The D slide, and the four-way cock, however, equally possess a great defect, that of wasting the steam that fills the passages of the movable portion of the valves. Watt, Hornhlower, Murdoch, and other steam mechanicians, devised modifications of the D and other valves, by which the waste of steam was nearly obviated. The invention of Mr. Murray in 1789, for the same purpose, rank, very high in our estimation, being attended with less friction than the others ; we accordingly give it insertion in this place.
o in the foregoing figure, is the pipe conveying steam from the boiler, and delivering it into the descending pipe p, which terminates in the valve q, opening to the lower part of the cylinder, by the side opening, marked as a shaded parallelogram, while the valve r opens a similar communication with the upper part of the cylinder; so that, by the successive opening and shutting of q and r, steam is admitted above and below the piston. a is the lower end of the eduction-pipe, joining on to the condenser, and this pipe opens first to the lower part of the cylinder by the valve t, and leads also by a perpendicular continu ation of the same pipe e, to a valve a, by which a connexion is formed with the upper part of the cylinder. The two apertures into the cylinder, called nozzles, are therefore common both to the admission of steam and the forma tion of a vacuum, which is regulated simply by the working of the valves. For as the figure now stands, r is the only open valve in the steam-pipe; consequently steam would enter above the piston to depress it, while a vacuum would exist be low it, on account of the valve t being open to the condenser. As soon as the piston reaches the bottom of the cylinder, the valves r and t must be shut, and u and q opened ; when the steam, being no longer able to g it through r, would pass down the pipe p, and enter the lower part of the cylinder through g. Mean time u being opened to the condenser by the pipe o, would cause the necessary vacuum above the piston to permit its ascent, which being completed, the valves must be again put into the position shown in the figure, to eroduce its descent, and so on._ It will be sufficient to state that those valves are operated upon either by levers, passing in a steam-tight manner through the side pipes, or that erne timea the spindles of the valves are made to act one through the •tthet, in stuffing, as in the present instance, when they are worked by =term./ applica
tions.
By this most ingenious contrivance no waste of steam arises, excepting In the small aperture between the valves ; and the friction is obviously much less than in either slides, cocks, or perhaps any other kind of valve; the only resistance to their motion being the pressure upon the upper side by the steam, when in their seats. Their cost, compared to slide-valves, is much greater; but as they are not liable to material wear, and work with great accuracy, the extra ex pense does not prevent their very general adoption in large engines.
Having thus briefly noticed a very important class of valves, we proceed to describe another kind, which have even stronger claims upon our attention, as will be immediately acknowledged by naming them, Safety-valoes ' • these are well-fitted covers or stops to apertures made in the upper part of a boiler, and loaded to such a degree only as the steam will over come when it exceeds the required pressure. The contrivance, in nearly its usual form, (the steelyard,) was invented by Dr. Pepin, in 1684, as an appendage to his apparatus for dissolving bones by steam at high pressure ; but the first applica tion of it to the steam-engine was by Savery. It received some improvement by Beighton in 1718, since whose time the same form continues to be used, as will be recognised in numerous steam apparatuses in various parts of this work. Mr. Tredgold, in his able work on the steam-engine, observes, that it would be a at improvement upon safety-valves, if they could be so constructed as to be relieved of a part of their load, when raised from their seat. With the view of efilicting this object in the simplest possible way, we suggested many years ago, (see Register of Arts, 4.e. for tannery, 1829) the employment of a bent lever, in stead of the straight one in common use, the action of which will be understood by reference to the subjoined diagram, wherein is also represented some other modifications of the safety-valve, which it is presumed are worthy the consider ation of the practical man.