the specific gravity 1·520, and brought forward by degrees to the lowest pan, where it is concentrated to 1.750. When soda is to be concentrated, or the solutions of common salt, a single iron pan may be used instead of the leaden pans above described. Since the flues conveying the sulphurousacid gas to the chimney are liable to get stopped by the sublimation of the volatile metallic oxides, it is necessary to have holes at the ends near the chimney for the purpose of cleaning them when foul. The claim in this case is, to the application of the heat produced by the combustion of bisulphurets of iron and copper, in making sulphuric acid, for concentrating that acid and other saline solutions. The specification is accompanied by drawings to show the nature of the apparatus employed, and the manner of conducting the process; but it is presumed, that practical chemists will perfectly understand the arrangement from the above description. CANAL STEAM NAVIGATION. On the 22nd ult., a trial was made on the Grand Junction Canal, of a small experimental steamer fitted with submerged propellers (not screws) on a plan recently patented by Captain W. H. Taylor. It was witnessed by the chairman and several of the directors of that navigation, and gave, we understand, the most unqualified satisfaction. No perceptible wave was produced by the boat when towing at the rate of four miles an hour, which is as great a speed as is required for the goods traffic on canals. We had ourselves, not long ago, an opportunity of seeing this boat at work on another canal, and were much struck with the absence of every external sign of the motivepower by which it was propelled. Not the slightest swell by which injury can be done to the banks, but an air bubble or two at the sides, which vanished as soon as generated. The success of this invention has led to the formation of an association for carrying goods by steam on the Grand Junction and other canals in connection with it; and so far as all the heavier kinds of goods are concerned, there can be little doubt of the canals being at length enabled, by this means, to compete effectually with their powerful railway rivals. In an early number we shall give a full description of Captain Taylor's invention. THE "METEOR" AND "FAIRY." Sir,-In your last Number, I see there is a letter from Mr. Cruden, of Gravesend, professing to give a true account of the tial of speed between the Fairy and the Meteor, when, in fact, it is nothing of the kind. The true particulars are these. The Meteor left Blackwall before the Fairy, and the latter was obliged to stop at Woolwich to take in stores, and not, as erroneously stated by Mr. Cruden, from " declining further contest." I think no man that knows how to judge the speed of one vessel as compared with another, would say (with such a trial as stated above) that the Meteor is the fastest boat by 1 mile per hour. The Fairy, after taking in stores, proceeded to Greenhithe, to have her compasses rectified. In the meantime, the Meteor had proceeded on her passage to Gravesend; and having landed her passengers, returned to Greenhithe to have a trial with the Fairy; but the latter was unable to accept the challenge, as it was of the greatest importance she should proceed to Portsmouth with as little delay as possible. I merely write this that the public may know what is really the case, and not be misled by that which is not the truth. Weston-super-Mare Suspension Bridge.-We perceive from the Gazette of this beautiful and thriving watering place, that measures are in full progress for connecting the mainland to the island of Bernbeck, by means of a suspension bridge on Mr. Dredge's principle. It is to be 1100 feet in length; the central span 545 feet; and the outside openings 272 feet. The "Fairy."-The cause of the Fairy having performed so indifferently in some of her recent trips, has been found to be that a rope had got entangled with the screw. The Rattler met with an accident of the same sort, when towing the Northern Discovery ships, which made it necessary to take her into Cromarty to be examined, and delayed for a short time the progress of the expedi tion. INTENDING PATENTEES may be supplied gratis with Instructions, by application (post paid) to Messrs. Robertson and Co., 166, Fleet - street, by whom is kept the only COMPLETE REGISTRY OF PATENTS. LONDON: Printed and Published by James Bounsall, at the Mechanics' Magazine Office, Mechanics' Magazine, MUSEUM, REGISTER, JOURNAL, AND GAZETTE. No. 1148.] SATURDAY, AUGUST 9, 1845. [Price 3d. SHERWIN, COPE, AND CO.'S IMPERIAL ROTARY PRESS. SHERWIN, COPE, and co.'s imPERIAL ROTARY PRESS. [Registered under the Act for Protection of Articles of Utility. Messrs. Sherwin, Cope, and Co., of Cumberland-street, Curtain-road, Shoreditch, proprietors.] THE present press is an improvement on the press of the same makers, well known to bookbinders and others, by the name of the "Imperial Arming Press." The improvement consists in the substitution of a rotary for a reciprocating movement; and for most pùrposes there is no doubt that the former is by far the better movement of the two. A is a fly-wheel (see fig. 1), the rotation of which gives motion to a pinion, B, which moves in its turn the cogwheel, C. The revolution of these wheels turns a crank, D, which, by means of a connecting-rod E, works an