_Rock Excavation._—If the trench or the place for the tank is to be in rock, the cost of excavation is much increased. The rock must be drilled and blasting powder or dynamite used to loosen the material so that it can be thrown out later by hand. In ordinary rock, a man will drill from six inches to twelve inches of hole per hour, that is, the hole will cost from ten to twenty cents per lineal foot. The depth of the hole determines the amount of rock loosened per charge. If the holes are three feet deep, about one-third of a cubic yard is loosened per hour, while if the holes are five feet deep, one cubic yard of rock is loosened per hour. This indicates at once the economical advantage of deep holes compared with shallow ones. In the first case, nine lineal feet of hole would have to be drilled in order to get one cubic yard of rock, nearly double the amount required where the hole is five feet deep. Usually the distance between the holes is made equal to the depth of the holes, although in some rock the depth can, with advantage, be made greater than that distance. If the rock is very loose and seamy, deep holes may sometimes not be warranted, because the effect of the blasting is taken up by the loose rock in such a way that the value of the explosive is not realized. Shallower holes, more frequently blasted, utilize the explosive gases more completely. The kind of explosive which may be used varies from slow, low-power black powder to rapid, high-power nitro-glycerine, the many forms of dynamite and high-grade powder in use being combinations of nitro-glycerine with some absorbent. In most cases, ordinary blasting powder is suitable for rock excavation in small quantity. It lifts the rock rather than shatters it, and is more convenient and safe to handle. Forty-per-cent dynamite is to be recommended where the rock is very seamy so that quick-acting explosive is essential, and also where the rock is very hard, so that black powder tends to blow out the hole rather than to shatter the rock. The cost of forty-per-cent dynamite is about twenty cents per pound, and the cost of powder is about twelve cents per pound. On the average, it may be assumed that it will require one pound of the former and one and a half pounds of the latter per cubic yard of ordinary rock excavated. The cost of lifting the blasted material out of the trench will be at about the same rate as that of earth.