and the formation of tubular files, which he acknowledges never occurred to him, must have followed the other at a short interval. Extract of a letter from Messrs. Johnson, Cammell & Co., Sheffield, to the late Sir John Robison, K. H., F.R.S.E., Nov. 7, 1843. We now have the pleasure to hand you, as requested, an extract from our letter of Feb. 13th last, to the late Sir John Robison, explaining the mode we then adopted in the manufacture of the half-round files from steel of parallel thickness, as suggested by him, viz., by means of a screw-press, and swages of copper, and which we see is the plan named in the paper communicated to the Society.

From the specimens sent you last week, you will perceive we have deviated from the plan first suggested, "of cutting them from blanks of steel prepared as for a thin equaling-file." We, however, did not give up this plan until, from practical experience, we found its working very uncertain and irregular, for the file being of uniform thickness, the edges presented an equal, or greater, degree of resistance to pressure, than the centre; and the top swage, or boss, coming in contact with the centre of the file previous to any other part, caused it to bend more freely than the edges, producing various degrees of curvature in the same file. Again, if our top boss was so shaped as to create an earlier and freer pressure on the edges, to ensure a more uniform curve, we then endangered the sharpness of the teeth on those parts of each side of the file, convex and concave, first receiving such undue pressure. These objections and difficulties are all overcome, or lessened, by our present mode of cutting and turning the files from steel with slightly tapered edges on one side, thus,

The flat surface is cut with a continuous tooth, and can be turned either convex or concave, and the tapered surface can either be cut in ridges, or left safe, or uncut, which, from the following extract, you will perceive, was Sir John's first idea. In Sir John Robison's first communication, November 10, 1842, after recounting the difficulty of obtaining a smooth half-round file, or one of equal continuous tooth, he says:

"I propose to overcome this difficulty, and to cut half-round files as truly smooth as flat ones are now struck, by making the blanks of rolled steel-plate; by striking them in the flat state, and by afterwards giving them the degree of curvature required, by means of a screwpress, and tin, or copper, swages; or else by passing them between. grooved and furrowed rolls (of soft metal for the struck side)," evidently implying that one side would be unstruck, or uncut. In fact, for some time, at first we only attempted to cut one side of the file, and frequently now we are requested to leave sometimes the concave, and others the convex surface, blank, or uncut.

Edin. New Philos. Journ.

Elkington's Process of Electro-Plating and Gilding. By Mr. PELLATT.

It is immaterial what metal is used for articles to be plated by this

process; a compound metal composed principally of nickel, however, is preferred, which, when plated with silver, can scarcely be distinguished from the solid metal.

The first thing before plating, or gilding, is thoroughly to cleanse the articles from all grease, or oxide, and this is done by boiling them in caustic alkali, and scouring them with sand and dilute acid; they are then washed and dried, and a copper wire being attached to them, they are placed in a metallic solution of the metal required to be deposited, the wire being connected with the negative pole, while a silver plate, suspended in the same solution, is connected with the positive pole of the battery.

The process of gilding is similar to that of silvering, except that the gold solution requires to be heated while the process is proceeding.

Great care is required in the arrangement of the batteries, the object being to decompose the solution easily, and at the same time to produce a firm, smooth, and regular deposit of the metal. The secret of the manipulation consists in the correct balance of power between the battery, on the one hand, and the strength of the solutions, and the number of articles to be deposited on, on the other.

The solution for gilding is prepared by dissolving the gold in a mixture of pure nitric and muriatic acids, the product being a chloride of gold; after evaporation, this is converted, by means of an alkali, into the oxide, which oxide is dissolved in pure cyanide of potassium. The solution of silver is prepared by dissolving pure silver in nitric acid diluted with distilled water, and similarly treated with the cyanide of potassium, as in the gold solution.

In forming articles of solid silver, the following process is employed:

Upon a wax model is first deposited a copper surface by the electroprocess; the wax is then melted out, and thus a perfect mould of copper is obtained, into which is deposited silver of any substance that may be required. The copper is then removed by dissolving it with acid, and the article required is obtained.

If the original model is in metal, an elastic mould made of glue and treacle is used, by pouring the composition, in a liquid state, upon the model.

By a late improvement the proprietors have the means of giving a metallic conducting medium to this composition, and to other substances, without the use of any external application, as black lead, &c. Trans. Soc. Arts, Manuf. and Com.

Substitute for a Transit Instrument.

A neat reflecting instrument called a Dissleidoscope, has been contrived by Mr. Dent, the well known chronometer maker, to be used instead of a transit instrument, for determining the time by the passage of the sun and stars over the meridian of any place.

Imagine a prism made of three plane glasses so fixed, that the plane of the upper one is at right angles to the plane of the meridian,

but the upper end declining in any angle towards the north. When the sun, or star, as it approaches the meridian, comes into the field of this instrument, its rays are reflected in a given angle, from the surface of the upper glass. This angle is sensibly the same during the time the object is transiting the field of the instrument, and the image of the object will appear to travel from east to west.

A portion of the sun's light passing through the upper glass, will fall on the glass forming the eastern side of the prism, and being reflected thereon to the glass forming the western side of the prism, will be by that sent up through the upper glass. The consequence is, there will be two images of the sun, one reflected from the upper glass, and the other by two reflections from the lower glass. The last image, as the object approaches the meridian, and middle of the instrument, will travel from west to east, or directly opposite to the first reflected image; and when the object is on the meridian, the two images will coincide.

