observations of the sun's apparent diameter, then we must allow that the earth, instead of continually approaching nearer the sun, is, in fact, receding from it. But these ancient observations are faulty on two accounts, even granting that their instruments had been perfect. 1st. They had no accurate knowledge of the sun's parallax. 2nd. Refraction was altogether unknown to them. But although allowances were made for parallax and refraction, this would not have the effect of showing that the sun's apparent diameter in ancient times was less than it is at present. In fact, when the earth is at her mean distance from the sun, that is when the centre of the earth passes the extremities of the axis minor of her orbit, in every semi-revolution the sun's semidiameter is getting less and less, and this will continue to be the case until the excentricity of the earth's orbit becomes a minimum; the consequence of which is, that the real circumference of the earth's path, instead of contracting is in reality expanding, and all this is fully established by accurate observations,

Mr. Mackintosh, notwithstanding his pretended veneration for Newton, still continues to sneer at some of his principles, gravitation among the rest. He seems to consider most of them as mere metaphysical or fanciful hypotheses, which have no existence in reality. Nothing can be more unfair than this. Newton was too profound a logician ever to be led astray by metaphysical jargon no supposition or conjecture he ever attached any value to; he felt the force and importance of that maxim so warmly inculcated by Bacon, "That nothing is to be received as proved, in the study of nature, that is not logically deduced from en observed fact; that accurate observation of phenomena must precede all theory; and that the only admissible theory is a proof that the phenomenon under consideration is included in some general fact or law in nature." Newton, in his principles of Natural Philosophy, uses the word gravitation as an expression, not of a quality, but of a fact-not of a cause, but of an event-when he shows, "that the primary planets are retained in their orbits round the sun, and the satellites in their path round their respective primaries, by a force tending toward the central body." Here it must be noticed, that this expression

ascertains nothing but the direction of the force; it gives no hint as to its manner of acting-" it may be the attraction of the central body"-" it may be a tendency inherent in the planet"-" it may be the influence of some ministering spirit;" but whatever it is, this is the direction of its effect.

Mr. Mackintosh perhaps has read the poem of "The Loves of the Triangles" if he cannot reconcile his mind to the term universal attraction, he may change it into that of the universal loves of the planets.

I have read Mr. Mackintosh's last letter (No. 693); it is a strange production. I have not made up my mind whether I shall answer it or not. If I should, I trust Mr. Mackintosh will not take it amiss if I should season my lucubrations with the same ingredients as he has thought proper to do in his last. KINCLAVEN.

CHALKLIN AND BONHAM'S PATENT

VICES.

Sir,—“ A Blacksmith" who kindly communicated to your Magazine, a few weeks since (No. 691), a description of Messrs. Chalklin and Bonham's patent vice, seems to have his doubts as to the ball and socket "answering its end under a great degree of pressure."

I respectfully beg to inform him, and any other person who may entertain similar doubts, that the patentees have had vices of their improved construction in active and hard work for many months past; and that there is not the slightest symptom or chance of a failure in those parts. Any person wishing to convince himself on this point, may do so by calling at No. 408, Oxford-street, where he may see them in use.

Your correspondent must have been misinformed, as the dock-yard at Woolwich has not, at present, been supplied with any of this improved kind.

I believe the parallel vice, mentioned in the same communication, is some five or six years old; therefore, its practical advantages and utility must be by this time well known and appreciated by the public, if it is possessed of any.

By giving the above a place in your very useful publication, you will greatly oblige the patentees, and yours, &c. JAMES PILBROW.

Barnet, Dec. 10, 1836.

THE FIRE-INSURANCE SYSTEM.

Sir, I have read Mr. Baddeley's communication in answer to the remarks I made on the conduct of the Fire-Companies, and though I am ready to defer to his knowledge and experience in matters connected with that on which we are at issue, I take leave to subjoin a few lines, which you will be pleased to give a place in your Magazine, if you consider them pertinent to the question.

