base AB, the whole wedge forces its way. It is generally understood, that the wedge acts upon the principle of a double inclined plane. The less the breadth of the base AB is, in proportion to the length of the two sides AC, BC, the greater is the acquired power. It is calculated accordingly in theory, that, if AC and BC, taken together, be four times the length of AB, or, which is the same thing, if AC be four times the length of AD, the half of AB, the power will be equal to four times the resistance; and if the wood cleave at a distance before the wedge (which is the case with most kinds of timber), the

advantage acquired is computed to be A D B still greater. But, in truth, where there is so much friction, it is difficult to attain a precise calculation upon the subject. Hatchets and chisels, and other sharp instruments, having both edges sloped, act upon the principle of the wedge. So also does the knife, in so far as it is used to split; but it acts also upon the principle of a fine saw, and therefore

it is that it is drawn backward and forward across the body to be cut. It has justly been remarked, as one of the many proofs of wisdom displayed in the Creation, that the beaks of birds are formed in the shape of wedges, for the purpose of enabling them to dig into the ground, or into the bark of trees, and to break the shells of fruit. VI. The last mechanical power we have mentioned is the SCREW. It consists of two parts, the screw more properly so called, and the nut. The screw is a

The

cylinder with a spiral protuberance, which is called its thread, apparently coiled round it in the same manner as the ivy twines round the oak, or a serpent twists itself round a pole. From this last circumstance the thread sometimes receives the name of a worm. nut, which is the weight to be moved, is generally a heavy piece of iron, with a hole perforated in the centre, which is so grooved as to accommodate itself to the spiral twistings of the screw, upon which the nut

moves. To the nut there is affixed a handle, which handle and nut taken together are called a winch. The screw acts upon the principle of an inclined plane, by which the body, in place of rising in a straight line, gradually ascends by a spiral curve to the

top. Cut a piece of paper in the shape
of the triangle ABC, of which the side
AC obviously represents an inclined
plane; apply the side AB to a stick or
other cylinder, and wrap the paper round
it; and you will see at once that the line
AC representing the inclined plane, has
become the spiral of a screw.
B

C

The closer the parts of the thread are to each other, the more is the advantage gained by the screw. The operation of this power will, I think, be well understood by the following familiar illustration. If, in place of attempting to ascend a high hill in a straight and perpendicular direction, we make use of a path, which winds spirally round the hill till we reach the summit, our ascent, as every one knows, will be rendered much easier and this facility will be more and more increased, as the different parts of the winding path approach more closely to each other.-Hitherto we have only considered the operation of that part of this mechanical power, which is more properly called the screw. You will, however, at once perceive, that the handle also acts as a lever, and that in proportion as its length is increased greater power is acquired. The power of a screw therefore may be augmented, either by diminishing the distance between the parts of the thread, or by lengthening the handle. This mechanical force is employed by bookbinders and printers, and in the manufacture of wine, cider, and sometimes cheese, &c.

MECHANICAL PROPERTIES OF FLUIDS.

A FLUID is a body, the particles of which yield to any impression, and are easily moved among each other. This defective cohesion, among the particles of fluids,

