far greater import, because it relates to a Company, or body of men, who would fain shelter themselves under this mysterious cognomen from all comment and censure, be it ever so justly merited. It is incompatible with existing circumstances to realise first principles, and act on the abstract law of meum and tuum; mutual concessions, nay, occasionally, sacrifices, are imperative to the security of the whole. 66 Mr. Baddeley comments freely on the reprehensible apathy" displayed by the public in neglecting to provide the means of protection against fire, and further informs us, that they seem rather to court than shun the evil. Whether this be so or not, I am unable to say. Mr. Baddeley has paid much attention to the subject, and doubtless makes this appalling statement of a fact upon sufficient evidence. In this instance I readily defer to his knowledge and opportunities for observing; but I claim a right, as well as he, to comment and pass a censure where I think it merited. If the neglect and supineness of the community is reprehensible, that of the Companies is still more so, as they incur a risk so enormous as to include not less, on Mr. Baddeley's own showing, than a third of the gross property of the metropolis, and are further, as they are, utterly unable to realise any thing like that amount, should they be called on to do SO. Here, then, a trust reposes in the Companies, in the due and just administration 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 communication from Mr. Ettrick, and states that he has for some years laboured unsuccessfully 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 describe, 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 between 15 and 16 to 32 of copper. If copper be clean and good, one may the be sure of a good metal by making the quantity of tin 154, 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 mixing. 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 perfectly 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 withdrawing 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 granulates 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 convenient 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, 18, Norton-street, Liverpool, Dec. 6, 1836. MR. HOWARD AND MR. SYMINGTON. Sir,-In his last communication, Mr. I have descended almost to Howard says 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. 9 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 concerns, Í have the honour to be, Sir, 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 horizontally, 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 centre. 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 impression 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. "From table, No. 1, it appears that the average drainage from June to December, 1835, inclusive (seven months) was 0.392 of the falling water, or nearly ; and the average from June to October, inclusive (five months), was 0.319, or nearly of the fall. August is the minimum month, and shows a drainage of 0192, or nearly of the fall. July is the maximum month (except December, which drained the snow of November), and a drainage of 5.414, or over of the fall. "From table, No. 2, it appears that the average drainage for the year 1835, including the snow on the ground on the 1st of January, was 0.449, or nearly half the falling water; January to May, inclusive (five months), 0-662, or say of the fall; June to October, inclusive (five months), 0.246, or of the fall. It will be observed that the quantity drained from June to October, inclusive, was very uniform, although the falling water was very different, which causes a great range in the ratios for the several months. This was produced by the mountains acting as reservoirs, causing the highest ratio to appear in the month of least fall, and the lowest ratio to appear in the month of the greatest fall of water. From which it is obvious, of June to October (five months), is required to give a proper view of the drainage during the season of the greatest evaporation." The experienced civil engineer may, by that superior tact which is obtained by extensive practice, be enabled in some degree to dispense with such a guide; but to the younger members of the profession it would be of essential service, and even the veteran tactician might find it useful as a reference occasionally. Sir, I have from time to time ad. mired the ingenuity with which Mr. Mackintosh has developed and defended his new theory; but I must confess, that two matters have lately appeared in your Magazine which have materially tended to undermine my opinion of the depth and soundness of his views. -The first is his denial of the moon's rotation on her axis; in support of which he adduces the example of a ball of lead at the extremity of one of the arms of a fly-wheel; and he says, that if the moon revolve upon her axis, so also may the ball of lead be said to do: most certainly it may, and so it does, and so do all the particles of the said wheel each on its own imaginary axis. To prove this, I also may be allowed to cite a few familiar examples; let Mr. Mackintosh pay a visit to Bartholomewfair, take a jaunt in one of those childdelighting roundabouts or vertical merrygo-rounds; so long as the chair in which he may be seated maintains its parallelism, or has no rotation on its axis, so long will the motion be quite agreeable; but let the chair bottom be made fast to the arms of the machine, and my word on't, Mr. Mackintosh will experience a practical illustration of a rotation on his own internal axis. If this suffice not, let Mr. Mackintosh take a bottle of crusted port; and, holding it by the neck, let him endeavour to make it describe an orbit