the square of two seconds, (the time) or four, I get sixty-four, the height of the plane. But the plane being three times as long as it is perpendicularly high, it must be three times as many seconds in rolling down the plane, as it was in descending freely by the force of gravity, that is, six seconds.

Emma. Pray, papa, what common instru ments are to be referred to this mechanical power, in the same way, as scissors, pincers, &c. are referred to the lever?

Father. Chisels, hatchets, and whatever other sharp instruments which are chamfered, or sloped down to an edge on one side only, may be referred to the principle of the inclined plane.

CONVERSATION XX.

Of the Wedge.

Father. The next mechanical power is the wedge, which is made up of the two inclined planes D E F G and C EFG (Plate Iv. Fig. 29.) joined together at their bases e EFG: DC is the whole thickness of the wedge at its back A B C D, where the power is applied, and DF and CF

are the length of its sides; now there will be an equilibrium between the power impelling the wedge downward, and the resistance of the wood, or other substance acting against its sides, when the thickness D C of the wedge is to the length of the two sides, or, which is the same thing, when half the thickness D E of the wedge at its back is to the length of D F one of its sides, as the power is to its resistance.

Charles. This is the principle of the inclined plane.

Father. It is, and notwithstanding all the disputes which the methods of calculating the advantage gained by the wedge have occasioned, I see no reason to depart from the opinion of those who consider the wedge as a double inclined plane.

Emma. I have seen people cleaving wood with wedges, but they seem to have no effect, unless great force and great velocity are also used.

Father. No, the power of the attraction of cohesion, by which the parts of wood stick together, is so great, as to require a considerable momentum to separate them. Did you observe nothing else in the operation worthy of your attention?

Charles. Yes, I also took notice that the wood generally split a little below the place to which the wedge reached.

Father. This happens in cleaving most kinds

of wood, and then the advantage gained by this mechanical power, must be in proportion as the length of the sides of the cleft in the wood is greater than the length of the whole back of the wedge. There are other varieties in the action of the wedge; but, at present, it is not necessary to refer to them.

Emma. Since you said that all instruments which sloped off to an edge on one side only, were to be explained by the principle of the inclined plane; so, I suppose, that those which decline to an edge on both sides, must be referred to the principle of the wedge.

Father. They must, which is the case with many chisels, and almost all sorts of axes, &c. Charles. Is the wedge much used as a mechanical power?

Father. It is of great importance in a vast variety of cases, in which the other mechanical powers are of no avail; and this arises from the momentum of the blow, which is greater, beyond comparison, than the application of any dead weight or pressure, such as is employed in the other mechanical powers. Hence it is used in splitting wood, rocks, &c. and even the largest ship may be raised to a small height by driving a wedge below it. It is also used for raising up the beam of a house, when the floor gives way, by reason of too great a burden being laid upon it. It is usual also in separating large mill stones from the siliceous sand-rocks

in some parts of Derbyshire, to bore horizontal holes under them in a circle, and fill these with pegs or wedges made of dry wood, which gradually swell by the moisture of the earth, and in a day or two lift up the mill-stone without breaking it; to this practice Dr. Darwin alludes:

Climb the rude steeps, the granite-cliffs surround,
Pierce with steel points, with wooden wedges wound.
BOTANIC GARDEN.

CONVERSATION XXI.

Of the Screw.

Father. Let us now examine the properties of the sixth and last mechanical power, the screw; which, however, cannot be called a simple mechanical power, since it is never used without the assistance of a lever or winch; by which it becomes a compound engine, of great power in pressing bodies together, or in raising great weights. AB (Plate IV. Fig. 30.) is the

representation of one, together with the lever

DF.

Emma. You said just now, papa, that all the mechanical powers were reducible either to the lever or inclined plane, how can the screw be referred to either?

Father. The screw is composed of two parts, one of which, A B, is called the screw, and consists of a spiral protuberance, called the thread, which may be supposed to be wrapt round a cylinder; the other part c D, called the nut, is perforated to the dimensions of the cylinder; and in the internal cavity is also a spiral groove adapted to receive the thread. Now if you cut a slip of writing-paper in the form of an inclined plane a b c (Fig. 30.) and then wrap it round a cylinder of wood, you will find that it makes a spiral answering to the spiral part of the screw; moreover, if you consider the ascent of the screw, it will be evident, that it is precisely the ascent of an inclined plane.

Charles. By what means do you calculate the advantage gained by the screw?

Father. There are, at first sight, evidently two things to be taken into consideration; the first is the distance between the threads of the screw; and the second is the length of the lever.

Charles. Now I comprehend pretty clearly how it is an inclined plane, and that its ascent