retically correct, and practically much more convenient. The general rule to find the offset is—" Divide the number of inches in the chain used by the number of such chains in the radius of the required curve; the quotient is the offset in inches." The paper is accompanied by a table of offsets for curves of different radii; which the author found extremely convenient for use in the field. The paper being altogether mathematical, is not adapted for publication in abstract; but it will be given at length, with examples and diagrams, in the Transactions of the Institution. "Description of an Instrument for describing the Profile of Roads." By Henry Carr, Grad. Inst. C. E. The object of the author was the construction of a machine, which, being drawn along any road of moderate even surface, should describe the section of the line over which it passed. It is evident, that if a pendulum be suspended from a frame, standing perpendicularly, when the machine rests on a horizontal plane, on passing over a plane inclined at any angle with the horizon, the pendulum must form the same angle with the frame, the tangent of which angle, in terms of the radius, will be the rise or fall of the plane. The duration of the tangent will be determined by the paper on which the section is drawn being made to traverse at a speed proportionate to the distance passed over; and the extent, by the difference of the speeds of a nut and screw, which are made to revolve in the same direction—the nut turning at a constant velocity, and the screw at a speed differing from that of the nut in proportion to the tangent, slower or faster as the tangent is plus or minus, raising or lowering the nut according to the deviation of the plane from the horizontal line. The machinery is set in motion by the wheels of the carriage, and a series of wheels and pinions, of given diameters, cause the ground line and datum line to be drawn simultaneously by two pencils on a paper, which gradually unfolds itself from one drum, and is transferred to another at the rate of 16 inches per mile passed over, or on a scale of 5 chains to the inch. A profile of a line of country may thus be obtained with sufficient accuracy for a preliminary survey. A comprehensive perspective drawing accompanies the paper and explains the construction of the machine. 66 May 12, 1840. The PRESIDENT in the Chair. Photography, as applicable to Engineering." The object of the author in this paper is to direct general attention to the advantages which may be expected to result to the profession of the Civil Engineer from the discoveries of Mons. Daguerre and others, in enabling copies of drawings, or views of buildings, works, or even of machinery, when not in motion, to be taken with perfect accuracy in a very short space of time and with comparatively small expense. This system of copying not only the outline, but the tints of light and shade, united with accurate linear perspective, he contends may be easily adapted to the purpose of the engineer, as well as to all those professions in which the art of drawing is used. The photographic apparatus has already been employed to bring before us exact copies of the most interesting monuments of antiquity, the French antiquarians and artists having found it more easy and correct to Daguerreotype the Egyptian monuments and decipher the hieroglyphics at their leisure, than to labour over the originals. The subject is divided into two branches; the first being the art of copying drawings and plans, by the transmission and absorption of light, by prepared paper. The drawing to be copied is placed between two pieces of plate glass, held down in close contact with a sheet of photogenic paper, prepared by being washed over on both sides with a neutral solution of nitrate of silver of a specific gravity of 1.066, and afterwards with a solution of common salt and water (1 lb. of salt to 25 pints of water). The paper thus prepared must be dried and kept in the dark, on account of its peculiar delicacy. The rays of the sun are then permitted to pass through the white portion of the drawing or print, while they are interrupted by the black lines, and more or less by the tinted portions. The rays of light thus act upon the prepared paper, and produce in a few minutes a reversed copy, reproducing the lights of the original in shadows; this can be remedied by taking a second copy from the first, and thus the shadows are restored to their original positions. To destroy the sensitiveness of the prepared paper and preserve the copy, it is soaked in pure water, which carries off the excess of nitrate of silver, then covered with a solution of hypo-sulphite of soda, of a specific gravity of 1.055, and again washed in pure water, so that when dried it is permanently fixed. It is evident that a copy thus obtained must be exactly like the original, and the value of such a process may be readily estimated by engineers. The second branch, which is named " Daguerreotype," after the distinguished artist who brought it to its present state of perfection, is of a much higher order. This is the art of fixing and preserving on the surface of a polished silvered plate the images collected in the focal plane of a camera obscura. The process is rather complicated, but it may be thus briefly described. The surface of the silvered plate being cleaned and polished very perfectly, by means of finely levigated pumice stone, olive oil, and cotton, is rubbed lightly over with diluted nitric acid, in the proportion of 1 pint of acid to 16 pints of distilled water; it is then subjected to the heat of charcoal or a spirit lamp, until a firm white coating is formed all over the surface of the silver. The plate is then suddenly cooled. This process is repeated three times. It is placed in a dark chamber, with the face or silver surface downwards, where it is acted upon by the spontaneous evaporation of iodine; this condenses upon the silver, and produces a fine gold-coloured surface, extremely sensitive to the impressions of light. It is then placed in a camera obscura, the light having been until then perfectly excluded from it. It there receives the impression of any images brought within the focal plane; and by subsequently exposing it in a dark, close chamber, with its silver surface downwards, to the fumes of heated mercury, the images are rendered visible; to fix the images so received, the iodine is removed by dipping the plate in pure water, and then washing it either with a weak solution of hypo-sulphite of soda, or a saturated solution of common salt, and finally dipping it in distilled water and drying it. It should then be framed and glazed, to preserve it from external injury, and the picture will remain unchanged. Attempts have been made to use this process for preparing the plates for engravers, as much time and cost would thereby be saved, but hitherto it has not been done to any extent. The author presses upon the Institution the applicability of these processes to engineering uses, and quotes the remarks of Mons. Arago-"That photographic delineations having been subjected during their formation to the rules of geometry, we may be enabled by the aid of a few simple data to ascertain the exact dimensions of the most elevated parts of the most inaccessible edifices." Mr. Cooper, Senior, introduced the subject of photography by explaining, and illustrating by instruments and diagrams, the principles of the division and dispersion of the rays of light, according to the Newtonian theory, as well as the most recent researches into the subject. He described the chemical properties of light-its affinity for certain combinations, such as chloride of silver-its heating powers-the different effects of the rays on vegetation-and the application of these known principles to photography. He then explained the chemical pro perties of the chloride of silver, iodine, and other substances used in the process. In alluding to the probable uses of the Daguerreotype, he observed that the process might be employed to make drawings of machinery, as graduated scales might be fixed to certain parts of the objects, and they would be copied in their relative proportions to the machine. Mr. Cooper, Junior, illustrated Mr. Gordon's communication by explaining the photographic apparatus, and in the process of obtaining a specimen of Daguerreotype by means of the oxyhydrogen light, He described, among other points, the difficulty of obtaining pure silver upon the copper plates, as, for the advantage in rolling, the manufacturer will introduce an alloy of to 1 per cent. On this account, acid is used so repeatedly in cleaning the plates, that any particles of copper which have been rolled into the surface may be carried off. He explained his improvement to the iodine box, which consists in spreading the iodine all over the bottom of a tray lined with glass, and covering it with a piece of card-board, which becomes saturated with the fumes of the iodine, and on the silvered plate being placed over it, acts equally over its surface, instead of partially, as in the old system of placing the iodine in a mass in the centre of the tray. He had found this to be a great improvement. The shortest time in which he had obtained a photographic picture in England was 11 minutes; while, during a gloomy day in November, it took an hour and a half to procure a moderately good one. "An Universal Screw-Jack." By George England. This machine, a model of which was presented to the Institution, is intended for raising heavy weights, and moving them in any required direction; the vertical motion is similar to that of a common screw-lifting jack, and the lateral motion is communicated by a ratchet lever to a horizontal screw, working in bear |