The ostensible motive to insure against fire, indeed the compact between the Insurance Companies and the public, is simply, that any loss occasioned by fire shall be made good by the Company, excepting, of course, in such cases as are proved to be fraudulent. It is, however, the interest of both parties that the loss should be as light as possible; for, in the case of the public, the destruction of property is attended with many circumstances which a remuneration to the amount of loss sustained does not compensate; thus in a manufactory positive injury is sustained by loss of time, the destruction of machinery, tools, &c.— and this holds good, mutatis mutandis, in every instance. Here, then, prevention is more than equivalent to the remedy proposed. All parties have a common interest in the preservation of property; and it is unreasonable, all cir#cumstances considered (I admit it is not SO specified in the bond"), that the insurers expect the utmost vigilance, and the employment of the most efficient means, in arresting the progress of fires, when such an interest is at stake so considerable as to include a third of the entire property of the metropolis.

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There is no analogy in the case of the Insurance-Companies and that of a private individual. By taking upon themselves the administration of a fire-police, the Companies tacitly guarantee protection to the insurers. For against accidents originating directly on their own premises or dwellings only. And the insurer understands from this circumstance that this is only a measure of selfdefence on the part of the Company; and as every one is liable to such calamities, calculates on a protection and prevention which, as has been already observed, both parties are equally interested in securing.

By the strict letter of the agreement,

In

the Companies are not bound to provide fire-engines or organise a fire-police for the protection of property. It is sufficient for our purpose, however, that they have done so, since by so doing a responsibility attaches to them. It is idle to argue that they are at liberty to be as careless as they please with their own property, as if the consideration was something altogether ordinary, and they were the only persons interested. equity, their relation to the public is of the nature of a trust. Suppose the GasCompanies thought fit to discontinue suddenly the supply of gas, and argue, "We are not bound to continue to light your streets, houses, and shops. We think it expedient to discontinue this manufacture; and you have no right to complain, since there is no violation of the compact between us, and you have had your money's worth." The proprietors of water-works might also, by a parity of reasoning, discontinue the supply of that indispensable article, at pleasure; they are not bound to consult the consequences of their act and deed; they also may do as they like with their own; there is no guarantee on their part, nor does any really exist between the community and public bodies in any instance. But is it to be asserted from such premises that they are upon the bare supply, and acting to the strict letter of the agreement-in other respects, free to act as they please? Nay, so jealous should the constituted authorities be of the general safety, that even individuals must be controlled and subjected to strict laws in their relation to the entire body. Thus we find man is not at liberty to do as he pleases in the erection of a dwelling. The addition of a party-wall is imperative, as the safety of the community is supposed to be secured thereby. The propriety of such an interference cannot be questioned, though it would seem, from the way in which Mr. Baddeley argues the case of the fire-offices, that provided the materials were duly paid for, and the labourers' hire apportioned, and no one prejudiced, the good man may build according to his judgment, or his want of judgment. Now, though this wholesome enactment emanates from the Legislature, the public are represented by that body, and the act and deed is their's; but, forsooth, they are to have no voice in a matter of

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SO.

Here, then, a trust reposes in the Companies, in the due and just adminis tration of which all are interested; and if the public may be censured when censure is due, Public Companies are surely not to be exempt, under similar circumstances. For my part, I can discover no just reason for the distinction.

In the course of this discussion we bave lost sight, of the subject which immediately gave rise to it; and though I have duly considered Mr. Baddeley's ob jections, I must take leave to revert to the question as it was originally mooted, and ask why Mr. Braithwaite's steam fire-engine is not employed for the purpose of extinguishing fires? Mr. Baddeley's strictures on the apathy of the public may justly be applied to the Insurance-Companies; and I assure him that I am as well persuaded as he is, that "much good will ultimately of necessity arise to all parties,, by a more correct, understanding of the true rela

CASTING AND GRINDING SPECULA.

Sir, In No. 678 of the Mechanics Magazine, dated 6th August last, which has only just come into my hands, I find a letter signed C. G. on the subject of casting and grinding specula, in which the writer refers to a long-expected com munication from Mr. Ettrick, and states that he has for some years laboured un successfully in endeavouring to discover a method of casting specula free from porosity and holes in the surface. I have for several years devoted a portion of my leisure time to the construction of reflecting telescopes with, I venture to assert; a large measure of success; and if you think the subject will be interesting to your readers, I shall be happy to communicate in a few letters, as my leisure may permit, such results of the experience I have acquired in the art, as it may occur to me to detail. I have not found in common with your correspondent, that greater difficulties attend the casting than the figuring and polishing of mirrors; indeed, I attained some years ago, by the process I am about to de scribe, such a degree of perfection in casting, as left me little to desire in that branch of the art, and therefore my late efforts have been chiefly directed to such improvements of the figure and polish of reflectors, as might make them rival the best refractors in the definition of the fixed stars.

