under water; the one, brought to excess, must always be injurious to the strength of the fabric; the other diminishes the speed; but where an overhang is rendered necessary by other unavoidable circumstances, I cannot agree with "F" where he says, "the form, therefore, could only lessen the evil its greater weight had first increased," for the bears greater weight of hull at that part but a small ratio to the gain of displacement when in the act of falling. I certainly agree with your able correspondent in most of his views; but as long as a determinate quantity of weights are to be carried at a certain distance from the perpendicular in the load water line, the most dangerous form for the structure must be that which takes away the already deficient buoyancy for those weights. I am, Sir, yours, &c., B. I. REMARKS UPON THE MOTION OF ROLLING WHEELS. Sir,-At page 512 of the Mechanics' Magazine, vol. 1., a paragraph is inserted from the proceedings of the "Institution of Mechanical Engineers," which appears to me invested with more than an ordinary share of interest on account of the many important mechanical principles it involves. The paragraph to which I allude has reference to the comparative effects of wheels revolving upon their axles, and at the same time rolling upon a plane as in the practical case of the wheels of railway carriages; and as the approximate results were only abstractly stated, I was led at the time they appeared to make a few remarks upon the points involved, which I herewith subjoin, and which I regret my time has not permitted me to mature and extend. Fig. 1. E The first member of this equation is the cosine of the angle which the tangent to the curve at any point P makes with the axis AB.-Draw therefore DL parallel to AB, and join PL, PD; then is PD a tangent, and PL a normal to the curve at the point P; hence the generating point P, at every instant of its motion revolves about the point of contact L, with a velocity varying directly as the length of the normal PL; and the velocity of translation of the axis being given, that of the point P in the cycloidal arc is directly as PL. Because the angle at the centre of a circle is double that at the circumference, ference at P, and HT perpendicular to PD, and let V-the velocity of translation. The PTH=/ PDK 2 and the To determine the position of the spoke at which its extremity Pattains this mean velocity; equating the second members of (6) and (7) and solving for cos 0, to rad. unity, we find, Making the calculation indicated in this last equation, and observing that the effect must take place in the second quadrant where cos e is negative, it will be found that the position sought is 15°. 51' above the horizontal line, or 105°. 51' from the point of contact with the rail. The paragraph (page 512, vol. 1., col. 1) before alluded to refers to most of these results, with many others of great interest and much importance, and which would amply repay the trouble of inves angular velocity of a point in the spoke tigation if time permitted. Believe me truly yours, at an unit of distance from the centre is T. SMITH. Bridgetown, Wexford, June 28, 1849. ROTARY ENGINES. The following account of an improved disc engine, lately fitted up and used in driving the Times printing machines, appears in the Builder. The engine gives direct motion to a crank upon which the shaft of the disc exerts a uniform force. It is of simple construction, and, amongst other advantages, is capable of being worked at high or low pressure, and at from 80 to 150 revolutions per minute : "Bishopp's Disc Engine.-In this engine, the advantages of which have been long known, the objections that alone kept it out of general use appear to have been successfully overcome. It is a 16-horse power engine, on the high-pressure and condensing principle; it is, however, equally suitable to be worked as a simple low-pressure condensing engine. It stands in the machine-room close to a wall, and occupies a singularly small space.* The shafting for driving the printing machines is carried by brackets fixed to the wall over the engine, and is driven by two bands; the drum on the engine-shaft is 30 inches diameter, and the two pulleys overhead 4 feet diameter. "Our impressions in favour of the engine were confirmed by inquiry. It seems that before being erected at the Times office, it was tested, during a month, by Mr. Penn, of Greenwich, and Mr. Farey (both good authorities), in a corn-mill belonging to the former. The comparison was made with a beam engine of the best construction, and, under similar circumstances, there was an important difference in favour of the disc engine, the engines driving alternately the same machinery, at equal speed, from the same boiler. Several disc engines have been fixed in various parts of the kingdom during the last eight years, but the arrangements lately patented by Mr. G. D. Bishopp have so much improved it as to open to it a much