has been explained by some philosophers upon the supposition, that these particles are not only small but smooth and round.-Fluids are of two kinds; what are called non-elastic fluids or liquids, such as water, oil, quicksilver; and elastic fluids, such as the atmospheric air, vapours, and gases of every description. It is the mechanical properties of liquids, that are to form the particular subject of the present article. Liquids are very little susceptible of being compressed into a smaller bulk than their natural state. A striking illustration of this was, on one occasion, supposed to be given, by means of a celebrated experiment made at Florence; in which a hollow globe of gold, being filled with water, and subjected to a very strong pressure, the water was seen to escape through the pores of the gold, and to cover the external parts of the globe with moisture. By other experiments, however, it has been shown, that liquids are not altogether unsusceptible of compression; and some doubts have even been expressed with regard to the accuracy of the Florence experiment. The pores of liquid bodies are too minute to be visible, but that these bodies are porous appears from the experiments last mentioned, as well as from other circumstances. In consequence of the attraction of cohesion operating less strongly in liquids than in solids, gravity, on the other hand, in the former, has a more perfect operation. We have seen that in solids, when a single point, called the centre of gravity, is supported, the whole body remains stationary, and that a huge mass of rock may project without being brought to the ground by the force of gravitation. In liquids, however, the prevalence of cohesion over gravity can only happen to a very limited extent. That it does happen, indeed, every hanging drop of water is a proof. But it is quite obvious, that, while gravity acts upon a solid body as one collective mass, it has a more independent operation upon each individual particle of a liquid body. Hence it is that a liquid always finds its level, and maintains a smooth and horizontal surface. For if, by any accident, one particle be, for a moment, raised above the others, it immediately, by the force of gravity operating separately upon itself, is brought down to the

surface of the fluid; and, from the readiness, with which the other parts make way for its reception, becomes incorporated with the liquid mass. All the particles of a liquid body, in consequence of this independent gravitation, press against each other not only downwards, but also laterally or sideways, and even upwards. Were there no lateral pressure in liquids, why is it that we every day see water run out of a vessel, when an opening is made in one of its sides? This lateral pressure, no less than that directly downwards, is the result of gravity, and is occasioned by the superior particles, in their attempt to descend, forcing aside those beneath them. If you conceive three particles of a liquid to be thus arranged [], the superior particle, in its descent, will obviously make way for itself, by forcing the two others aside. As the lateral pressure of liquid bodies is thus the result of gravity, and of the consequent downward pressure of the superior particles, it follows, that the lower an opening is made in the side of a vessel containing such a body, the greater in proportion is the pressure, with which the water is forced out; and this circumstance is not at all affected by the breadth or width of the vessel. Take a vessel of whatever shape you please; it matters not whether it be quadrangular, or cylindrical, or conical; and make openings in it at different heights; any liquid, contained in the vessel, will issue from each of these openings, with a force proportioned to its distance from the top of the vessel; so that it will be discharged much more rapidly by the lowest orifice, than by any one at a greater height. The reason is obvious. At the lower orifice the fluid is forced out by the weight of the whole column of particles above it, while, at any higher orifice, it is pressed only by the column above that orifice, which is plainly no more than a part of the column that presses at the lower one. But we said that the particles of liquids have a pressure not only downwards, and sideways, but also upwards. If into an opening, made in the side of a vessel filled with any liquid, a tube be inserted, like the spout of a teapot, sloping upwards, the liquid will immediately ascend in the tube, till it stand at the same height with the sur

face of the liquid in the vessel. This, however contradictory it may appear to the doctrine of gravitation, is in truth an additional illustration of it, as it is the consequence of the pressure from above which immediately causes the fluid to issue at the only outlet left for its escape. This pressure operates equally at the same point in all directions, and so enables the fluid to retain its level. Let AB, CD, EF, represent three tubes left open at top, of which one is placed vertically, one in an inclined position, the third is of a curve figure, and all are connected by a horizontal tube DF. If water be poured into these tubes, it will rise to the same level GHK in all of them. It matters not what is the shape, or length, or diameter of the tubes, or how different they may happen to be from each other in all these respects. Though AB, for example, were a capacious vessel of vast dimensions, and the others only slender tubes, the result would be precisely the same; the water would stand as high in AB as in the other tubes; and it will rise no higher. By pouring water into the tube CD, you will in vain attempt to fill the higher vessel AB to the top: as soon as it has reached its level, the water, in place of continuing to rise will overflow at C, because a fluid will not rise above its source. If oil had been poured

into any of the tubes, it would have stood higher than the water would have done in the other tubes, because oil is lighter than water. If, on the other hand, quicksilver had been poured in, it would have stood a great deal lower, because it is so much heavier than water. The principle, that a fluid will always find its