The metal of which specula are made is a composition of copper, grain or bar tin, and arsenic, in the following proportions:-Copper, 32; tin, from 15 to 16; arsenic, 1 by weight. The qualities of this composition are extreme hardness, brittleness, whiteness, and a great susceptibility of polish, which seem all to depend as to their maxima upon the accurate proportions between the copper and tin. It would appear that different qualities of copper require different quantities of tin to produce the maximum of perfection, and that this causes the proportion of tin to vary be tween 15 and 16 to 32 of copper. If the copper be clean and good, one may

be sure of a good metal by making the quantity of tin 151, and adding the due proportion of arsenic, although after a little experience it will be found better to try the metal itself in the process of mix ing

Old copper bolts of ships are very good and convenient for melting. The proper quantity of copper must first be put into the crucible, and rendered per fectly fluid. The proportion of tin should then be melted in a separate crucible, the crucible containing the copper withdrawn from the furnace, the tin poured into it and well stirred with a clean iron rod or dry stick of wood. As the mixture is fusible at a much lower heat than the copper alone, it will be quite hot enough after the stirring and withdraw ing from the furnace to pour into the moulds, and I have made very good metals by this process. But I think it much better for the compactness of the composition and freedom from pores to introduce a second melting before the casting, and pour this first mixture immediately after it is made and well stirred over a birch-broom into a large tub of water, which instantly cools and granu lates the metal; thus preventing, it is said, the calcination of any part of the tin by its remaining any length of time at the great heat imparted to it by the copper. Somewhat less than the due proportion of tin should, in the first instance, be added, not more than in the proportion of 15 to 32; and in the second melting, when the composition is ready for pouring, a small quantity should be taken out in a ladle and instantly cooled in water. It will most likely crumble almost to pieces, or, at any rate, will easily break with the fingers- If the fracture is extremely white and brilliant, like quicksilver, the arsenic may be now added, wrapped up in a piece of paper, and well stirred in, until the fumes have disappeared. It is, however, most likely that the composition will bear the addition of a little tin, and it will be conve nient to have small portions weighed ready, such as would raise the proportion of tin from 15 by tenths up to 16, if necessary. Very small portions will make an obvious difference in the fracture, and it may perhaps be well to add one dose beyond what may be considered the ne plus ultra of brilliancy, to insure a somewhat firmer texture. A very little experience will enable the operator to ascer

tain pretty accurately by the fracture the proper quantity of tin, but the copper should never be in excess, which would render the metal liable to tarnish. Too much tin renders the fracture dull and grainy, and the metal very tough and difficult to work. I do not consider the addition of arsenic, absolutely essential; I have made good metals without it, but was generally accustomed to its use, and so far as my experience went, its effect seemed to be to whiten the metal, and indeed to be somewhat similar to that of an additional small, portion of tin.. The furnace I used was built upon the plan of that described in Henry's "Chemistry," 11th edition, vol. i., p. 680, and there stated to be the invention of Mr. Knight, of Foster-lane. Its size, however, of nine inches square inside,, was rather too small for melting with quickness the quantity of metal necessary for a nineinch speculum, though I have cast a larger size out of it. Its construction is convenient for allowing the fumes of the arsenic to escape, which otherwise might be pernicious.

To describe the process of modelling, the metal in the sand would, I fear, extend this letter to an unreasonable length, I shall therefore reserve that subject for my next communication, and in the mean time,

Remain, Sir,

Your most obedient servant,
WM. LASSELL, Jun.

18, Norton-street, Liverpool,
Dec., 6, 1836.

MR. HOWARD AND MR. SYMINGTON.

