S.S. Fairy Dell, at sea, 16th February, 1871.

Steam 125 lbs.; Vacuum 26; Revolutions 80; Stroke 20". High Pressure Cylinder 84"; Low Pressure Cylinder 241′′; I.H.P. 146. Fig. 90.

ON THE FORM OF THE FIGURE WHEN THE EXPANSION

GEAR ALONE IS USED.

If the steam be cut off at any part of the stroke, the steam pressure will decrease and the pencil begin to descend. Now, if the temperature of the steam be unaltered, the pressure, as before stated (see page 326), will vary inversely as the space it occupies, and the theoretical figure will be F a b c d (see Fig. 16, page 327). But since the slide-valve also acts we have the modification this would produce too, for the slide-valve is placed between the expansion-valve and the cylinder; it follows, therefore, that the effective volume of the steam intercepted by the expansion-valve is the whole of the space between it and the piston, and the slide-valve interposes an additional barrier when it begins to cut off the steam. The case, therefore, is somewhat similar to what it would be if there were two expansion valves, one nearer to the cylinder than the other, and the outer one acting first.†

This figure exhibits a series of diagrams representing various grades of expansion; here o gives the full power of steam without using the expansion gear; 1, that produced by the first grade of expansion; 2, that produced by the second grade, and so on. It is worthy of remark, that the diagram

Except with engines fitted with SEAWARD's slide.

The condensation of steam by radiation of heat from the cylinder, the leakage of steam through the piston or valves, or the expansion of the steam in the nozzles of the cylinder, the steam pipes, or the extra length of cylinder allowed for clearance of the piston, will vary the shape and termination of the curve. It should, therefore, always be an inquiry, upon the production of a diagram, whether the curve be theoretically correct or not. If it is not, a satisfactory reason should be found for it, in the construction of the engine or the valves, and the piston should be thoroughly examined. A ready method to determine at what point this curve should terminate, is to find the capacity of nozzles, pipe, &c., from the expansion-valve to the cylinder, and adding it to the clearance, call it so much additional length of stroke. Extend the diagram in the same proportion and then divide it into any number of equal parts that will divide it equally, and have one of the divisions cut at the point where the valve closed. Take the number of these divisions on the steam line for the numerator of a fraction, and the whole number of them for the denominator, and divide with it the whole pressure upon the piston.

marked o, which is that resulting from the slide-valve, closely assimilates to that marked 1, produced by the first grade of expansion, as it should do in well constructed engines.

Each of the following figures exhibits a series of diagrams representing different grades of expansion, it is required to determine the best point at which to cut off the steam.

Fig. 99. This is from the top side of the engines of a screw steamer, the engines being connected to the screw shaft, without intermediate gearing. The stroke is 2 feet 3 inches; the screw shaft, and of course the engines also, making from 90 to 98 revolutions per minute. This figure shows the steam cut off with the slide-valves, and also with the first grade of expansion; and it will be observed that the power is very little reduced by putting on the expansion, as, with the speed of the piston and the condenser being small, the vacuum is much improved by cutting off the steam. In this example the valves are very

well adjusted.

Indicator diagram from the engines of H.M.S.S. Thunderbolt, and shows the different

Ans. Average of 9'84 lbs.; 114 lbs.; 12.85 lbs.; § 14.22

lbs.