round a candle, at the same time taking care to keep the crusted side always towards the flame; if he do not speedily discover the want of a universal joint at his shoulder to admit of the rotation of the bottle on its axis, I am much mistaken. Lastly, let Mr. Mackintosh take a small globe, and having tied a string round its equator, let him affix a piece of yarn to one of its poles; let him then swing round the globe by means of the string, and he will soon find a thread of fairly spun yarn formed at the pole; could this take place unless the globe revolved upon its axis? The other stumbling-block in the way of my high opinion of Mr. Mackintosh's reasoning powers, is the plan proposed by him for perpetual motion at page 149, and which, be it remembered, he only distrusted, when he distrusted the eternity of the universe itself! Now, Mr. Editor, I honestly aver, that of all the many gropings after the great ignis fatuus with which your publication has teemed since its commencement, this attempt of Mr. Mackintosh's appears to me the most absurd, the most unmechanical, and the least plausible. Mr. Mackintosh thought, that if a squirrel could turn round a tread-wheel, so also ought his bullet, in like manner attracted by its magnet; but in the first place, it is necessary to make the two cases parallel. In order to do so, a cord must be noosed round the neck of the squirrel to represent the magnet, and the other end of the said cord being made fast to the ceiling, let the squirrel be hauled up to the top of his wheel; will he then turn it? No, "he'll be hanged first." How then could Mr. Mackintosh, his magnet being powerful enough to suspend his bullet and thereby nullify its gravity, still expect that the said gravity, so annihilated, would exert its power in causing the descent of the wheel? There is not the slightest intimation that Mr. Mackintosh ever thought of reversing or cutting off the poles of the magnet, which alone could have made his plan even commonly plausible, but which even then would have been immeasurably inferior in point of ingenuity to Mr. Baddeley's pendulum, or to a thousand other similar ideas which have long since appeared in your Magazine. Had not Mr. Mackintosh's name and authority been appended to this plan, and did he not evidently, fron the whole tenor of his communication, still retain all a father's fondness for the bantling, I had never wasted so many words upon it. And here let me remark, that I was rather surprised to read the elaborate refutation which Kinclaven, at page 148, thought it necessary to give to O. N.'s most absurd conclusion about the latitude of London and the pole-star (page 116); surely in this case, at least," Le jeu n'en vaut pas la chandelle." Never in this world was absurdity branded in more glaring colours on any proposition, being neither more nor less than that the polestar would, in a certain year, be at the same instant both north and south of the zenith of London. First, it is stated that in the year 15,064, the pole-star will be 8° 56′ 49′′ south of London; and next, that this very zenith distance will regulate the latitude of London, which will then be 90-8.56 49-81° 3′ 11′′. Now, this last could only be the case on the supposition, that the north pole and the north star are one and the same; and if so, the north star must then be to the north, at the same time that it is stated to be to the south. Hence, it is clear that this "practical" astronomer confounds the pole of the earth with the star which is now called, "by courtesy," the pole-star, but which must resign that distinction long before the period he refers to; and, as if to afford ample proof of the utter confusion of ideas with which he approached the subject, after stating that the latitude of London in the aforesaid year will be 81° 3' 11", a little lower down he makes the declination of the zenith of London 51° 32′ at the very same time. By-the-bye, Mr. Kinclaven himself may be caught napping now and then; for instance, what does he mean by talking of the pole-star's distance from the tropic of Cancer? The tropic of Cancer is a small circle, while he means it for a great one, and a parallel of declination while he means it for a circle of longitude. And again, like O. N., he talks of the pole's declination; but, most probably, this latter is an error of the press. I am, Mr. Editor, Your obedient servant, Dec. 11th, 1836. PRINTING IN COLOURS. We are glad to see that Mr. Baxter has, at length, adopted a plan of applying his new invention of printing engravings in colours, which must at last bring it fairly before the public. The attention of every one who professes the slightest regard for art, must be attracted by his new Pictorial Annual, with eleven plates, in which the minutest shades and tints in the paintings of eleven distinguished artists are re-produced, with a spirit and delicacy which must render it difficult to distinguish the copy from the original. The invention has now, indeed, been applied for some time to the illustration of a series of works on natural history, &c., by Mr. Mudie and others; but so little noise hitherto has been made about the matter, that apprehensions might reasonably be entertained that the process might be invented anew at Paris, amidst a flourish of trumpets, and some Parisian company present a petition to Parliament (backed of course by the Athenæum) for a few thousands, by way of recompense, for borrowing the idea. In this case, however, the Parisians might lay some claim of priority, for twelve or thirteen years ago they actually did spread a report of having invented a process for printing lithography in colours, to which they gave the name of "Lithochromy." The secret history of this affair has been |