The angle in which the lower glasses are inclined to each other on the lower angle of the prism, is not necessarily confined to any limits, but it should not be too acute. The angle, however, in which the axis of the prism is inclined to the horizon, should be such as never to have the ray at right angles to it, or to the surface of the upper glass, because if it was the head, it would be in the way of viewing the contact, or coincidence, of the two images.

By putting the axis of the instrument parallel to the horizon, Mr. Dent has also contrived to make it an instrument of equal altitudes, and thus, with the aid of a good watch and clock, to determine its own meridian.

Mr. Dent generally places the axis of his prism parallel to the axis of the earth, but if that axis is made to move about another at right angles to the plane of the meridian, the transit of stars below, as well as above, the pole, might be observed. He also has the face of the prism, not—as we have for the better exemplification of the modus operandi described it-perpendicular to the plane of the meridian, but declining from it towards the west in a small angle. By this means he avoids the interception of the rays by the head.

The rectification and verification of the instrument are likewise very easy, and its trifling expense (two guineas) and portability, will, doubtless, render it a favorite substitute for the more costly transit instrument. It is, of course, a patented invention.

Lond. Railway Journ.

Gilding and Silvering by Immersion.

The following new methods of gilding and silvering, by immersion, have been adopted on the Continent. Their easy execution puts them within the reach of persons who have hitherto been strangers to this kind of operation:

Gilding on Silver.-Silver is gilt very readily by means of neutral chloride of gold, added to a solution of sulpho-cyanide of potassium,

till the precipitate formed at first is redissolved. It is necessary that this liquid should preserve a slightly acid reaction, and if it has lost it by too great an addition of sulpho-cyanide, it must be rendered so by adding a few drops of hydro-chloride acid. In order to gild, the silver is plunged into this liquid nearly boiling, and tolerably concentrated, in which state it is kept by pouring, from time to time, some hot water to replace that which has evaporated. In this manner, inconveniences which would result from too great concentration of the acid is avoided, whose pressure is, nevertheless, useful to oppose the formation of an auriferous precipitate which takes place by elevation of temperature, when alkali predominates.

To Gild and Silver on Copper, Brass, and Bronze.-The solution of the cyanide of gold, or silver, has been already pointed out for silvering and gilding under the influence of electric forces, but it has been found that the same solutions, brought to a temperature near their point of ebullition, can also gild and silver by dipping. With regard to their preparation, if it were necessary to obtain them chemically pure, it would be expensive, without any advantage being obtained; the operation can be simplified, and rendered much less expensive, by adding directly, either to the chloride of gold, or to the nitrate of silver, neutral, the cyanide of potassium in excess, so as to obtain the soluble double cyanides.

Silver cannot be gilt by this method, but as has been already stated, the sulpho-cyanide of gold and potassium gilds this metal very well. The solution of the cyanide of copper in the cyanide of potassium, will not copper silver, even in contact with zinc; however, it will copper this latter metal in a very solid and perfect manner.

It must, however, be stated that these processes, though so very convenient, because they always succeed, and require but a few minutes for their preparation, deposit, unfortunately, but a very thin coating of the precipitated metal. This is an inconvenience common to all methods of coating by simple immersion.

Civ. Eng. & Arch. Journ.

Hullmandel's Lithotint Process. By B. ROTCH, Esq., V. P. From the time of Senefelder, who invented lithography, in 1796, up to the present day, this beautiful art has gone on gradually improving; but in no instance has so important a step been made as in that of Mr. Hullmandel's process of producing original drawings, or fac-simile copies as may be required. Previously to the lithotint process being introduced, crayons made of a composition of grease, wax, shellac, soap, and black, were used for a similar purpose.

Mr. Hullmandel's process, which is the result of numerous experiments, may be thus described:-The drawing having been sketched, tinted, and finished by the artist on stone, with lithographic ink mixed with water, to produce the various shades, which is as easily done as on paper, is covered over with gum-water, and weak nitric acid to fix it. After remaining a sufficient time to dry, a solution of rosin in

spirits of wine, is poured over the stone, and as this ground contracts by drying, it cracks into thousands of reticulations, which can only be discovered by the use of a microscope. Very strong acid is then poured over the aquatint coating, which, entering all the fissures, produces the same effect on the stone as the granulations of the chalk by the ordinary process. The rosin protects the drawing in every part except in the cracks. The acid having remained a sufficient time to act on the unprotected parts of the drawing, is removed, the ground is washed off, and all appearance of the subject on the stone vanishes, until ink being applied by a roller in the ordinary way, it is reproduced, and ready for taking off the required number of impressions, which, in some cases, have extended to the number of 2000. Trans. Soc. Arts, Manuf. and Com.

Lunar Occultations.

Lunar Occultations visible in Philadelphia during the month of April, 1844; computed by MRS. CHARLOTTE S. DOWNES, from the Elements published with the Occultation list of the United States Almanac.

The Immersions and Emersions are for Philadelphia, mean astronomical time. Im. for Immersion, Em. for Emersion. These abbreviations in Italics refer to those Immersions and Emersions which take place on the Moon's dark limb. N. App. for Near Approach.

The angles are for inverted image, or as seen in an astronomical telescope, and reckoned from the Moon's North point and from its Vertex around through East, South, West, to North and Vertex again. For direct vision add 180°.