larger sphere of action. This at the Times office was manufactured by Messrs. Joseph Whitworth and Co., of Manchester. The peculiarity of the disc engine is, that it gives direct motion to a crank on the engine shaft, and exerts a perfectly uniform force on it throughout the revolution. There are, therefore, no "dead points;" and when driving by gearing, without a fly-wheel, there is no back-lash in the wheels. Moreover, the steam can be cut off at a very early part of the stroke, without materially affecting the regularity of the driving force. Other advantages besides the little space occupied are, that it can be fixed on the beams of a floor, or on a slight foundation, and that, although the speed of the piston (i. e., of the disc rings) is only 200 feet per minute, the engine makes three times as many revolutions per minute as a common engine, and, consequently, in most cases, much expensive gearing is dispensed with. It appears to us admirably adapted for driving the screw propeller direct, as the engine shaft has only to be extended through the vessel, and have the propeller fixed to it; it would thus enable sailing vessels which cannot spare much room to adopt the screw as auxiliary power. The disc engines are now made entirely from wooden patterns, and every wearing surface, it is said, can be Seven feet long, and four feet wide; and the highest part of the engine is only three feet above the floor of the room. refaced or renewed, as in engines of the common construction. We cannot but think that this engine ought to come into general use." The rotary engine referred to in the preceding article, and called "Bishopp's Disc Engine," is the old disc engine of Davies and Taylor revived, which some eight or ten years ago caused some stir in Birmingham, but proved in the hands of a Company which was formed for working the invention an utter failure. Mr. A. Bishopp has lately invented and patented some improvements in the engine, which are supposed to have given quite a new face to the affair; and hence the new name, under which it now presents itself once more as a candidate for public favour. [It may be here proper, in order to prevent mistakes, to state that the Mr. Davies of the "Disc" engine is a totally different person from Mr. Davies, the inventor of the rotary engine, which has recently occupied so prominent a place in our pages; and that the inventions are also very different. The former is a Mr. Henry Davies, of Stoke Prior, Worcestershire-the latter Mr. Isaiah Davies, of Birmingham.] Whether Mr. Bishopp's improvements are likely to obviate the objections which existed to the disc engine in its original state, we are unable to say, not having as yet had an opportunity of seeing it in its improved form; but we shall, at all events, require some better evidence than a mere trial of "a month"-even though conducted under the superintendence of such eminently competent judges as Mr. Penn and Mr. Farey-before we put much faith in its capabilities. Let it work for sixteen months uninterruptedly, as Mr. Isaiah Davies's rotary engine has done at Messrs. Edelstein and Williams's, Birmingham (see last vol., p. 124)-without any perceptible change, and with entire satisfaction to those using it, and then we shall say that Mr. Bishopp has indeed achieved something as extraordinary as unexpected. On the authority quoted by the Times we place, for obvious reasons, no reliance. The language and reasoning used are evi dently such as could proceed from no professional pen; e. g., the writer talks of direct action being a "peculiarity of the disc engine;" having evidently not the least notion of its being a peculiarity which it shares with all the rotary engines ever constructed. We gather from the specification of Mr. Bishopp that his improvements have mainly for their object the better packing of the moving parts. No doubt these are improvements in the right direction, but they do not, we apprehend, meet all the difficulties of the case. Be the merits, however, of this "modern antique" what they may, it is to us a source of great gratification to find that we have now with us such high authorities as Mr. Penn, Mr. Farey, Messrs. Whitworth and Co., of Manchester, and (though last, not least,) the proprietors of the Times newspaper, in thinking that the rotary principle of action is by no means that illusory conceit which it has been commonly pronounced to be; but, on the contrary, a rationality deserving of all possible favour and encouragement. Mr. Bishopp is lucky in having been honoured with such high patronage, and will richly merit every personal advantage which complete success may bring him. Come what may, too, the friends and advocates of the rotary system must own themselves deeply indebted to Mr. Bishopp, for obtaining for their common principle of action a degree of public attention, which they might otherwise have long looked for in vain. ON THE SPECIFIC HEAT OF CERTAIN