Mr. Hornblower saw that this must be a beneficial employment of steam, and preferable to the practice of condensing it while its full elasticity remained; but he has not considered it with the attention necessary for ascertaining the advantage with precision.
Dr. Robison then proceeds to an investigation of the effect of this engine, and he finds it to be exactly the same with the accumulated pressure of a quantity of steam admitted in the beginning, and stopped in Mr. Watt's method, when the piston has descended through the mth part of the cylinder. In considering, says he, Mr. Hornblower's engine, the thing was pre sented in so different a form that we did not perceive the analogy at first, and we were surprised at the re sult. We could not help even regretting it, because it had the appearance of a new principle and an im provement; and we doubt not but that it appeared so to the ingenious author; for we have had such proof of his liberality of mind as permit us not to suppose that he saw it from the beginning, and availed himself of the difficulty of tracing the analogy. And as the thing may mislead others in the same way, we have done a service to the public by showing that this en gine, so costly and so difficult in its construction, is no way superior in power to Mr. Watt's simple me thod of stopping the steam. It is even inferior, be cause there must be a condensation in the communi cating passages. We may add, that if the condensa tion is performed in the cylinder A, which it must be unless with the permission of Watt and Boulton, the engine cannot be much superior to a common engine; for much of the steam from below B will be condensed between the pistons by the coldness of the cylinder A; and this diminishes the downward pressure on A more than it increases the downward pressure on B. The disposition and connection of the cylinders, and the whole condensing apparatus, are contrived with pecu liar neatness. The cocks are very ingenious; they are composed of two flat circular plates ground very true to each other, and one of them turns round on a pin through their centres; each is pierced with three sec lora) apertures, exactly corresponding with each other, and occupying a little less than one half of their surfaces. By turning the moveable plate so that the apertures coincide, a large passage is opened for the steam; and by turning it so that the solid of the one covers the aperture of the other, the cock is shut. Such regulators are now very common in the cast iron stoves fur warming rooms.
Mr. Ilornblowees contrivance for making the col lars for the piston rods air tight is also uncommonly ingenious. This collar is in fact two, at a small dis tance from each other. A small pipe, branching off from the main steam pipe,communicates with the space betwee., the collars. This steam, being a little stronger than the pressure of the atmosphere, effectually hin ders the air from penetrating by the upper collar; and though a little steam should get through the lower collar into the cylinder A, it can do no harm. We see many cases in which this pretty contrivance may be of signal service.
But it is in the framing of the great working beam that Mr. Hornblower's scientific knowledge is most conspicuous; and we have no hesitation in affirming that it is stronger than a beam of the common form, and containing twenty times its quantity of timber. There is hardly a part or it exposed to a transverse strain, if we except the strain of the pump V on the strutt by which it is worked. Every piece is either pushed or pulled in the direction of its length. We only fear that the bolts which connect the upper beam with the two iron bars under its ends will work loose in their holes, and tear out the wood which lies be tween them. We would propose to substitute an iron bar for the whole of this upper beam. This working beam highly deserves the attention of all carpenters and engineers." Before concluding this account of Mr. Hornblower's engine, we shall describe a very ingenious skeleton valve of his invention, which has been found of great advantage. It is shown in Plate DIX. Fig. 2, 3, where AAAA is the box containing the valves, BB is an inverted valve firmly fixed to the bottom of the socket S ; this socket serves as a guide to the part of the valve which is to be lifted by a short cylindrical rod as in common cases. The part which is to be lifted is DDEE, and the lifting is to be performed by any of the usual methods attached to the eye F, which is part of the cross bar EE in Fig. 3, which is a plan of the upper surface of the valve and its upper seat. The valve has two seats, and the principal passage for the steam is at the lower seat, for the steam in its passage goes through the body of the valves, having always access to the lower seating, as the body of the valve is entirely open, excepting what is taken up by the cross bar EE ; so that in this sense we lift the thickness of the metal only, of which the body of the valve is constructed. In order to understand the operation of the valve better, we must conceive the upper space in the box to be always full of steam, and consequently the inner part 00 of the valve also full; then the lower space of the box will be a vacuum, when upon lifting the valve (which is a cylinder open at both ends) the steam will pass through it and into the lower space at the inverted lid BB. The cylindrical part is raised a little in the figure to show how it sepa rates From the lower lid BB. An improvement in this valve has been suggested by Mr. Tredgold. The practical difficulty being to make it fit steam tight on two seats, he proposes to make the outside of the cylinder to slide in a stuffing box, or in au elastic packing of metal. When this is done, the largest valves will present no other resistance to being opened than the pressure on the scat, and the friction of the surface of the cylinder. " It is simply," says Mr. Tredgold, " the common conical valve inverted, and that which formed the seat in the common valve moves instead or the plate, and should obviously slide in a steam-tight case." .account of Trevithick's high Pressure Engine.