CIPITATION OF IRON BY AMMONIA. been describing the engine himself, INFLUENCE OF SUGAR IN THE PREfrom a view of it in operation, without wishing to describe the principlé of its operation, he could scarcely. have used other terms, than those used in the journal of Cosmo of Medici." BURSTING OF STEAM BOILERS. Sir, The disastrous events which have repeatedly happened, owing to the bursting of steam boilers, naturally turns the attention of humane minds towards any thing which may tend to prevent or mitigate such dreadful evils. An idea having occurred to me on the subject, I hasten to communicate it to you. I propose to have an outer and an inner boiler; the inner one to contain the water, whilst the space between it and the outer one is intended to contain steam only, which will be supplied by a communication at the top. The pressure will, of course, be upon the external boiler, and as that contains steam only, I presume that but little damage would occur, in case an explosion should take place, as the force of such a small quantity of steam would be immediately destroyed by its expansion. As an additional security, the space between the two boilers might be divided into different compartments, all communicating at the top, which would naturally confine an explosion to that compartment which might happen to be weakest, and principally to the force of the small quantity of steam contained therein. All danger from the hot water would be avoided, as it would still be retained in the inner boiler; I think, too, that the top of the inner boiler might be so contrived, as, in case of an accident, to be easily fixed to the steam pipe, and the vessel might proceed on her voyage as if nothing had happened, though not with equal safety. I need not enter into further particulars, as any engineer who may adopt the plan will easily be able to supply the rest. I am, Sir, An old Working Mechanic. The power possessed by tartaric acid, of preventing the precipitation of iron from its solution in muriatic acid by ammonia, is now well known, from the observations of M. Rose and others. M. Peschia has remarked a similar effect produced by sugar. Peroxide of iron being dissolved in nitro-muriatic acid, with the addition of sugar, ammonia, on being added, produced no effect. A solution of iron in muriatic acid, to which sugar had been added before ebullition, presented the same appearances with ammonia; but if the muriate of iron and sugar had not been boiled together, then the ammonia precipitated the oxide. Gum arabic, which scarcely differs from sugar in its composition, has not this property.-Ann. de Chemie." ILLUMINATED CLOCK FACES. The face of the new clock, which Church, Fleet Street, is nightly has been put up at St. Bride's lighted up with gas, an improvement versally adopted throughout the which we should be glad to see unimetropolis; and, indeed, in all towns and cities. It will not only make our church clocks twice as useful as they are their dial plates being, hours (on an average) out of every at present, invisible for about twelve nights of winter half their gloom. twenty-four-but take from the dark The clocks of Venice have been illuminated in this manner (though tury; but the first instance of the not with gas) for upwards of a cen example being followed in this counGlasgow, who, about two or three try, was furnished by the people of years ago, lighted up their Tron church with gas. The plan, elevation, and station, being given to delineate the interior of a room, with a French casement and sky-light, in parallel perspective. The principal variation in this figure, is the situation of the vanishing point, which is here placed in the centre, (this is no rule, though it is frequently the best position in. interior views, large landscapes, &c.) To find its place, having drawn the rays to the station, and the intersecting line, I N, according to problem 1, draw from S the ray, V L, cutting IN at right angles, and its intersection gives V. the vanishing point. The sky-light must be drawn on the plan, (and if, as in the present instance, circular, enclosed in a square,) and rays drawn from each The rays, 1,4,5, and 6, relate to the room; 2 and 3, to the window, and the dotted rays, 7, 8, 10, and 9, to the sky-light. To find the representation of the room, having raised the perpendiculars as in problem 1, set off from HL, on GH, 2, the geometrical height of the walls, A D, and L, on GH, 1, the same, and also A C, the height of the window. Then draw the lines, A V, AV, DV, and D V; and where they cut the perpendiculars, 1, 4, 5 and 6, draw the parallel lines, E E, F F, K K, and M M, and the height of the window will be found by the intersections of the line CV, with the perpendiculars, 2 and 3.0 To find the situation of the skylight, draw on the ceiling the dotted diagonal lines, and where they intersect is. centre; then, angle to the station; to which, also, where the diculars cut the rays must proceed from the angles of diagonals, draw the square, 7, 8, 9, doors, windows, projections, &c. 10, and find the circle by problem 4. GRATUITOUS $ LECTURING. It may be here observed, that whether in parallel, oblique, or mechanical perspective, the intersecting line may be placed not only between the plan and the station, but on or beyond the plan, as in fig. 28, where, for illustration, I have drawn the lines, IN, 1, and I N, 2. In this case, the visual rays must be continued beyond the angles, till they cut the intersecting line. The points of elevation are found either by the intersecting of I N, 1, with the plan, (as A,) or by continuing the lines, as in problem 1, (as B.) Thus, however small the plan, the representation may be made of any required size. In placing the horizontal line, its natural position is the height of the eye, or about 5 ft. 6 in. but in machinery, panoramic views, (sometimes called bird's eye perspective,) &c. it is often convenient to elevate it above the object to be delineated, and, under which circumstances, the drawer must use his own discretion. When the student is thoroughly versed in the science, it will occasionally be productive of advantage, to deviate slightly from its exact rules, when a striking effect is required; as though the drawing may be perfectly correct, when viewed from the proper station, many objects would appear bad to the general observer, particularly columns, cylinders, &c. but such deviations must be made with the utmost caution, and in the best taste; and in representations of machinery, or where great accuracy is required, should rarely be attempted. M&H. S. (To be Concluded in our next.) madong ed ela od kan bas GRATUITOUS LECTURING. 523 Sir,-"Let the galled jade wince, my withers are unwrung." If my letter in refutation and exposure of a misstatement of facts, contained in Aurum's epistle, has raised in him so considerable a degree of warmth of feeling, I cannot but help attributing it to the truths which I have there stated. Aurum is angry because he is wrong; and in the whole of his gentle attack upon the contents of my letter, he does not attempt, in one single instance, to contradict me. He knows very well, that he could not support his case; so off he flies in a tangent, and complains most pathetically of being denounced as a false friend of the Mechanics' Institution; being made the vehicle of personalities and so on. After all this show, what does it amount to? Let us strip the jackdaw of his borrowed plumes, and see how he then looks. He complains of personalities-then we look to him for the virtue of abstaining from the use of them-but, no: gross," we find the epithets of "spleen,"" unseemly," "intemperate," and, further on, he politely calls me fool," after saying he shall not, and in the quotation of poetry, I am called " and acarrogant" and senseless," cused of "sinister views." Now, supposing I had abused the worthy gentleman, is he not even with me? yet he declares that he holds such conduct in abhorrence- admirable consistency! 66 er 66 The point on which Aurum seems most to feel, is that of being denounced! as a false friend. If he wished to rebut this charge, and to show that I accused him wrongfully, why did he not disprove the facts I advanced to expose his misstatement? No, this he could not do with any kind of propriety, he therefore contents himself with characterising my reply to him as unbecoming. Not one word does he offer in refuta-i tion of the facts I brought forward; his are mere assertions, mine are positive truths. He asserts that we differ only in opinion, and that my letter only proves his assertion, but let us examine the case, and see how a plain tale shall, set him down. Aurum, in his first letter, stated, that the committee of the London Mechanics' Institution managed their affairs badly; to prove this, he observed, that the lectures were bad; to prove that the lectures were bad, he offered as evidence, "the consequent decrease in the attendance of the members." Now, I stated, and Aurum has |