plate, in which the same letters of reference are annexed to the same parts, in order to explain those in the second, which are either partly or wholly hid in the first. Of the Pile-engine. A is the great upright shaft or axle, on which are the great wheel B and drum C, turned by horses joined to the bars S, S. The wheel B turns the trundle X, on the top of whose axis is the fly O, which serves to regulate the motion, and also to act against the horses, and keep them from falling when the heavy ram Q is discharged to drive the pile P down into the mud in the bottom of the river. The drum C is loose upon the shaft A, but is locked to the wheel B.by the bolt F. On this drum the great rope HH is wound; one end of the rope being fixed to the drum, and the other to the follower G, to which it is conveyed over the pulleys I and K. In the follower G is contained the tongs F (see Fig. 3.) that take hold of the ram by the staple R for drawing it up. D is a spiral or fusy fixed to the drum, on which is wound the small rope T that goes over the pulley U, under the pulley V, and is fastened to the top of the frame at 7. To the pulleyblock Vis hung the counterpoise W, which hinders the follower from accelerating as it goes down to take hold of the ram; for, as the follower tends to acquire velocity in its descent, the line T winds downward upon the fusy, on a larger and larger radius, by which means the counterpoise W acts stronger and stronger against it; and so allows it to come down with only a moderate and uniform velocity. The bolt I locks the drum to the great wheel, being pushed upward by the small lever 2, which goes through a mortise in the shaft A, turns upon a pin in the bar 3, fixed to the great wheel B, and has a weight 4, which always tends to push up the bolt I through the wheel into the drum. L is the great lever turning on the axis m, and reting upon the forcing bar 5, 5, which goes down through a hollow in the shaft A, and bears up the little lever 2. By the horses going round, the rope His wound about the drum C, and the ram Q is drawn up by the tongs F in the follower G, until the tongs come between the inclined planes E; which, by shutting the tongs at the top, opens them at the foot, and discharges the ram, which falls down between the guides bb upon the pile P, and drive it by a few strokes as far into the mud as it can go; after which the top part is sawed off close to the mud, by an engine for that purpose. Immediately after the ram is discharged, the piece 6 upon the follower G takes hold of the ropes aa, which raise the end of the lever L, and cause its end N to descend and press down the forcing bar 5 upon the little lever 2, which, by pulling down the bolt I, unlocks the drum C from the great wheel B; and then the follower, being at liberty, comes down by its own weight to the ram; and the lower ends of the tongs slip over the staple R, and the weight of its heads causes them to fall outward, and shut upon it. Then the weight 4 pushes up the bolt I into the drum, which locks it to the great wheel, and so the ram is drawn up as before, As the follower comes down, it causes the drum to turn backward, and unwinds the rope from it, while the horses, great wheel, trundle, and fly, go on with an uninterrupted motion; and as the drum is turning backward, the coun terpoise W is drawn up, and its rope Twound upon the spiral fusy D. There are several holes in the under side of the drum, and the bolt I always takes the first one that it finds when the drum stops by the falling of the follower upon the ram; until which stoppage, the bolt has not time to slip into any of the holes. This engine was placed upon a barge on the water, and so was easily conveyed to any place desired. I never had the good fortune to see it, but drew this figure from a model which I made from a print of it, not being quite satisfied with the view which the print gives. I have been told that the ram was a ton weight, and that the guides bb, between which it was drawn up and let fall down, were 30 feet high. I suppose the great wheel may have had 100 cogs, and the trundle 10 staves or rounds; so that the fly would make 10 revolutions for one of the great wheel. 114 Definition LECTURE V. Of Hydrostatics, and Hydraulic Machines. THE science of hydrostatics treats of the nature, gravity, pressure, and motion of fluids in general, and of weighing solids in them.* A fluid is a body that yields to the least parof a fluid. tial pressure or difference of pressures. Its particles are so small, that they cannot be discerned by the best microscopes; they are hard, since no fluid, except air or steam, can be pressed into a less space than it naturally possesses; and they must be round and smooth, * The doctrine of fluids is generally divided into three branches. Hydrostatics, which treats of the gravity and equilibrium of fluids at rest; Hydrodynamics, which treats of fluids in motion; and Hydraulics, which treats of the construction of machines in which fluids are chiefly concerned.-E. ED. It is now found, from many unquestionable experiments, made both in this country and on the continent, that most fluids are to a certain degree compressible. Mr. Canton having placed a glass tube filled with water, beneath the receiver of an airpump, and removed the pressure of the atmosphere, found that the water expanded itself and rose in the tube. When the same glass tube was placed under the receiver of a condensing engine, and the air in the receiver greatly condensed, the water was compressed, and sunk in the tube. When other fluids were subjected to similar experiments, he found them compressible in the following proportions: Spirit of wine 66 millionth parts of the whole; oil of olives 48 millionth parts; rain-water 46; seawater 40; and mercury 3.-For a further account of the experiments of this ingenious philosopher, see Philo. Trans. vols. 52 & 54.-The elasticity of water is also evident from the reflection of stones, which strike the surface of the water in an oblique direction.-E. ED. seeing they are so easily moved among one an- PLATE X. other.* All bodies, both fluid and solid, press downward by the force of gravity: but fluids have this wonderful property, that their pressure upward, or sidewise, is equal to their pressure downward; and this is always in proportion to their perpendicular height, without any regard to their quantity; for, as each particle is quite free to move, it will move toward that part or side on which the pressure is least: and hence, no particle or quantity of a fluid can be at rest, till it is every way equally pressed. Fluids downward To show by experiment that fluids press up- Fig. 1. ward as well as downward, let AB be a long press as upright tube filled with water near to its top; much upand CD a small tube open at both ends, and ward as immersed into the water at the large one; if the immersion be quick, you will see the water rise in the small tube to the same height that it stands in the great one, or until the surfaces of the water in both are on the same level; which shows that the water is pressed upward into the small tube by the weight of what is in the great one; otherwise it could never rise therein, contrary to its natural gravity, unless the diameter of the bore were so small, that the attraction of the tube would raise the water; which will never happen if the tube be as wide as that in a common barometer. And, as the water rises no higher in the small tube after its surface is on a level with the surface of the water in the * That fluidity depends on the particles of the fluid being small, round, hard, and smooth, is a mere bypothesis, and one on which all the phenomina cannot be explained; especially those of elastic fluids. The more modern, and probable hypothesis is, that fluidity depends on that principle which chemists term ca loric.-A. ED. |