Definition and Object. -interior ballistics treats of the\ formation, temperature and volume of the gases into which the powder charge, in the chamber of a gun, is converted by com bustion, and the work performed by the expansion of these gases upon the gun, carriage and projectile. Its object is the' deduction and discussion of rules and formulas for calculating; the velocity, both of translation and of rotation, which the gases of a given weight of powder of known composition and quality are able to impart to a projectile and their reaction upon the gun and carriage. The discussion of the formulas deduced willebring out many important questions, such as the proper relation of weight of charge to weight of projectile and length of bore, the best size and shape of the powder grains for different guns and their effect upon the maximum and muzzle/ pressures, the velocity of recoil, etc. The most approved formulas for calculating the pressures upon the surface Oof the bore will be given; but the methods which have been devised for building up the gun, so as best to resist these pressures, will not be entered upon here as their consideration belongs to another branch of the subject.