If the disembarkation is to be performed in front of the enemy, some of the field pieces should be so placed that they can be disembarked immediately, with their carriages, implements and ammunition; also the tools and materials for throwing up temporary intrenchments on landing. 7. Some vessels should be laden solely with such powder and ammunition as may not be required for the immediate service of the pieces. 8. On a smooth, sandy beach, heavy pieces, &c., may be landed by rolling them overboard as soon as the boats ground, and hauling them up with sling-carts. [Sidenote: 163]] APPENDIX. RIFLE CANNON. A _rifle_ is a firearm which has spiral grooves cut into the surface of its bore, for the purpose of communicating a rotary motion to a projectile around an axis coinciding with the direction of its flight. The object of this rotation is to increase the range of a projectile, by causing it to move through the air in the direction of its least resistance, and to correct the cause of deviation by distributing it uniformly around the line of flight. Various plans have been tried to secure the safest and surest means of causing the projectile to follow the spiral grooves as it passes along the bore of a rifled piece. Those projectiles, which promise to be the most successful for heavy guns, may be ranged under two heads, viz: 1st. Those which have flanges or projections on them to fit into the grooves of the gun in loading. The flanges are made of softer metal than the body of the projectile. 2d. Those which are constructed on an expanding principle. [Sidenote: 164]] The body is generally made of cast-iron; and the expanding portion is a band or cup of some softer metal, as _pewter_, _copper_, or _wrought iron_, which enters the bore of the piece freely when it is loaded, but which is forced into the grooves by the discharge. The grooves are of different forms, determined by the angle made by the tangent line at any point with the corresponding element of the bore. If the angles be equal at all points, the groove is said to be _uniform_. If they increase from the breech to the muzzle, the grooves are called _increasing_; if the reverse, _decreasing_ grooves. The practical method of cutting grooves consists in moving a rod armed with a cutter, back and forth in the bore, and at the same time revolving it around its axis. If the velocities of translation and rotation be both uniform, the grooves will have a uniform twist; if one of the velocities be variable, the grooves will be either increasing or decreasing, depending on the relative velocities in the two directions. _Twist_ is the term employed by gunmakers to express the inclination of a groove at any point, and is measured by the tangent of the angle which the groove makes with the axis of the bore; and this is _always equal to the circumference of the bore divided by the length of a single revolution of the spiral measured in the direction of the axis_.