As a free communication among the different parts of the fortification is of the greatest importance, it becomes necessary to have ramps or slopes, by which the artillery, &c. may be brought upon the rampart. If the bastion be full, ramps in the gorge will be sufficient ; if empty, one will be required at each flank, at the faces, and also at the cuat,ins. The breadth of the ramp may be from 10 to 19 feet, according to circumstances; and its perpendicular as cent, from 1 inch to 21 inches per horizontal foot.
Soutcrrains, or caserns, are places constructed under the rampart, for the purpose of lodging the soldiers, and con taining ammunition, Ste. The invention of shells has ren dered souterrains much more necessary now than they were in former times. They are most advantageously placed in the curtains and cavaliers, as being there least exposed to the enemy's fire. Their sizes are different, according to the purpose for which they arc intended. Such as arc in tended for powder magazines, ought to be in a dry place, and as far as possible from buildings. It is calculated that 18 tons of powder occupy a space of about eight cubic yards.
Gates, in a fortified town, should be as few as possible, to save the garrison unnecessary duty, and to prevent the expense of keeping bridges in repair. Gates are general ly placed in the curtain; and according to Carnot, ought to be 9 to 10 feet in width, and 13 to 14 in height. They are shut by a draw-bridge, and also by a strong door at each extremity of the passage. On the right and left of the gate, and within the town, are watch-houses for centincls, which ought to be shell•proef. Formerly, the bridges be fore the gates were of wood, but in the new fortresses, they are of stone. The former had the advantage of being easi ly burned, in case of emergency.
In those curtains that have no gates, there are suhterra neous passages called Posterns, which communicate with the ditch by two flights of stairs or ramps. They are use ful in holding ammunition for the outworks. The extremi ty next the field ought to be covered by a tenaille, or clo sed up by a very strong gate.
Engineers have differed in opinion with regard to the comparative merits of a dry and a wet ditch. There can be no doubt, that one capable of being made either dry or wet, according to circumstances, is to be preferred : at the same time, it seems to be generally admitted now, that a dry one is the more advantageous, particularly if the en ceinte has a revetement. It not only enables the garrison
to lend more prompt and effectual assistance to the outworks, but it also encourages the defenders of these to maintain their position with more obstinacy, knowing that they have a reach and safe retreat in case of extremity. Besides, it is only in a dry ditch that the garrison can make sorties, the communication by means of boats being altogether un fit for rapid movements.
In a dry ditch, the communication between the fortress and the ravelin consists of a passage b, Plate CCIXI I. Fig. 3, about nine feet wide, and inclosed by a parapet on each side, terminating in a glacis. A passage of this kind is called a cajzonnier. When they have a parapet only on one side, they are called half caponniers. Both kinds arc some times carried between the shoulder of the bastion and the ravelin, as well as between various other parts of the fortifi cation, and are useful in defending the ditch. They are always perpendicular to the lines which they enflank.
In moist ditches which are intended to be dry, it is fre quently necessary to have a small ditch called a cunette, for carrying off the superfluous water. It is also sometimes used to prevent the enemy from surprising a work that has no revetement or wet ditch. In this case, it is 3 feet deep, and 12 wide.
If a place he so situated, that a current of water can be brought through the ditch at pleasure, it gives the garri son a prodigious advantage over the besiegers, and always obliges the latter to be very cautious how they cross the ditch for the purpose of making an assault. In order more effectually to inundate the ditch, dams or batardeaux, a b, c d. Fig. 1, are sometimes employed. They are construct ed of stone, and of a sufficient thickness to resist the pres sure of the water contained in them. The water is admit ted and discharged by means of sluices, flom which it flows over a long slope, so as not to injure the bottom of the ditches. Batardeaux ought to be constructed in places as much as possible without the reach of the enemy's fire ; and the sluices ought to be well protected. When the batardeau is coast ucted before the curtain, it serves as a communication between the place and the ravelin, by means of a gallery in the interior.