oscillating cross-head, F, from the extreme end of which an arm, G, descends, which turns on a pin, H, and projects a little way down beyond that pin, as shown in the separate view, fig. 2. The under projecting end of this arm G is of the curved form represented in the figure, and acts against a knuckle, H', which presses down the head of the piston, so that, as the cross-head F is pulled in the direction of the arrow, the arm G presses down the knuckle and piston; and on the action being reversed, the piston is released, and returns to its original posi Fig. 2. tion. M is a strong spring, attached at one end to the back of the frame, and at the other to the piston, to assist in throwing up the piston each time it is released from the pressure of the arm, G. GEOMETRICAL CONVERSION OF CONVEX SURFACES. There is a numerous class of propositions in elementary geometry, which are of the greatest utility in practical constructions; yet singular as it may appear, they are, for the most part, either totally neglected by our artizans or but partially brought into use. It would be difficult to assign a reason for this state of things, were we not prepared to attribute it to the circumstance, that, in consequence of the small share of geometrical knowledge possessed by the generality of our operatives, the application of many of those propositions to objects of a practical nature is not very obvious. This is undoubtedly the case, and moreover, as we have frequently had occasion to observe, when a series of principles on a subject that is naturally of a dry and uninviting character are grouped together or brought into one place, without being fully discussed, and at the same time having their practical utility pointed out, that utility, however extensive it may be, is liable to escape detection by the mere practical man, and the principles themselves become, as it were, a dead letter, at least to those for whom the original selection and arrangement were more especially intended; but when a particular problem is separated from the group and considered by itself, without regard to its dependence on other problems of a kindred nature, the application becomes manifest, and the operator, by rendering himself familiar with the process of construction, finds no difficulty, in the course of subsequent enquiries, of singling out the cases to which the particular problem is applicable. Taking this view of the subject, we propose from time to time in the pages of our Magazine, to consider some of the more important problems of practical geometry, and by treating them severally according to the manner specified, we GEOMETRICAL CONVERSION OF CONVEX SURFACES. 83 face on a plane, according to the usual way, would give a rectangular parallelogram, and not a circle as required by the problem, and the subsequent reduction of this parallelogram to an equivalent circle would only be approximative, whereas the problem is capable of a rigorous determination, since no incommensurable quantities are involved in its construction. hope to invest them with all the generality and importance of which they are susceptible. This being premised, we straightway proceed to consider a few of the cases that more particularly constitute the subject of the present paper. PROBLEM I. Having given the diameter of the ends of a right cylinder, and also its axis or length, to determine geometrically the radius of a circle of which the area shall be equal to the convex surface of the cylinder. The algebraic solution of this problem is simple enough, but the geometrical solution, which is here required, is not so obvious; for the development of the surPut d = the diameter of the cylinder's base, or the diameter of its circular ends, 1 = the length of the cylinder, or the distance between its circular ends or bases, By reasoning on the method by which the convex surface of a right cylinder is calculated, and comparing it with the mode of determining the area of a circle, we shall be led directly to the discovery of the process by which the required object is to be effected, and for this purpose. and r = the radius of the circle whose area is equivalent to the convex surface of the cylinder; Then, because the convex surface or superficies of a right cylinder, is equal to the rectangle under the altitude and the circumference of its base; we have 3.1416 dl for the value of the convex surface; and because the areas of circles are to each other as the squares of their radii, the expression for the area of the equivalent circle, is 3.1416 r2, where 3.1416 is the area of a circle whose radius is unity; but by the conditions of the problem, these values are to be equal; therefore by comparison, we get 314162=3·1416dl; that is, r2=dl; by casting out the common factor 3.1416, and by converting the remaining terms into an analogy, we get d:r::r:l; where it appears, that the radius of the circle required, is a mean proportional between the length of the cylinder and the diameter of its base, and having discovered this relation, we deduce from it the following construction :— Let A B C D, fig. 1, be the given cylinder, A B or D C the diameter of its circular ends, and E F its altitude or length; produce A D directly forward till DQ becomes equal to DC, the diameter of the base; then is A Q equal to AD+DC, the altitude or length, and the diameter