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The instrument is similar in form to the common sliding rule, and is adapted to perform involution and evolution, both with integral and fractional indices, with the same facility as multiplication and division by the comThe substance of the paper, together with the engraving, is copied into Rees' Cyclopædia, Art. Sliding Rules.

mon one.

As in the case of the circular instrument, this ingenious contrivance of Dr. Roget has hitherto been lost to the practitioner. Recently, however, a twelve-inch pattern has been laid down at the expence of Messrs. W. and T. Gilbert, opticians, Leadenhallstreet, of whom copies may be obtained.

Last year there was published "The Slide Ruler's Guide, being a practical Treatise on the use of the Artificer's Common Slide Rule," by George Oakley Lucas, 12mo. pp. 40. This little work affords an instance of what is but too common, viz. a person's undertaking to assist the public, before he has made himself acquainted with the extent to which others have preceded him in the same department.

I am, &c.

J. W. WOOLLGAR.

IMPROVEMENT ON THE HYDROSTATIC
PRESS.

Sir, I have often heard it mentioned that the Hydrostatic Press has not been found to answer by bookbinders so well as simple beating. Though no bookbinder myself, the reason which suggested itself to my mind is this, (the fault complained of being that it does not expel the air) that in beating, the instrument used does not fall with such uniform pressure on the edges and outside of a book, as the press does, and so leaves more room for the air to escape. I propose, therefore, that the press be made in a very trifling degree convex, whereby the air would be gradually pressed from the centre to the openings between the leaves and escape. This to me appears so obvious a remedy, that I am induced to offer it to the notice of bookbinders through the medium of your work.

QUICK COINING.

There are eight presses at the Mint, which, on cases of emergency, can all be put in action, and each press coins 40 sovereigns in a minute, making 320 sovereigns by the whole eight presses in a minute, or equal to 19,200 in an hour. Allowance must, however, be made for the breaking of dies, &c. As many as 150,000 sovereigns per day have been coined.

C. A toothed wheel revolving within the brass frame B.

D. A rack working into the teeth of C, and of sufficient length to turn C once round; it has a pin 5, working in a slit in B, and has a slit in which a pin 6 on B works, and thus its parallel motion is preserved.

F. A ruler, which may be retained in any position by the tightening screws F F, its upper corner being brought to coincide with the line H, of the scale of divisions, and its side with the point K. The distance from H to K, must be exactly equal to the length of the rack, or to the circumference of the wheel C.

L. A bar fixed on to a square part of the axis of C, below the frame, and carrying a sliding tracer, and being divided into any number of equal parts. The scale H, must be divided into a similar number of parts, but the length of a division on H, must be to that of a division on K, as 628 to 1, or as the circumference of a circle to a radius.

To use the instrument, set the tracer on L, to the diameter of the generating circle, and the bar E, to a division on H, corresponding with the division on L, at which the tracer stands, then bring the angular point of the rack to the point K, when the tracer will be immediately below the axis of C, and slide the point of the rack along the edge of the bar E, and the tracer will describe a cycloid by the time that the point of the rack arrives at the line H. I am, Sir,

Your obedient servant, J. MURDOCH, Mech. Draftsman. 4, Vittoria-place, Mile-End-road, August 19, 1829.

NOTES OF A COURSE OF LECTURES ON MECHANICS, DELIVERED BY DR. LARDNER, AT THE LONDON UNIVERSITY.

(Continued from last No. p. 12.) It is supposed that all matter is composed of elementary monicules, which are quite hard and infinitely minute, and of which there are three things to be observed.

1. No particles are in actual contact with each other.

2. They are so minute as not to be distinguishable.

3. They are indestructible by human or natural agency.

To prove the first it need only be shown that a body is compressible, for it cannot be said that the particles themselves are compressed. Thus, when a piece of metal is contracted by cold, the weight remains the same although the size is different. It is the pores, or spaces between the particles that are contracted.

The second observation can easily be imagined, by reference to the description of divisibility before given.

The third property is indestructibility. It has been very justly and truly observed, that "It is as impossible to annihilate one of the parcles, of which a body is composed, as to form one, and there is not one single particle less in the world now than there were at its creation." The whole power of man consists in making three changes in matter, viz. by combustion, separation, or evaporation. Thus fuel is changed by combustion, some of the particles escape up the chimney in the form of smoke, (and are dispersed over the earth to conduce to the use or pleasure of man) others remain in the chimney in that of soot, while the rest fall in ashes below, or are dispersed about the room as dust.

All substances or materials are divided into three kinds according to the different strengths of cohesion.

1. When the cohesion of a body is so much stronger than its repulsion as to keep it in a firm position, it is called a solid.

2. When the cohesion is equal to the repulsion, or the difference between them on either side is very small, it is a liquid.

3. But if the cohesion be much less than the repulsion, the particles will actually repel each other, in which case they are called gases.

Every thing in nature may be classed under one of these three heads, with only four exceptions, viz. heat, light, electricity, and magnetism; the study of which is too abstract to be treated of in the present course of lectures.

A solid is the only class required to be understood in mechanics; the

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 tota! 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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Third Definition. If in motion, a body cannot stop, then it would require the same force in an opposite direction.

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

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.)