The great advantage of this system is, that, as it dispenses with apparatus: where there is time for forming a work of this sort, of considerable length, the quantity of fire, that may be thrown in a given time, is limited only by the length of the work: thus, as the Rockets may be laid in embrasures cut in the bank, at every two feet, a battery of this description, 200 feet in length, will fire 100 Rockets in a volley, and so on; or an incessant and heavy fire may, by such a battery, be kept up from one flank to the other, by replacing the Rockets as fast as they are fired in succession. The rule for forming this battery is as follows. “The length of the interior slope of this work is half formed by the excavation, and half by the earth thrown out; for the base therefore of the interior slope of the part to be raised, at an angle of 55°, set off two thirds of the intended perpendicular height--cut down the slope to a perpendicular depth equal to the above mentioned height--then setting off, for the breadth of the interior excavation, one third more than the intended thickness of the work, carry down a regular ramp from the back part of this excavation to the foot of the slope, and the excavation will supply the quantity of earth necessary to give the exterior face a slope of 45°.” Fig. 2 is a perspective view of a common epaulement converted into a Rocket battery. In this case, as the epaulement is not of sufficient length to support the Rocket and stick, holes must be bored in the ground, with a miner’s borer, of a sufficient depth to receive the sticks, and at such distances, and such an angle, as it is intended to place the Rockets for firing. The inside of the epaulement must be pared away to correspond with this angle, say 55°. The Rockets are then to be laid in embrasures, formed in the bank, as in the last case. Where the ground is such as to admit of using the borer, this latter system, of course, is the easiest operation; and for such ground as would be likely to crumble into the holes, slight tubes are provided, about two feet long, to preserve the opening; in fact, these tubes will be found advantageous in all ground. Fig. 2 also shews a powerful mode of defending a field work by means of Rockets, in addition to the defences of the present system; merely by cutting embrasures in the glacis, for horizontal firing. [Illustration: _Plate 7_  Fig. 1  Fig. 2] A ROCKET AMBUSCADE.