regulating forces, springs, a verge, and a balance. 10. A very remarkable Wooden Domewas that of Halle du Blé, at Paris, which was destroyed by fire some years ago. Its construction, invented by a carpenter of the name of Molineau, was extremely simple, and surprised many that it had not been thought of before. It occurred to this man, that a thin shell of timber might be made so as to be nearly in equilibrio, and also when firmly hooped horizontally, to have all the stiffness that was requisite. The circular ribs comprising this dome, consisted of planks, nine feet long, thirteen inches broad, and three inches thick,-three such planks, bolted together, so that two joints met, forming each rib. All the pieces being small, no machinery was required for carrying them up. The ribs were connected together by horizontal timbers, and iron straps, which answered the purpose of hoops. When the work had reached a certain height, every third rib was discontinued, and the open space was glazed. At a certain height above this, again, every second rib was discontinued, and the vacuity in like manner glazed. Higher up, still, the heads of ribs were framed into a circular ring, so as to form a wide opening, and over this was placed a glazed canopy, in which was left a space for the escape of heated air. This dome, which was 200 feet in diameter, and only a foot thick in the sides, was extremely beautiful, and used to be spoken of with admiration by all who had seen it. SOLUTIONS AND QUERIES. THE question proposed by Rusticus, in page 286, is not properly put, for a rectangular board cannot be broader at one end than the other; but the board, as it is, may be cut to answer the conditions of the question, viz. at 51 inches from the thick end, the board will be (according to its description) 16.9 inches broad, and at 9 inches farther it will be 15.1 inches broad, 16.9 + 15.1 = 32, the half of which is 16, which multiplied by 9, is 144, a square foot. G. D. Query. A weaver's beam, 24 inches in circumference, has on it 95 rounds of cloth, of an inch thick-what is the length of the web? G. D. 1. What is the best method of removing yellow spots from books or prints, that have been contracted by lying in damp places? 2. What is the best mode of cleaning books or prints that have been soiled by being carelessly handled? 3. What are the best ingredients for making a varnish for violins, their respective quantities, and what is the best method of laying it on? 4. What is the most improved method of blackening and polishing picture frames? 5. How is a colour to be composed that has the nearest resemblance to gold leaf? 6. What is the most improved method of mixing up, and painting a mahogany colour? Unusual Parhelia seen at the Cape.-On the 7th May, 1823, when the sun's lower limb had just dipped the water's edge, the Reverend Mr. Fallows observed several parhelia, viz. 4 on the left, and 3 on the right hand of the sun, and all cut by the horizon like the real sun. They had the same shape as the real sun, and were as high, but not so long. When the upper limb of the sun came in contact with the horizon, it and the mock suns appeared as bright points upon the water's edge, and then one of them instantaneously vanished. The barometer was at 30.2 inches, and the thermometer at 64°. Dr. Young supposes that these parhelia were only fragments of corona, formed by the defraction of a cloud ris ing but little above the horizon; and he attributes the absence of colours to the absorption of all the light, except the red, in its long passage through a hazy atmosphere. The phenomenon, now described, we consider as very remarkable, and as one hitherto unobserved; but we cannot, for the following reasons, concur in the ingenious explanation of it given by Dr. Young: 1. Mr. Fallows distinctly states, that not a cloud was visible. 2. He distinctly describes, and draws the mock suns as having exactly the shape of the real sun,—a form which fragments of coronæ could not be supposed to as sume. 4:00 THE GLASGOW MECHANICS' MAGAZINE. 3. He describes them as equal in brightness to the real sun,-which they could not possibly be, if they were fragments of coronæ, unless some obstructing medium obscured the light of the real sun, without affecting that of the coronæ. Mr. Fallows describes the sky as delightfully clear, without a cloud visible, and the sea horizon as remarkably distinct, so that there appears to be no ground for a physical explanation of the phenomenon, unless we could suppose a recurrence in different azimuths on each side of the sun, of that condition of the atmosphere which produces lateral mirage. It deserves to be noticed, that according to the diagram of Mr. Fallows, all the mock suns were included in a space extending only one degree and a half on each side of the sun's centre.