IMITATION GRANITE.

Sir,-For the information of Lambda, as to the object sought in strewing sand over outside painted work, be it known that in the western part of York State (U. S.) brother Jonathan is awfully given to be "architectural" in the construction of his churches, but makes a woeful failure in his classic aspirations, for want of books and models to work by. Moreover, the materials, not of his choice, but of his necessity, are only wood and brick. He therefore makes a brick church, with a wooden tower or spire, or any other thing he fancies. Sometimes the whole erection is of wood, which is painted and then strewn with coloured sand, according to taste, which at a distance leads the traveller to suppose that it is a stone building, till a nearer approach exhibits the gaping glued joints. The sand also answers another purpose than mere show. It acts as a screen to the paint, and prevents the sun from consuming the oil so fast as it does when left bare. In

ordinary painting, three years of hot sun leaves a painted building, only the dry lead, which forms the body of the colour, and which will then wash off like whitening. In damp climates, the sand will, of course, form a lodgment for mosses.

Yours, &c. JUNIUS REDIVIVUS.

RULE FOR WEIGHING GAS COKE.

Sir, I forward you, below, a practical rule, which may, perhaps, be con sidered worth insertion in your useful Journal. In order to ascertain the exact weight of a sack of gas coke, 12 were weighed separately, and the average was 144 lbs. net per sack. Hence the following simple rule for fixing this result in the memory. The square of the number of sacks in a chaldron is equal to the number of pounds in a sack e. g. 12=144.

Yours, &c.

J. B.

PRINCIPLES OF LOCOMOTION.

To Saxula.

Sir,-My time being very much occupied with other matters, I must beg to decline entering into any argument on the merits of a scheme, which, as a professional mechanic, I may be allowed to

say I know to be worthless. But I shall offer a few observations which I hope will convince you, Sir, of your mistake, and thereby prevent a further expenditure of time and ingenuity.

I must be understood to speak in what follows, without regard to friction, or such other matters as belong more properly, to the practical application of a principle, than to the principle itself.

A locomotive machine must contain within itself a first mover (as a steamengine, &c. &c.) and also the abutment or fixed point against which that first mover acts, and by means of which the first mover gives motion to parts of the machine, which act upon other fixed points, or abutments, which are not parts of the machine, but separate and distinct from it; by means of which the first mover moves the machine from place to place.

I conceive the greatest obstacle to the general introduction of locomotive-carriages to be, the difficulty of obtaining those outward abutments. As to the

length of the cranks, you may depend upon it, Sir, the same power will produce the same effect, whether the crank is six inches long or six feet.

Thus far all is mere assertion; and I leave it to its fate, and proceed at once to notice what appears to me to be the grand error into which you have fallen. If I am not deceived, you have mistaken some such proposition as the following for a demonstrable truth.

"Whatever the power may be which is applied to produce locomotion; that power may be represented by a weight fixed on the horizontal spoke of the wheel; and whatever is true of the weight, will also be true of the power.

1 must trouble you to refer to vol. xv. p. 152, fig. 10, and to imagine the weight hung twice as far as it is drawn from the centre of the axle; and also that the wheel cannot turn round without rising over the obstacle C. Having found what weight is sufficient to move the carriage forward over the block C, we will call it one. If we hang four times this weight between the block and the axle, or at one-fourth of the distance from the centre of the axle that the other was hung, its power to turn the wheel round will be precisely the same as that of the weight one. But no weight, however large, could possibly move the

wheel if suspended from the point we are supposing; because the only tendency of the weight is to descend; whereas the point it is suspended from must ascend when the wheel is moved.

Now, Sir, let us consider how the carriage came to be moved by the small weight. The weight was moved by the mutual attraction between it and the earth; the fixed point which caused the first mover to act was in the earth, and not in the machine. The machine was affected in the same manner it would have been if a man had pulled at the place the weight was hung to, with the same force as the weight! Or, if one end of a spring of equal power had been fixed to the arm, and the other end fixed to the earth. In either case it is moved by something without and distinct from itself; and in either case is no more a locomotive-machine than a mail-coach.

