The most striking illustration of the adaptation of machinery to pile driving, is perhaps the application of the Nasmyth's steam hammer to this purpose. In this case the steam is admitted into the bottom of a cylinder bearing a platen-rod connected directly with the monkey, so as to raise it vertically until the piston reaches the head of the stroke ; the steam then escapes, and the piston falls. The weight of the monkey in these engines need only be limited by the capacity of the cylinder, or by the elasticity of the steam ; and it is in practice made to range between 32 and 45 cwt.; but the height of the fall is limited by the length of the stroke, and it rarely excmtla 2 feet 6 inches. In the ordinary pile engines, the number of blows does not exceed 35 or 40 per hour; but with the Nasinytha hammer it le easy to increase that number to between 50 or 60 per minute. It follows from the dynamical conditions of this engine, that in ordinary clays, sands, or alluvtai deposits, where the resistance to the descent of the piles ie not very great, and Is tolerably uniform, that the Nasmyth's pile-engine drives piles with a rapidity far greater than any other machine hitherto applied to such a purpose ; but when the resistance becomes consider. able, the piles are very much exposed to be beaten up, to be set on fire, or to be broken by the frequent blows; or if these results should not occur, they may be set into such a state of vibration as to oppose a serious resistance to their descent. The writer of this article has actually seen 5880 blows struck in 11 hour upon a 14 square pile, by a monkey of 49 cwt. falling 2 feet 6 inches, without causing it to advance more than 3 inches ; but the head of the pile was literally beaten up into a sponge : an ordinary pile-engine with a monkey of 16 cwt. falling 18 feet, would, in six hours, have produced a greater
dynamical efltsct under far more advantageous conditions. The use of the Natenyth's steam pile-driver should therefore be limited to the clam of soils above-named ; but in driving through hard gravel, or tufaceous limestone, the monkeys with large falls and slower action should be used: in fact, the mechanical effect produced by a weight falling on the head of a pile is such that the descent of the pile is proportional to the fall of the monkey ; and it is admitted that economically there is an advantage in the adoption of heavy monkeys falling through heights ranging between 12 and 20 feet. It has been shown, by direct experiment, that the descent of a pile is proportional to the mass of the monkey, added to that of the pile, multiplied by the square of the common velocity of those two bodies after the shock. For if the mass of the monkey be represented by m, and that of the pile by in', the common velocity of the monkey and of the pile by is, and the velocity of the monkey before the shock by V, the descent may bo derived from the following formula :— + de= + rnrlx= m211 (et +In') but as in this case Vi= 2gh, iu which g= the accelerating force of gravity (32S feet in London), and It = the fall of the monkey, the descent of the pile is proportional to = 1 -I- til The nature of the soil to be traversed, and the number of piles to be driven in a given time, must be the determining motives for the mice. tion of the pile-driving machinery to be adopted. For camp-sheeting the ringing engine will suffice; the Nastnyth's hammer is admirably adapted to soft, equable, soils; but the ordinary pile-engine worked by crabs is the best fitted for hard irregular soils.