"Expanding pistons, composed entirely of metal, have accordingly been suggested, have been the subject of patents, and have been brought more or less into use. In one of the best of these the circumference is divided into segments, between every two of which is a triangular wedge, having its point towards the circumference, and its base towards the centre. Each of these wedges is backed by a strong spring, the tendency of which is to force outwards the point of the wedge, and consequently the segments against which it bears. But, in practice, the point of the wedge has been found very liable to mark, and finally to score and destroy the cylinder, against the sides of which it is continually pressing with a force greater than that exerted by the segments.

"Mr. Motters head's piston is composed of two layers, each of which consists of segments; of these, three alternate ones are pinned so as to form immoveable abutments for the support of the three moveable ones, which are curved internally, and have each a strong spring, placed like a bowstring across the internal curve, and bearing against the sides of a cylindrical cove that fills up the inside of the piston. Hence, it is evident, that these moveable segments are continually pressed outwards by the action of their respective springs. The moveable segments occupy a little larger part of the exterior curvature than the fixed ones do; and, therefore, by so placing the two layers of segments, that the three moveable ones in the upper layer shall correspond with the three fixed ones in the lower layer, an expanding piston is produced, every part of which that touches the cylinder is a segment of a circle of the same magnitude as the cylinder itself, and while it prevents the escape of steam, has no sensible wearing on the sides of the cylinder.

"Messrs. Helford and Thomas, engine makers, in whose employ Mr. Mottershead is, have made trial of one of his expanding pistons for the last seven months; the inside of the cylinder was found, at the end of that time, not to be in the least degree scored or marked, but retained its polish perfectly. It works without oil, and has been proved by a water pressure of 700lbs. on the inch. The composition of which the piston is made is bronze, consisting of seven parts copper and one part tin. Three other engines have more recently been fitted up by the same makers with Mr. Mottershead's expanding piston."

The circumference of this piston, as exhibited in the elevation fig. 1, plate 4, is divided into six sliding segments, three of them, hh, forming an upper layer, and three fff, a lower layer, which alternately overlap and underlap each other. Fig. 2 shews a plan of the lower half of the piston, fff being the segments which slide between the fixed wedges m mm; these wedges are fixed to the bottom plate; at k k k are pin bolts which enter slot-mortices in the bottom plate, and limit the range of the sliding segments, and prevent them from falling out of their places when the piston is put together; ggg are the springs which abut against the fixed central projection, and thence by their elasticity protrude the segments against the cylinder, as the former or latter wear away by friction. The

piston is bound together on the piston rod c, by the nat e, and the shoulder d."

If, instead of copying the erroneous or interested remarks of others, the writer of the preceding paper had examined a little into the truth of them, he would have acted wiser, and more to the credit of the respectable and patriotic Society on whose behalf he wrote them. Alluding to Barton's piston, the writer echoes, that "the point of the wedge has been found very liable to mark, and finally to score and destroy the cylinder, against the sides of which it is continually pressing with a force greater than that exerted by the seg'ments.' In the first place, we would observe, in reply to this uucalled-for attack, that the wedges are not pointed, the sharp extremities being taken off in all the pistons that are manufactured. With respect to the statement of their scoring and destroying the cylinder, we have already shewn, in a previous notice of this admirable invention (see vol. 3, new series), that they have no such tendency, and that all the persons making such observations in print have committed themselves in the grossest inconsistencies; we shall therefore merely state in this place, that the objection (if it be good for any thing) which has been urged against Barton's piston by the Editor of the Society's work, applies to Mottershead's piston (as we have before observed) in a greater degree.