Now, Sir, let us fix a spring between the point of the arm at which we hung our first, or smallest weight, and the frame of the machine. In this case, the fixed point which enables our power to act, is within the machine; now, our machine is locomotive, and all that is necessary is, that our spring should be strong enough to separate the two points it is fixed to, for the purpose of moving the machine over the block. Suppose the necessary strength ascertained, and a spring of four times the strength, or density, placed at onefourth the distance from the centre of the axle, or at the point where we hung our weight, four. The moving power we now employ does not depend upon the points it is fixed between, moving in any particular direction, as regards the earth; all that is necessary is, that these two points should separate, which is effected as easily by the strong spring and the short crank, as by the weak spring and the long crank; and with precisely the same effect, and with the same expenditure of mechanical power.

As to the carriage and weights represented on page 72, vol. xv., it is demonstrable that it will be passed over the block by a less weight, circumstanced as the one is marked four, than by any of the others. I trust, Sir, your experiments confirm this; as, if they do not, they must be very ill made. Yours, &c.

S. Y., an Engineer.

95

P.S. I hope Trebor Valentine will not be driven from the pages of the Mechanics' Magazine, by a similar paper to one which Mr. W. Baddeley treated with well-deserved contempt ?

KEY TO THE SETTLEMENT OF A TIRESOME CONTROVERSY.

Sir,-The following problem, I believe, has no pretensions to originality. The solution of it only requires a slight knowledge of geometry and of a simple but well-known proposition in mechanics; but I believe the whole of Saxula's new theory on locomotion will be found to be involved in its solution.

Problem. Suppose the radius of a circular wheel is R, and that it is loaded with lead near the circumference (the lead forming part of the wheel), so that the distance between the centres of mag

nitude and gravity is m Required, the greatest inclination of a plane, on which the wheel, when placed in its stable position, will remain at rest. And, supposing the above found_angle of inclination to be diminished P degrees, determine how far the wheel will roll up the plane, the motion being supposed to commence from the unstable position.

State the result in numbers, when R 30 m 12 inches, and P13°.

This year, Mr. Whiston, having calculated the return of a comet, which was to make its appearance to-day, Wednesday, at five minutes after five in the morning, gave notice to the public accordingly, with this terrifying addition, that a total dissolution of the world by fire was to take place on the Friday following. The reputation Mr. W. had long maintained, both as a divine and a philosopher, left little or no doubt with the populace of the truth of his prediction. Several ludicrous events now took place. A number of persons in and about London, seized all the barges and boats they could lay hands on in the Thames, very rationally concluding, that when the conflagration took place, there would be the most safety on the water. A gentleman

for

who had neglected family prayer better than five years, informed his wife, that it was his determination to resume that laudable practice the same evening; but his wife having engaged a ball at her house, persuaded her husband to put it off till they saw whether the comet appeared or not. The South-sea stock immediately fell to five per cent., and the India to eleven, and the captain of a Dutch ship threw all his powder into the river, that the ship might not be endangered. The next morning, however, the comet appeared according to the prediction, aud before noon the belief was universal that the day of judgment was at hand. About

this time 123 clergymen were ferried over to Lambeth, it was said, to petition that a short prayer might be penned and ordered, there being none in the church service on that occasion. Three maids of honour burnt their collections of novels and plays, and sent to a bookseller to buy each of them a Bible, and Bishop Taylor's Holy Living and Dying. The run upon the Bank was so prodigious, that all hauds were employed from morning till night in discounting notes and handing out the specie. 'On Thursday, 'considerably more than 7000 kept mistresses were legally married in the face of several congregations.

And to crown the whole farce, Sir Gilbert Heathcote, at that time head Director of the Bank, issued orders to all the fire-offices in London, requiring them to keep a good look out, and have a particular eye on the Bauk of England.

