other two classes of substances form distinct sciences.

The word solid has three distinct meanings.

In Geometry, it stands opposed to superficies, and signifies that which has three dimensions, viz. height, length, and breadth.

In Mechanics, it has the definition before used, and in which sense only it will be now used.

In Metaphysics, it means any thing which occupies a space, or which must be removed before any other substance can occupy the same space. In this sense air is a solid, as may be proved by plunging an inverted glass into a vessel containing water, which cannot enter the glass until the air is displaced.

With regard to a solid, the first property to be considered is inertia, which may be described as a total absence of activity, an incapacity of any spontaneous action. It is a principle that must be evident to all. It is not a body itself that is inert, but every particle of it is so. to be proved,

There are four kinds

1. If a body be at rest it cannot put itself into motion.

2. If a body be in motion it cannot alter its direction or increase its motion.

3. If a body be in motion it cannot stop.

4. Nor diminish that motion.

In illustration of the first definition, it may be observed, that a mass of matter in a state of rest will remain so for ever unless acted upon by some force. It is impossible to imagine that a stone or other substance could move by itself. Now, it can be proved that every other definition can be resolved to this first one.

Second Definition. If in motion, a body cannot increase that motion. Let a body moving with a given velocity suddenly increase at any given rate, say two miles an hour, the same extra force must have been used then which would have moved a body (at rest) two miles an hour, but as no force was used, it would have moved contrary to the first definition. Neither can it alter its direction. In this case the only difference consists in the extra force being exerted in a lateral direction.

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Fourth Definition. If in motion, a body cannot diminish that motion, for the force which when applied against a body in motion, would diminish that motion, would, if it acted upon a body at rest, move it with a velocity equal to the diminution, and which would be contrary to the first definition.

There are three distinct data to be considered, viz. the application, the direction, and the quantity or velocity of a force, all of which are essential.

The property of the third definition of inertia, viz. an inability to stop when in motion, may be shewn in many familiar effects. Stages when they are running with a certain velocity, give the same to the passengers, and when the coach stops suddenly, the passengers would be thrown off were they not prepared. It is in consequence of this property, that a person can leap further by taking a run before doing so; the impetus is gained by the run.

All bodies, as far as have been tried, have inertia, consequently, we may suppose it an universal property of matter. Now, this supposition is made by virtue of what is called mechanical induction, in contradistinction to physical induction, for which, before any conclusions can be drawn, every particle of matter must be tried on. Thus, before it can be said that sand is inert, experiments must be tried on every single particle of the sand; but it is otherwise in mechanical induction, for there if inertia or any other property be discovered in all bodies on which experiments have been made, it is assumed as universal.

It must be remembered that there are two kinds of rests, distinguished by the names of absolute rest and equilibrium; the former produced by its inertia, and the latter by the neutralisation of forces exerted in different directions against the body.

Force is whatever produces or tends to produce motion in a body.

There are two classes of problems derived from the effects produced by the influence of several forces. When under the influence of several forces, a body will be either at rest or it will

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move. If at rest, the forces must neutralise each other's effects, and the body will be in equilibrium. The study of this branch of forces is called statics, and its object is to investigate what forces will keep a body in equilibrium. But if these forces do not neutralise each other's effects, it forms another problem in mechanics called dynamics, when a certain number of forces are given in different directions, to determine what will be the position of the body on which these forces act. (To be continued.)

the range of the ball with three pounds: and consequently there will be a saying of one-third of the powder that is commonly used.

4th. The momentum of the sphe roid is double that of the ball, which is fired from the same gun.

By the momentum of a projectile is meant the power which it has to act against, or to overcome, an impediment; and this arises from its quantity of matter and velocity taken conjointly. Thus the momentum of two hammers with equal faces, when striking the same body, will be equal: though the weight of the one should be six pounds, and that of the other twelve pounds; provided the velocity of the first is double that of the second. And if the velocity of both is equal, the momentum of the second will be double that of the first: and therefore the momentum of the spheroid is double that of the ball.

5th. When it is said that the momentum of the spheroid is double that of the ball, the supposition is that it goes straight through the impediment which it strikes, whether it be of earth, or stone, or wood. But this will seldom be the case, because if its fore part meets with unequal resistance. from the body it strikes, the longer axis will turn round, and the shot will act against the obstacle length-wise, and consequently produce a greater effect than if it acted in the first direction. Now the resistance from the sides of ships is unequal, on account of the position of the bolts and nails, and different texture of the beams and planks, and a small difference in the firmness of the resistance, or in its position, will give a great deflection to the shot.

It is well known that large shot have a great advantage over small shot, because they break out holes in solid bodies in a greater proportion than their weights. The diameter, for instance, of a twenty-four pound ball is double the diameter of a three pound ball; and if both have the same velocity, the first will make a hole in a rampart or wall above eight times larger than the second, and it will penetrate to more than twice the depth. But if they strike wooden defences, or the sides of ships, the difference of the

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holes will be still greater, because the holes do not retain their original pro portions on account of the springing of the wood. Every one will be sensible of this, who has fired cannon at a bulkhead, or who has examined a ship. after a smart engagement.

In consequence of the greater momentum and size of heavy balls, ramparts are cut through by them which would suffer little from small balls;. and they fracture beams and masts of ships, to which a great number of small balls would do little harm.

Now the spheroid will not only have a greater momentum, and make a larger hole than the common ball, but the hole will in general be more irregular, and so cause more splinters, and be stopped up with more difficulty on account of its shape.

6th. Firing with spheroids does not require more time, nor more accuracy, than the usual practice. Balls and grape-shot may be used as at, present in quick firing upon troops, that are on a plane, or near at hand. Though the spheroid should soon be known to our enemies, the addition and the wadding may be kept secret for a long time; and all improvements in war are of the greatest importance to the nation which uses them first? Vary the instruments of destruction and the species of terror," said an old General," and I will gain every battle and take every town."

7th. If shells were made of a spheroidical form, it would seem, that they would produce two important effectsa double momentum, and a greater range than has ever yet been known.

From what has been said, three consequences may be drawn, which are of vast importance to this country at present.

A great saving may be made on gunpowder.

The power of our artillery may be doubled; all the guns in our navy may be converted, as it were, into those of double the force; a sixpounder into a twelve, a nine-pounder into an eighteen, and so on, to the thirty-two pounder, the largest in our navy, which may be converted into a sixty-four; and all this at a trifling expence, in a short time, and without making the least change in the guns.