force of the charge it is nece ary that the inflammation be nearly completed before the gas begins to escape through the windage, and the projectile is sensibly moved from its place, and as the tension depends much upon the heat evolved by the combustion, the absorbing surface should be a minimum compared with the volume. In cannon where the charge of powder is large, the form of the seat of the charge is simply that of the bore prolonged; this arrangement, when compared with the chamber, makes the absorbing surface of the metal a minimum and reduces the length of the charge, so that its inflammation will be as complete as po ible before the gas escapes and the projectile is moved. To give additional strength to the breech, and to prevent the angle formed by the plane of the bottom and sides of the bore from becoming a receptacle for dirt and burning fragments of the cartridge-bag, it is rounded with the arc of a circle, whose radius is one-fourth the diameter of the bore at this point. Instead of being a plane bottom it is sometimes made hemispherical, tangent to the surface of the bore. In all United States cannon of the most recent model, the bottom of the bore is a semi-ellipsoid; this is thought to fulfill the condition of strength more fully than the hemisphere. With light pieces, in which it is nece ary to use small charges of powder, if the charge were made into a cartridge of a form to fit the bore its length would be le than its diameter, and being ignited at the top, a considerable portion of the gas generated in the first instance of inflammation would pa through the windage, and a part of the force of the charge would be lost. To obviate this defect, to give the cartridge a more manageable form in loading, and to make the surface a minimum as regards the volume, the diameter of this part of the bore is reduced so as to form a chamber. The shape of the chambers of fire-arms is either cylindrical, conical, or spherical; the effect of these different forms of chambers on the velocity of the projectile will be modified by the size of the charge and the length of the bore. Up to a charge of powder equal to one-seventh of the weight of the projectile, and a length of bore equal to 9 or 10 calibers, experience shows that the presence of a chamber is advantageous, but beyond these it po e es no advantages to compensate for its inconvenience. For very small charges of powder and short lengths of bore, the cylindrical chamber gives better results than the conical chamber. For the same capacity, the conical chamber gives a shorter cartridge, and is therefore better suited to the rapid inflammation of a large charge of powder than the cylindrical chamber. The Gomer chamber belongs to this cla . (See Gomer Chamber .) The spherical chamber was formerly used particularly in mortars, but owing to the inconveniences which attend its construction and use, and its liability to deterioration, it is now entirely abandoned. In all the regulation guns of the U. S. land service, the bottom of the bore is a semi-ellipsoid. The adoption of this form simplifies the whole subject of chambers, and it is found to give increased ranges for small charges. No very careful experiments have been made to determine in a general way the effect of chambers on the strength of cannon; but late experience indicates that cylindrical chambers in heavy iron guns have an injurious effect on their endurance, and they have consequently been abandoned in these pieces.