The present modes adopted for reducing the pressure on the back of the valve, are simply a ring encompassing a raised portion on the valve; a fitting projection on the inside of the case enables a perfect joint to be made; and a communication with the condenser causes a vacuum at the back of the valve, in place of the pressure of the steam. For small valves, springs are used to assist the pressure, the area being small there is no cause to reduce the pressure at the back.

The means for securing the valve rod to the valve are numerous; some makers prefer a clip or ring, others adopt a screwed fixture; a third will put the rod through the valve, while a fourth authority will make a solid T head to the rod, and fit the same in the valve. The best mode is to secure the rod into a loose nut of gun-metal, sufficient play being allowed for wear and repairs. In some instances two rods are adopted.

CHAP. II.

THE PROPORTION OF VALVES AND PORTS IN THE CYLINDER. COMMON AND EXHAUST RELIEF SLIDE VALVES.

SLIDE valves of the present day are divided into three classes or kinds-common, exhaust relief, and equilibrium. It is now proposed to describe the two former.

The common slide, Fig. 1, will be readily understood by the diagram, as to its shape, &c. As,

perhaps, it will not be out of place to give examples for future reference, allusion will be made to the proportions of the valve relatively

to those of the ports of the cylinder. Let it be presumed that a valve, &c., is required for an engine of 65-horse power nominal, adapted for the screw propeller. Now, allowing 17.5 square inches per horse power, the cylinder would equal about 38in. in diameter, having a stroke of 2ft. The size of the steam port supply being generally 1 square inch (in area) per horse power, the result would be 65 square inches. Thus far having determined this proportion, it now remains to decide the proportion of the ports, &c., not at a guess, but by correct data, certain for all dimensions or ratios. It will be noticed in the diagram, Fig. 2, that the slide, when at full

stroke, moves the full width of the port, hence the term common slide. Now, it is obvious that the area of the port being given, that same being

divided by its length, will produce the requisite width, which latter, in the present example, will be 24in., outside lap (presumed) 1in., inside lap in.; having thus far agreed, the following rules will supply what is required :—

Width of bar =

a minimum.

width of port 5, but lin. as

Width of exhaust space in valve

= width of

port supply 1.5+half travel of the valve+width of bar, minus inside lap.

Width of exhaust port in cylinder = width of bars minus inside laps, deducted from the width of exhaust space in the valve.

Now, as the two last rules may seem a little confused, it will be well to explain their origin. The exhaust space in the valve is that part that allows the steam to escape into the waste port. It is the general practice that the area of the exhaust port should exceed that of the supply; some engineers prefer supply 1, exhaust 2; others, the former 1, the latter 15. It is clear that when the valve is at full stroke, as in Fig. 2, one bar only is in question as to the area for the exhaust; the half travel of any valve, it is almost needless to say, is the outside lap, plus the width of the supply port; thus it will be understood that the inside lap must be also taken into

consideration to produce the exact proportion, although it may be said this last is not of vital importance, yet to attain the correct result, it must be noticed.

For explanation of the last rule, Fig. 1 will readily convey an idea of its origin and the truth of its composition; the inside laps, it will be seen, determine the dimension required. Simple as this rule may appear, it is of as much importance as those preceding it, more especially when requiring the width of the exhaust port by calculation in the absence of a diagram.

It is not here presumed that there is any new formula given; on the contrary, the author rather wonders that such a simple and correct mode of producing the proper proportions or distances is not more often alluded to. Many young engineers of clear perception look on this portion of the data requisite for the steam engine as the most difficult of all; but first determining the ratio of the length of the connecting rod to the stroke of the engine, and the grade to cut off the steam at, the remainder may be said to be a simple fact. Yet, withal, there is, perhaps, an atmosphere of mystery pervading the region alluded to, only to be dispelled by practice and thought. It may as well be added that