Temporary galleries are only made 4½ feet high, and 2½ or 3 feet wide. The branches, at the ends of which the chambers are to be placed, are only made 2½ or 3 feet high, and 2 feet, or 2 feet 3 inches wide. The first of these is dug on the knees; the second sitting or lying. The miners are divided into brigades of 4 each; and the rate of work for each brigade is 3 feet of the temporary gallery in 4 hours. The first brigade is relieved by a second, after having worked 4 hours, or laid one frame; which second brigade is again relieved by the first, at the expiration of the same time. In the most easy ground to work, a miner may be heard to the distance of 14 or 15 fathoms under ground; and the noise made by fixing the frames of the galleries may often be heard as far as 20 or 25 fathoms. A drum braced, standing on the ground, with a few peas or other round substances on the head, will be very sensibly affected by an approaching miner. It is of the most essential consequence to place the entrances to the countermines beyond the reach of any surprise from the enemy. To prevent an enemy gaining possession of the galleries of the countermines, they should be well secured by strong doors, at every 15 fathoms. These should be musquet proof. A glacis, properly countermined, and every advantage taken of it to retard the besiegers, may, with proper management, prolong a siege at least 2 months; and if the rest of the works are also countermined, and properly defended, they may add another month to the siege. Every system of countermines must depend upon the system of fortification to which they are to be adapted; the general principle for their regulation is, that the galleries should occupy situations, from which branches can be most readily run out under the most probable points of the besieger’s batteries and approaches. The general system of countermines commonly used in a place prepared beforehand, is as follows:—The principal or _magistral_ gallery runs all round the work, under the banquette of the covert way, and across the places of arms, having the entrances at the re-entering places of arms. Nearly parallel to this at 20, 25, or 30 fathoms distance is another gallery, called the _envellope_. These two galleries are connected by galleries of _communication_, under the gutters of the re-entering parts of the glacis, and under the ridges of the salient parts. From the envellope are run out about 15 or 16 fathoms, galleries, in directions parallel to the capitals of the works, and at 23 fathoms distance from each other. These are called _listeners_.