The reason given for this effect of" scoring and destroying," is, the inequality of the pressure of the component parts of the piston against the circumference of the cylinder. Now, it would naturally have been supposed, by reasonable people, that the "Society for the Encouragement of the Arts, &c." would not have rewarded as an improvement upon Barton's, a contrivance in which the very inequality of pressure spoken of is still greater; but a glance at the drawing of Mottershead's piston will convince every observer, that where the segments overlap each other, there is exactly twice as much pressure against the cylinder as where they lie singly; consequently, the effect will be to wear the cylinder into alternate grooves and ledges. Mottershead makes no provision against this effect; * no notice whatever is taken of its existence; the Society are blind to the fact; their optics, however, become magnifying lenses in the examination of Barton's, but, like some unskilful microscopists, with high magnifying powers, they mistake the dust that has been artfully thrown into their eyes for real objects. This is the most favourable view we can take of their mistatement of facts; but to Mr. Barton, the consequence is of serious moment. That worthy and ingenious man has struggled through numerous difficulties in bringing various mechanical improvements to perfection; in his patent piston, he has experienced a host of opposition, and, we have heard, that some parties, while condemning his invention, were at the same time pirating and using it privately. To avoid paying Barton a reasonable compensation for

* Barton, in his pistons, makes due provision against the effect of any essential inequality of pressure, by making variations in the metals, or by cutting away certain parts of the rubbing surface of the wedges, to equalize the pressure throughout.

his patent-right, many attempts have been made to modify the invention in such a manner as to evade his claim; but not one of them have succeeded; and after repeated failure, these very persons have been compelled to avail themselves of his skilful assistance. In defence of his patent-right he has also been drawn into expensive law-suits and whilst forwarding, during twelve years, this valuable improvement in the steam engine, (which ought, by this time, to have yielded him an ample fortune) he has experienced little else but vexation and trouble; but just now, when the proprietors of steam engines are generally becoming sensible of the value of the piston, which has made its own way through evil report, and good report," and there is a probability of the inventor being ultimately recompensed, the " Society for the encouragement of the Arts, Manufactures and Commerce" make use of their influence to destroy its reputation, and recommend to the public instead, a slight modification of the Cartwright piston, the principle of which is condemned by all the best mechanics of the present day.

Since writing the foregoing, we have spoken to Mr. Thomas, (of the late firm of Helford and Thomas, alluded to by the Society) who informs us, that further experience of Mottershead's piston has proved to him that they do not answer, and that Barton's is the best metallic piston hitherto made,

STEAM COACH.

TO THE EDITOR.

SIR,-I presume, from the great public attention excited by Mr. Gurney's steam carriage, that a short account of the means by which the whole is propelled, may prove interesting to your readers. I confess, that, previous to the opportunity afforded me by the liberality of the patentees at their late visit to Bath, that I contemplated all attempts to substitute mechanical powers for living action as impracticable, unless the resisting medium is in an uniform measured degree, as in rail roads, &c. It appeared to me, that the constant changes arising from the inequalities of roads, and those ascents and descents which are in the line of communication between Bath and London, could never be counteracted by that perpetual alteration in the force of steam, as such conditions would require. The horse in an instant accommodates his force to the degree of resistance to which he is exposed; the construction of an inanimate machine, competent to produce corresponding effects, appeared to me to be attended with insuperable difficulties. Mr. Gurney's journey to Bath, and his return to London, have induced me to believe that the principal point is attained; and I think the patentees have judiciously concluded to confine its operation to short stages from London, and which will afford them opportunities of rendering it more perfect before any extensive line of road is proceeded on.-The merit of every machine principally depends on its simplicity. In this respect Mr. Gurney's plan is entitled to every admiration. The power he generates is very little reduced by friction, and its whole force is nearly employed, as the propelling agent. The boiler, or generator,