For

MINOR CORRESPONDENCE. Ventilation of Coal Mines.-Sir, I feel that an apology is necessary to an old and talented contributor to your pages, Mr. Michael Garvey. some time past, I have been in possession of a scheme. by Mr. G. for ventilating coal mines; but pressing engagements have prevented me from laying it before the public, through the medium of the Mechanics' Magazine. Under all circumstances, bowever, I cannot say that I experience any regret on account of this delay, since, perhaps, but for the. silence observed, by every body else, on the subject of the collieries, we might not have been favoured with the interesting letter of Humanitas, inserted in your last number. The communication that I shall have the satisfaction of placing in your hands, will show that Humanitas does not stand alone in his endeavours to rescue a deserving body of mechanics from personal danger in their avocatious, and personal insult from their fellow men. Where, indeed, is the man that should say to them, "Stand off; I' am more useful than thou?" Let but such writers as Ilumanitas persevere in their endeavours to dissipate those divisive prejudices so amongst all mechanical trades, and such practical men as Mr. Garvey occasionally lay aside their working tools, to give free scope to their inventive faculties, and we shall soon find whether science is unheeded or declining to that state among practical men, that some of our modern philosophers would lead us to imagine. In a few days, I shall send you a drawing and description of Mr. Garvey's plan; and, immediately following, some account of

common

manner.

Sail-Worked Paddles.-Mr. Editor, I beg leave to acquaint you, that the plan of the vessel with sailworked paddles, given in the last Number of the Mechanics' Magazine, has a very considerable time since been invented and subjected to my mechanical labours. I, therefore, must acquaint you of it; that in case I should come forward with a similar invention, your correspondent, A. B. W. may not accuse me of having copied from him through the medium of your pages, I shall be happy to prove my assertion to you, Sir, in submitting to your personal view the model thereof, if desired.-Yours, &c. S. T. a Constant Reader of your valuable Magazine, London, Nov. 1, 1831.

Mr. Utting and Mr. Mackinnon.-We gather from a letter of explanation which Mr. Mackinnon has sent us, that he was led by a grainmatical misconstruction of a sentence in a communication from Mr. U. (vol. xiii. p. 112.) to ascribe to him that opinion respecting a future state which Mr. U. has disclaimed (vol. xv. p. 304). We deem it unnecessary to give the letter at length..

Plate Powder.-Sir, Having received many valuable hints from your useful work, I am sorry to find fault with it in any way; but what I wish to advert to is the plate powder, for which a recipe was given by "A Jeweller" in a late Number. This I tried upon a pair of watch-cases, making the mixture as near the proportions as I could with my clumsy scales; but the effect was a total discoloration, and I was obliged to send them to a watchmaker's to be cleaned, when it appeared that a drop of spirit of wine with a little of the powder had found its way into the works, and covered the place where it touched with verdigris.-Yours, &c. A TEA DEALER, Lisson Grove North.

Cause of the Report in Explosions." An Old Sportsinan" thinks the question on this subject "might be set at rest by asking Mr. Baddeley how any noise is made when a gun bursts, and there remains nothing for the air' to rush' to?"

INTERIM NOTICES.

The Supplement to Vol. XV. containing. Title, Preface, and Index, with a Portrait of Jacob Perkins, Esq., Civil Engineer, engraved on Steel by Roffe. from an original Painting by Howard, is now ready, price 6d.

We shall endeavour to furnish G. L. S. with the information he wishes in our next.

Communications received from N. E. P.-Mr. Bartlett-Iron Base-A Well-wisher-W. R.-Laicifrepus T. D.-H. H. W.-P. T.-Trebor Valentime A Constant Reader-Mr. Robert Twiss, Jun. X, Y, Z.-Junior,

LONDON: Published by M. SALMON, at the Mechanics Magazine Office, Wine office-court, (between 145 and 146) Fleet Street, where Com'munications (post paid) are requested to be addressed. Sold by G. G. BENNIS, 55, Rue Neuve, St. Augustin, Paris.

M. SALMON, Printer, Fleet Street.