considered as one line. Bisect A Qin P, and on the point P as a centre, with PA or P Q as a radius, describe the semicircle Am R n Q, and produce the diameter C D to intersect the semicircle so described in the point R; Fig. 1. m between A D the altitude, and D C the diameter of the base, for by construction DQ is equal to D C. Draw the straight lines A R and QR; then, because, by the nature and conditions of the problem, the triangles QRD and R A D are similar, it follows that QD: DR::DR: AD, so that DR is manifestly a mean proportional between A D and D Q; but we have seen above that the radius of the equivalent circle is a mean proportional between A D and D Q, or rather between A D and DC; consequently, D R is the radius of the equivalent circle. Upon R as a centre, with R D as a radius, describe the circle Dm Sn; then will the circle so described be equal in area to the con vex surface of the cylinder whose length is E For BD, and diameter A B or DC. Here we have a very simple and elegant method of converting the convex superficies of a right cylinder into a circle containing an equal area, and it is easy to perceive that such a conversion must frequently be required in the mechanic arts, and more especially by those to whom the method is known; for it is only when the ways and means of effecting a particular object are known, that they are made available for accomplishing other purposes of a similar nature. If it were required to determine the diameter of the circle, which should be equal in area to the convex surface of an oblique cylinder, the thing could not be done by means of elementary geometry; that is, by straight lines and circles, but would require the delineation of some curve of the higher orders, depending for its description upon the determination of principles, which under certain conditions constitute a problem of very difficult solution. (To be continued.) IMPROVED MODE OF CASTING METALLIC REVERSES FOR THE ELECTROTYPE. Sir,-As a material for taking reverses of coins, &c., to receive the electro-deposit, the fusible alloy (3 tin, 5 lead, 8 bismuth) is preferable to those substances requiring a conducting film, on account of the greater certainty of result and simplicity of management attending the use of metallic moulds, and the superior sharpness of the deposited fac-simile. But hitherto the production of a perfect reverse in metal, according to the methods in use, has been a matter of sufficient difficulty to detract much from the subsequent facility of manipulation. The simplest methods given by writers on the electrotype, is, to pour the melted alloy on a piece of paper, clear its surface by scraping, and just at its point of cooling to dash" the medal forcibly upon it. This mode requires too much practice, and is attended by great uncertainty: the scattering of the metal and the consequent thinness of the mould are serious inconveniences, arising from the difficulty of finding the proper moment to strike the impression. In fact, this is nothing but an imperfect clichée process, in which the deficiencies are, the want of the pasty condition of the alloy, and the absence of efficient mechanical arrangements. In any process of casting by impact, success depends on the cooling of the alloy as soon as it is in full contact with the coin to be copied. If the alloy remain fluid after this contact, air is generally retained, and no perfect cast possible. Thus, simply placing the coin on the alloy, or pouring this.upon the first, are useless methods, for this reason, independent of the absence of the force necessary to expel the air at first. consequence of the low fusing point of the alloy, it is commonly poured upon paper, or any non-conducting surface, previous to use, on which it will remain fluid for a long time-a plan which occasions all the difficulties in this kind of casting. In It became apparent to me, that improvement on these methods might be effected by merely expediting the cooling of the alloy. To determine this, taking a small coin in one hand, I poured a portion of the alloy on a cold iron slab, and immediately dropped the coin upon it. The alloy became solid on the instant of contact with the coin, and on its removal, exhibited an excellent reverse. To prove the advantage of quick cooling, when a sheet of paper was interposed, the usual difficulties recurred. The In order to adapt this mode to partial purposes, the following simple directions are to be observed. A sufficient quantity of the fusible alloy is to be held in a ladle over a fire until it just melts: it is then to be quickly poured upon the metallic slab; and the coin, held ready, instantly allowed to fall flat upon it. proper height of the fall will vary, of course, with the weight of the coin, and is a matter of easy adjustment: usually two or three inches is sufficient. Sometimes it is convenient to attach another coin, &c., to increase the weight of that to be copied, the objects being the expulsion of air and sufficient depth of impression. The alloy should have a larger surface than the coin, so as to extend around it. If this be not the case, it will be found to spread under the impact, and the marginal parts of the coin will be indistinctly impressed. For the same purpose, when the medallion is large and heavy, a flat piece of wood, having a round hole cut out, may be placed upon |