as it is termed, consists of about twenty gun-barrels, three feet in length, communicating at each end with an iron tube, by which arrangement a general communication is effected with every tube, and the barrels are about one inch from each other, and serve as a grating for the fire above, which is fed with coke from the inside of the carriage; a second fire is placed under the barrels, and supplied from the outside; so that this chain of tubes is placed between two fires; these tubes are completely filled with water, so that whatever degree of heat is employed, the water retains its fluid form, and does not assume a vaporific character before its liberation from the heated barrels; if any additional portion of water be determined into these barrels with a force capable of overcoming the resistance of the valve, then a corresponding portion of water passes into an iron cylinder, about seven or eight inches wide, and two feet long, and immediately flashes into steam; this part of the machine is called the separatory, and answers an important purpose in regulating the force of steam; suppose ten drops of water be converted into steam, at a temperature of 212 degrees, that steam, subsequently exposed to any higher degree of temperature, will not have its powers of pressure increased without an additional dose of water-the separatory supplies the deficiency; so, on the contrary, if the fire be not sufficiently active, the steam contains a surplus of water, and which excess is deposited in the separatory, and prevents the working cylinders from being overcharged with that fluid; from the separatory the steam is alternately determined into two cylinders, which are six inches in diameter, and a stroke of 20 inches; the rods of the pistons are attached to cranks on the axle of the hind wheels, these cranks are reverse to each other, and hence by the reciprocating action of the pistons, a complete revolution of the axis takes place; at each end of the axis is a bar of iron the width of the wheel, and secured at each end by bolts to the circumference; by this adjustment the powers of the pistons are advantageously applied to the circumference of the wheel, and prevents the strain which would be the result, if nearer the nave. As the pistons are elevated by the steam, two pumps are worked by rods going through stuffed boxes, and are attached to the piston at the other end; these pumps communicate with the cistern of water, a small portion of which is determined into the generator, to produce the effect already noticed; the stream is afterwards directed into a cooling cylinder, through a worm, part of which is condensed, and returned to the cistern, and the other portion passes immediately through the chimney into the atmosphere.

The average pressure is about eighty pounds to the square inch of the surface of the piston: it is ascertained that double the pressure does not require the same proportionate increase of fuel; and as economy in this respect is of great importance, the steam in the first instance possesses a power equal to one hundred and sixty pounds, and the communication with the separatory is cut off when the piston has moved through half its range, the expansion of the steam is competent to effect the passage of the pistons through the remaining part of the barrel.

Close by the engineer is a governor, by which he is enabled to

regulate the steam, either by increasing its force or diminishing its quantity; and this important part of the operation is managed with great facility.

From the coke being employed, and the steam considerably cooled, there is no perceptible smoke, nor clouds of vapour, and the cylinders working about thirty times a minute, the saine number of revolutions of the hind wheels take place, supposing the circumference to be fifteen feet, the velocity would be nearly six miles per hour; on level ground, it will move through three times this space, so that there would be ninety strokes per minute; under such cir cumstances, the steam has a less pressing power, from the water not remaining so long in the regenerator. The metal which is employed is forged iron; if from any circumstance the pressure of the steam should overcome the resistance of the metal, no explosive effects would be the result, and no dangerous consequences would follow; the iron would rend, and the steam would escape through this small separation, and the action of the machine is necessarily stopped, and it is only by such a cessation that a solution of the continuity in any part of the apparatus is suspended.

One bushel of coke will suffice for two miles, and one gallon of water is consumed each minute: hence every eight or ten miles a fresh supply of water is required, and every twenty or thirty miles, coke.

From a comparative statement of the relative expences, if we estimate the apparatus and wear equal to four horses, the engineer and stoker as corresponding to the coachinau and guard, then the expences will be reduced to the small compass of travelling. I understand that four horses are estimated at 2s. per mile; while the coke required would not exceed 3d. so that the difference of expences would be as 3d. to 2s.

Probably a more important result would arise whenever the time shall arrive for the general employment of this mode of travelling: upon a moderate computation, in the British dominions, a million of horses are appropriated to this purpose, as each horse consumes that produce from land equal to the support of seven men, a change of agriculture would be the result, and food for seven millions of inhabitants thus created, From such a reflection, I am induced to conclude with the noble Duke at the head of our Government, that the advantages are probably incalculable.

No. 11, Sydney Place, Bath.

C. H. WILKINSON.

NEW FURNACE BOILER.

BY THE EDITOR.

IN fig. 4, plate 4, is represented a vertical section of a steam engine boiler, and smoke consuming furnace, which the Editor designed some years ago, and had almost forgotten, until lately, when he stumbled upon the sketch, from which the figure herein given is copied. As, upon consideration, it appeared likely to furnish some useful hints in constructing such apparatus, as well as in the formation of distillatory apparatus, the Editor has not thought it, amiss to lay the idea before the public.