PILE-DRIVER, a machine for driving piles into the ground, uf which there are many kinds; some are worked by a great number of men, who raise a heavy weight to a small height, and then let it fall upon the pile, till, by reiterated blows, they drive it to the required depth. This machine is extremely simple. A long thick plank of wood is fixed up close to the pile, having a mortise through the upper end, in which a pul ley is fitted ; a rope goes over this to suspend the rammer, which is a large block of hard wood, properly hooped, to pre vent it from splitting. In rising and falling, it slides against the thee of the plank, and is guided by irons, which are fixed to the ram, and bent round the edges of the plank, in the manner of hooks. The plank, when placed upright, is secured by guy-ropes, in the manner of the mast of a ship; the end of the great rope which suspends the ram, has ten or twelve small ropes spliced into it, for as many men to take hold, and work it by ; they raise the ram up two or three feet by pull ing the ropes all together, and then letting them go, the ram falls upon the pile-head. When the pile becomes firm enough to cause the rain to rebound, they take care to pull the ropes instantly after the blow, that they may avail them selves of the leap it makes.
'Phis is the simplest form of the machine. Others, instead of a plank, have two upright beams attached together, at such a distance as to leave an opening between them for the recep tion of a piece of wood which is,allixed to the ram, and by this means it is guided. Instead of guy-ropes, these are usually fixed upon a base, consisting of a triangular frame, upon one.angle of which the uprights are erected ; and from the other two angles, braces arise, inclined so as to reach the uprights at one-half or two-thirds of their height, to steady them. This plan is very convenient for driving piles in cor ners; but for driving in rows, it is more advantageous to have the uprights fixed at the middle of one side of the tri angular base, and have stays from all the three angles. A
machine of this kind, with a ram of beech, four feet long and one foot square, may be worked by ten or twelve men, at the rate of twenty-four blows per minute, and fixes the pile very quickly. To estimate the force of the rammer, its weight ought to be multiplied into the velocity it acquires in falling. Thus, if a rammer weighing 5001bs. drop from four feet, it will fall in half a second, and have at the time of percussion a velocity capable of carrying it uniformly eight feet in half a second, without any thrther help from gravity ; so that we must multiply 500 by 10, or its weight by the number of feet it would fall in a second, and the product, 8,000, gives the mo mentum of the stroke. If a capstan, pulley. or windlass, be made to raise the rammer to a considerable height, and then, by an easy contrivance, loosen it at once from its hook, the momentum of the stroke will always be as the square root of the height from which the rammer fell.
Notwithstanding the momentum, or force of a body in motion, is as the weight multiplied by the velocity, or simply as its velocity, when the weight is given or constant ; yet the effect of the blow will be nearly as the square of that velo city ; the effect being the quantity the pile is driven into the ground by the stroke. For the force of the blow, transferred to the pile, being destroyed in some certain definite time by the friction of the part within the earth, which is nearly a constant quantity, and the spaces in constant forces being as the squares of the velocities; therefore, the efThets, which are those spaces sunk, are nearly as the square of the velo. cities, or, which is the same thing, nearly as the heights fallen by the ram or hammer to the head of the pile.
For large works, such as bridges, &c., the piles are driven by a different kind of machine : this has a very heavy iron ram, with mechanical powers, by which it is raised to a very considerable height, and then let fall, instead of continually repeating small blows. These are sometimes worked by horses, or steam-engines.