-Edin. Phil. Jour. SHEFFIELD PHILOSOPHICAL SOCIETY. At a meeting of the Philosophical Society last month, Mr. Abraham gave an account of an instrument which he had lately invented, for the purpose of extracting particles of iron and steel from the eye. From the paper which was read, it appeared that the attention of this gentleman had frequently been drawn to the practice of extracting particles of steel from the eyes of the workmen by means of a penknife, or a lancet, which instruments Mr. Abraham naturally considered as dangerous, particularly when used by unskilful and unexperienced persons. Having been applied to by a young man (a die sinker) who had had a particle of steel firmly fixed in the centre of his eye for the space of 18 hours, Mr. Abraham applied a fine, but powerful magnet, which immediately attracted the particle, and afforded the sufferer instantaneous relief. The success attendant on this experiment, induced Mr. Abraham to construct an instrument which any person may use in cases of the most distressing kindthereby affording relief to the most delicate of our senses, without the risk which is necessarily involved in the application of a penknife or a lancet.― New Monthly Mag. PATENTS LATELY GRANTED. (Continued from page 384.) To E. Jordan, of Norwich, for an improvement in the form and construction of water closets, or of the apparatus connected therewith. March 27, 1824. To T. Gettien, of Pentonville, for improvements in the machinery and process of making metallic rollers, pipes, cylinders, and other articles.-April 15, 1824. To D. Tonge, of Liverpool, Shipowner, for an apparatus, by means of which, an improved method of reefing sails is effected.-April 15, 1824. NOTICES TO CORRESPONDENTS. If B. will look into this Number, he will find a shorter method of calculating discount than the one he proposes.-L. M'L. won't do; we think he might employ his ingenuity to better purpose.-J. P. may know enough, but he has told us nothing. The Scrap Gatherer must send off his despatches before Tuesday evening, if he wishes them inserted on the following Saturday.-J. W.'s queries are inserted, and his useful discoveries will be inserted when we get them, if they are really " useful and original.' -We do not think G. D.'s solution of the clock question, sufficiently distinct for insertion, and the walking one is run down. His other communications are inserted. If A Constant Reader' would specify what kind of articles he wishes to whiten, he may expect his query to be inserted, and perhaps answered, but not otherwise.-We have received A. N., W., and M. S., Lancaster, but they are unavoidably postponed. Published every Saturday, by W. R. M PHUN, 155, Trongate, Glasgow, to whom Communications (post paid) must be addressed J. CURLL, PRINTER. THE CAMERA LUCIDA, Invented by Dr. WOLLASTON, For Drawing objects from Nature in true Perspective, and also for copying, reducing, or enlarging Drawings already made. THIS ingenious instrument is represented in operation at fig. 1, and consists of a glass prism, ɑ, mounted upon a stand, but seen detached at fig. 2, and in section at fig. 3; which prism receives the picture of any objects placed before it in the direction of the rays shown by dotted lines, b, b; the picture being very evidently seen by looking down perpendicularly into the prism, in the same manner as the eye situate at c. Use of the Instrument. The intention of the Camera Lucida is to throw the picture formed in the prism, down upon the table or paper below, for the purpose of tracing its outline with a pencil. This is effected by a particular adjustment of the eyehole in the sliding-plate, d; attention to which circumstance, is, above all others, necessary in using this instrument. Fixing of the Instrument. The stem which supports the prism is connected by a double joint to the clamp at the bottom, so that its inclination may be altered in any direction. The clamp is intended either to screw upon "the edge of a table, or to lie down flat as a fulcrum. Adjustment of the Prism. The prism is attached to the stem by means of its pin, which slides into a hole at top; and as the squareness of the picture depends upon the truly horizontal position of the axis of the prism, the joint, f, will allow of this adjustment. The prism must be turned vertically upon its pin, and horizontally by the sliding of the stem in its tube, until the flat glass face is directly opposite the object to be