The structures usually destroyed are houses, walls, stockades, bridges, tunnels, canal-locks, railroads, rolling-stock, etc., etc. The time does not usually allow the charge to be placed in the most advantageous position or to be properly tamped. For this reason the high explosives are best suited for this kind of work, and large charges are a necessity. =103. Houses and Magazines= are best destroyed by placing several charges with connecting trains inside and along the walls, laying strong timbers upon them, with struts from the timbers to the floors and roof above; barricading the doors and windows from within, and firing the powder from a safe distance without. =104. Walls.=--A wall not exceeding 3 or 4 feet in thickness may be breached by charges of gunpowder placed at intervals along it. Calling the thickness of the wall in feet _t_, the charge in pounds may be 3_t_^3, placed at intervals of 2_t_. For gun-cotton the Woolwich rule calls for charges in pounds of from ⅓_t_^2 to ½_t_^2 per running foot. Experiments made in New York with dynamite indicate that the charges should be at least ½_t_^2 per running foot, and for very good masonry should exceed this. A charge of dynamite of ½_t_^2 per running foot will be given by a cylindrical cartridge whose diameter in inches equals the thickness of the wall in feet.[22] The effect of the charge will be very much increased by throwing over it even a very light tamping of earth or sand. =105. Stockades.=--A strong stockade or palisade may be broken down by charges of from 40 to 60 lbs. of gunpowder placed in contact with it, and preferably covered with sand-bags. 10 or 15 lbs. of high explosive should produce about the same effect. =106. Bridges.=--Arched bridges are best attacked in the piers if high and thin, or at the haunches and crown of the arch. Two or more charges in the length of the pier, or width of the roadway, will be more effective than the same amount in a single charge at the middle. The charges should be placed in chambers cut in the piers or down through the roadway to the back of the arch. The abutments of single-span arches are generally very strong, and the haunches well covered with earth and masonry. In hurried work, therefore, the crown will generally be selected, a trench dug down to it across the roadway, the charge placed in the trench, tamped if possible, and fired.[23] High explosives, from their shattering effect, are perhaps most advantageously used by suspending them beneath and in contact with the arch at the crown and haunches. The plank or timber upon which they are placed should be as heavy as possible, in order to act as a partial tamping, and should be drawn up so that the explosive will be in actual contact with the soffit of the arch. Under these circumstances they should produce as great an effect as four or five times their weight of gunpowder.