Sir, In his last communication, Mr. Howard says I have descended, almost to personal scurrility against him. I am not aware of having done so; and suspect Mr. Howard is himself the party against whom such an accusation might with justice be brought. And to convince him that my suspicion is justified, let me remind him he described as "twaddle" my thanking him for a piece of information, which he had given me through the pages of your Magazine, concerning his share in depriving me, of the Alban

In reference to the completeness of Mr. Howard's plans, as exemplified in the Vesta, I must take the liberty of saying that it does, seem rather strange

to me, if they be so very complete, that she should be so often withdrawn from public view, for the purpose of having perfection perfected. While Mr. Howard chooses to amuse himself without interfering with my patent rights, he need fear no interruption from me; but if he interfere with them, he will find me just as ready to defend them in any other quarter as I have been in your Journal. I remain, Sir,

Your most obedient servant,

WILLIAM SYMINGTON.

1, King William-street, Dec. 8, 1836.

ASBESTOS GAUZE SAFETY-LAMP. 1]

Sir, The safety-lamp of Davy, not. withstanding its oft demonstrated insecurity, being still very extensively employed in the mining districts, it follows, that any improvement in its construction must necessarily be attended with considerable public benefit: and I therefore beg, through the medium of your Magazine, to offer the following suggestion regarding it. I propose, then, that the flame of the lamp be surrounded with a very fine gauze of asbestos, instead of the wire one with which it is, at present, invested. The advantage arising from this modification will be, that owing to the extreme slowness of asbestos in conducting or receiving heat, the gauze proposed could never be heated beyond a very trifling temperature; and that, consequently, the dreadful explosions, to which the lamp in its present form is so frequently giving rise, would thus be, in a great measure, prevented.

Humbly hoping that the above hint may prove valuable to that useful body of men whom it more particularly con

cerns,

Í have the honour to be, Sir,
Your most obedient servant,
W. H. O.

St. George's East, Dec. 9, 1836.

REPORT ON MR. PRUTZMAN'S LEVER LOCK AND KEY.

(The Committee on Science and the Arts, constituted by the Franklin Institute of the State of Pennsylvania for the promotion of the Mechanic Arts, to whom was referred for examination the Lever Lock and Key, invented by Mr. Prutzman, of Philadelphia, Report:)

That they have examined the lock and key and find it to be a specimen of both ingenuity and workmanship.

The main feature in the lock is the manner of securing the bolt, so as to prevent its being operated on by means of a pick, and? to prevent a key being fitted to it unless in detached parts. The lock is arranged with tumblers working vertically, and horizon tally, so as to secure the bolt in its position, when locked or unlocked. The tumblers are operated on by means of a lever inserted in the bit of the key, and working on a centres This lever is put in motion by a plate so arranged in the lock, as to pass into one of the wards of the key, and press the lever, towards the barrel or stem. One end of this lever acts on a projection raised on one of the tumblers, causing it to descend, whilst a portion of the key acts on a parallel tumbler, causing it to ascend, the opposite end of the lever acts on a horizontal tumbler, and thereby relieves the bolt. When the bolt is shut the tumblers resume their former position, and secure it in its place. A lever is placed between the two parallel tumblers and working on a centre, the bolt is secured at one end and moved by means of a key at the other, causing the bolt to move in an opposite direction to that of the key.

The general arrangement of the lock is simple and good, and we may add the best that is within the knowledge of the Committee. From the common key an impres sion may be taken so as to form a duplicate; but from the present arrangement of a key, with a lever inserted, it will not be practicable, owing to the great accuracy required in the formation of the lever.

Mr. Prutzman deserves a great deal of credit for his ingenuity. The Committee understand that several of the locks are at present in use, one in the Counting-house of the United States' Bank, and others in the City of Baltimore.

May 12, 1836.

W. HAMILTON, Actuary.

PROPORTION OF DRAINAGE TO FALL OF RAIN.

In civil engineering, a knowledge of the relative proportion which the drainage bears to the actual fall of rain, is sometimes of the utmost importance. The following observations (from the Franklin Journal) were made in America, and may serve as a basis for a similar set of tables adapted to the climate of England:

"The following deductions are made from two interesting tables, containing the fall of rain and the amount of drainage in Eatonbrook and Mudison-brook valleys, in the State of New York. They are taken from a report of John B. Jervis, Esq., chief engineer of the Cherrango canal, to the canal commissioners of the State of New York,