delineated. Position of the Eye-hole. In the horizontal representation of the plate, d, at, fig. 2, a small hole will be seen, through which the picture is to be viewed in the prism; and if this slider is so adjusted, by turning upon its pivot, that the edge of the glass prism intercepts about half the hole, then by looking down perpendicularly through the hole, the picture will be seen in the prism, and at the same time, the fingers and pencil of the draftsman below. By these means the point is enabled to trace accurately the outlines of the picture apparently depicted upon the table or paper, as shown at fig. 1. Magnitude of the Drawing. As the dotted lines, e, e, which descend from the prism to the table or paper, diverge, the magnitude, or scale of the drawing under the hand of the draftsman, will depend upon its distance from the prism, which may be regulated at pleasure by drawing the sliding stem, g, more or less out of the tube; observing, that the stem must be slightly inclined from the perpendicular, for the purpose of bringing the picture clear off the clamp at bottom. It is recommended, that for all distant objects, the stem should be drawn out of the tube to the mark D, engraved upon it; and for objects that are two, three, four, or five feet from the prism, the stem should be drawn out only as far as the respective marks, two, three, four, and five; but after a little practice it will be found that such exactness need not be attended to. Distribution of Light. If the strength of light which falls upon the object to be drawn, and the light which falls upon the tracing pencil be equal, then the eye-hole may, as above, be cut by the prism exactly in the middle. But if the light upon the object be more intense than that which falls ́upon the pencil, the sliding plate must be drawn back a very small distance, in order to make the tracing point visible. Or if the light which falls upon the pencil be the strongest, then the sliding plate must be pushed forward a small distance over the prism, in order to make the object more distinct, which a little practice will make familiar. Bright day-light (but out of the sun-shine,) will always be most favourable to the use of this instrument: it may, however, be used by candle-light, attention being paid to the distribution of light, as above. Copying, Reducing, or Enlarging. If an original drawing be placed at one foot from the face of the prism, and the stem be extended so that a perpendicular line from the table to the prism be one foot, then the outline traced by the pencil will be a copy upon the same scale as the original. If the original be at two feet distance, and the table only one, then the copy will be half the scale. If the original be at one foot, and the table be at three feet, then the copy will be enlarged to three times the scale. In this application of the instrument, care must be taken to place the centre of the original drawing directly opposite the face of the prism; and it is also to be observed, that the drawing to be copied should not be very large, or it would in that case become distorted towards the extremities. Long and Short-sighted Eyes. The instrument may be used with or without the lenses, h and i, attached to the prism. For an old or long-sighted person, the convex lens, i, should be placed under the prism, as at fig. 1. For a shortsighted person, the concave lens, h, should be turned before the prism; or, for a perfect-sighted person, they may both be dispensed with, and folded back. DESCRIPTION OF AN APPARATUS FOR SINGEING MUSLIN BY MEANS OF GAS.-BY MR. JOHN HART. Mr. EDITOR,-As your Correspondent, Mr. John Fergusson, has alluded to the machine for singeing muslins he erected for Messrs. Chisholms, from a sketch which I gave him for that purpose, an account of it may be acceptable to some of your readers, especially those interested in the business of singeing. Sir Humphrey Davy, in his researches on the nature of flame, has proved, that the illuminating quality of carburretted hydrogen gas, depends entirely on the carbon contained in the gas, being kept at an intense white heat in the envelope of flame; and, if we interrupt the complete combustion of the gas, by the introduction of any body of lower temperature into the flame, part of the carbon will be deposited on that body in the form of soot, while the heat and light will be diminished. He also proved, that, if we mix gas with such a quantity of air, as will afford oxygen to combine with all the carbon to the very centre of the flame, we will destroy the illuminating power of the gas, but we will gain an increase of heat, without the possibi |