ALLOYS, M. Regnault has discovered, in alloys fusible at about 212° Fahr., a specific heat much greater than the mean of the metals composing them, and proposes to examine whether this anomaly does not disappear at lower temperatures. The experiments reported in my memoir show that it does, in effect, disappear. Thus, in the alloy of d'Arcet, I have c=0·096 from 201.2°, and c=0.037 from 122°; as the calculated mean is 0.036, it will be perceived that the difference is very trifling. I make it appear that the excess of heat observed in this alloy, near its melting point, is not owing to the commencement of fusion, as might be imagined, but to a new kind of latent heat, of which the following experiment shows the evolution :-A glass bulb, filled with the alloy in question, is isolated in such manner that the cooling may be observed; and for that purpose there is fixed in the alloy a thermometer, of which the changes may be noted with an eye-glass and stop-watch. Suppose the glass vessel to contain 150 grammes of the alloy of d'Arcet, the thermometer, which, at about 266°, takes five or six seconds to fall two degrees when the alloy is in a liquid state, takes more than 400 seconds to fall 3.6° between 204.8° and 201·2°. This is very simple: the latent heat of fusion is very disengaged during this interval. The solidification being completed, the thermometer resumes a regular rate, falling one degree in five or six seconds, until about 134.6° where it suddenly stops, and even rises two or three degrees. At the same time, the glass bulb is broken by a considerable expansion of the whole mass, and this expansion continues after the cooling, so that the thermometer, which was before strongly pressed, becomes free and movable. There is, therefore, in this, a change of constitution in the alloy, and the heat which was disengaged during this modification was sufficient to sustain the thermometer between 130-4° and 132.8° for more than 400 seconds; but it then immediately fell one degree in five or six seconds. The disengagement of heat afterwards continues for a long time, of which we have proof by the extraordinary slowness of the cooling. If, after having melted the alloy, it be suddenly cooled by plunging it into water, and then withdrawn as soon as it may be safely handled, it will, in a few moments, become again so warm as to burn the fingers. Here, the sudden cooling is at first opposed to the change of constitution; but a moment arrives at which the disposition of the molecules ceases to be compatible with so slow a temperature, and then the new arrangement takes place. And, on account of having been thus retarded, it recurs with much more energy, that is, in a much shorter This alloy is composed of bismuth, 8; lead, 5; and tin, 3. It is known as Babbet's Fusible Metal," and melts at a temperature below that of boiling water. time; it is not only a slower cooling which is observed, but a reheating which may carry the mass to 158° To recapitulate the excess of caloric given out by alloys, when heated nearly to their melting point, does not proceed from the latent heat of fusion; nor should it be con sidered as being simply specific heat-it is heat due in a great measure to change of constitution, which may take place in an alloy completely solidified and considerably below its melting point.-Comptes Rendus, Sept. 27, 1847. MESSES. BROOKE AND SON'S IMPROVED WATER-TIGHT NIPPLE AND PERCUSSION CAP. [Registered under the Act for the Protection of Articles of Utility. Edward Brooke and Son, of Russell street, Birmingham, Proprietors.] Fig. 1 of the above engravings is an external elevation of this improved nipple and cap; fig. 2, a sectional elevation, and fig. 3, a plan of the nipple alone. A, the pillar of the nipple is made in one piece with the "squares" B B; C is a circular bevelled cutting formed in the upper part of the cap on B, and immediately surrounding the base of the pillar A; D is a screw, by which the nipple is attached to the breech of the barrel, having a flange, E, at its upper part, which rests upon the breech; Fis the cap, the peculiarity of which consists in the lower part, a, being bevelled in a manner corresponding to the cutting, C, formed in the squares." 66 The object of this improvement is, that when the cap is placed upon the pillar, A, as represented in figs. 4 and 5, the bevelled part, a, of the cap, shall, when pressure is applied to it, be forced into the bevelled cutting, c, and form a perfectly water-tight joint. Another advantage is, that when the cap is fired, the cutting prevents the spread of the cap, and thus obviates all danger arising from splinters, and also renders the discharge of the firearm certain, as the whole of the fire of the cap is discharged into the nipple, and not as in ordinary cases, little more than a fourth. Fig. 6, is a view of this improved cap after it has been used. Fig. 7, a view of one of the ordinary caps under the same circumstances. |