In splitting wood (fig. 20), the sides of the opening act as levers, and thus rend the parts in advance of the point of the

wedge.

The wedge is useful in dockyards, where large vessels are raised by its agency.

The heads of

hammers are fastened on by wedges driven in at the part of the handles near the heads.

Fig. 20.

Nails, knives, awls, needles, swords, razors, hatchets, chisels, and other similar instruments, are in their operations on the principle of wedges. A saw is a series of wedges, which act by drawing them along, and pressing them on the object to be cut. When the edge of a razor is examined by a microscope it is seen to be a saw in formation, which, by being drawn along the beard, enters the hair, and thus cuts it off. A scythe acts in the same manner on grass. The saw-nature of fine edges may be illustrated by pressing the thumb against a sharp penknive; the skin is not cut, but the slightest movement of the edge across the skin immediately cuts it. "Mechanics and Mechanism," by R. S. BURN.

THE SCREW.

THE screw is placed under the heading of simple machines, but it cannot be used without the applica

tion of a lever or some other contrivance, when it be comes a compound engine of great power either in pressing bodies closer together or in raising great weights.

A screw is in principle a projecting inclined plane bb (fig. 21) winding round a cylinder aa; for were it un

wreathed, it would form an inclined plane, the length of which would be to its height as the circle of the cylinder is to the distance of one incline or thread, as they are called, from the other. This spiral thread or screw aa (fig. 22) works in another which is cut in the inner surface of a hollow cylinder cc, called a nut or box. This portion is generally fixed. The one is formed exactly to fit the other. A lever b is placed in the head or other part of the screw, and every turn carries it forward upon the nut or box, or draws it upon it to the extent of two turns of the thread. If the circle of the screw be 3 inches, and the distance of the threads an inch, then the power gained will be as 6 to 1, as seen in the inclined plane; the height raised will be an inch, but the whole cylinder, 3 inches or 6 half inches, has been passed over by the power, while the weight has only moved an inch. Thus it is as 1 to 6 of power gained. But as the distance apart of the

threads of the screw is lessened, so is the power increased. Suppose the distance of the threads apart to be of an inch, and this to be turned by a lever 36 inches long, then the circle described by the lever will be about 216 inches, which, multiplied by 4, for the inch of the screw, gives 864 for the power gained, being 864 times as great as the distance between the spirals; therefore a power of 1 lb. at the lever would balance 864 lbs. acting against the screw, and the velocity of the power will be to the velocity of the weight as 864 to 1. Saying a man's pull or pressure is equal to 120 lbs., and four men are employed at the lever, then the pressure would be 864 multiplied by 120 four times over, equal to 414,720 lbs.

Formerly, in the paper mills where it was requisite to have an enormous pressure, the lever was frequently sixteen or twenty feet long, worked by eight or ten men assisted by a winch and pulleys.

A corkscrew is a screw without a central spindle or cylinder.

The screw is applied in pressing books, letters, &c. (fig. 23), in packing

light substances, as cotton, flax, and blankets, by which they are made to occupy a comparatively small space; also in winemaking to squeeze the grapes, in cheese-making, and by the smith, carpenter, turner, and other artisans.

Fig. 23.

It has also been the power on the principle of which the hand printing-press has been made. To effect the

S

operation of printing with rapidity, the thread of the screw is made very wide, and others placed between, so that there are three screws; the great incline of the plane giving velocity, and the number of screws power. Thus, in a moment, by a pull at the press-bar or lever, a pressure of a ton is given on the paper and type.

The screw is the means by which coin is formed, letters copied, and dies imprinted on letter paper and envelopes. The beautiful embossed boards displaying great artistic taste in design and execution, so much used in the elegant ornaments conceived by ladies for the adornment of the drawing-room, are impressed by a large and much inclined screw, similar to that of the printing-press, having a huge horizontal wheel as a lever power, swiftly turned by the strength of several

men.

The screw also regulates many of the instruments of the mathematician, astronomer, operative chemist, and engineer, and is an invaluable assistant to the maker of delicate instruments, as by certain turns he can adjust his tools so as to mark in an inch a hundred thousand lines, the exactitude of which is all-important in the pursuit of scientific truths.

"Mechanics and Mechanism," by R. S. BURN.

PUMPS.

A PUMP, in the common acceptation of the term, is a contrivance for raising fluids by atmospheric pressure. There are three kinds of pumps used for raising water, of all of which there are various modifications. The simplest and most common pump is the ordinary lift, or sucking, or household pump.

It is of great antiquity, its invention being ascribed to Ctesebes1 of Alexandria, about 120 B.C. The annexed figure is a section of the common suction pump. This pump consists of a hollow cylinder,2 A, of wood or metal, which contains a piston,3 B, stuffed so as to move up or down in the cylinder easily, and yet be airtight to this piston there is attached a rod, which will reach at least to the top of the cylinder, when the piston is at the bottom. In the piston there is a valve which opens upwards, and at the bottom of the cylinder there is another valve C, also rising upwards, and which covers

:

B

the orifice of a tube fixed to the bottom of the cylinder, and reaching to the well from whence the water is to be drawn. This tube is commonly called the suction tube, and the cylinder the body of the pump.

When the piston is at the bottom of the cylinder, there can be no air, or very little, between it and the valve C, for as the piston was pushed down, the valve in it would allow the air to escape, instead of being condensed, and when it is drawn up, the pressure of the air above would shut the valve, and there would be a vacuum produced in the body of the cylinder, when the piston arrived at the top. But the air in the cylinder being very much rarefied, the pressure of the valve C on the water at the bottom will be greatly less than that of the external atmosphere on the surface of the water in the well; therefore the water will be pressed up the pump to a height not exceeding 32 or 33 feet.

As the valves shut downwards, the water is prevented