so readily, and in consequence, the steam pressure is better maintained ;* and there is generally a little more lead to the slide, to allow a freer ingress to the steam.

There is also another reason why the one diagram is better than the other. The eccentric and crank are so connected that they revolve together, and on account of the smallness of the throw of the former compared with the length of the eccentric-rod, if the steam be cut off when the crank has described a certain angle from the top centre, it will, if the lead be the same on both ports, be again cut off when it has described the same angle from the bottom centre, but because of the shortness of the connecting-rod, the piston will not have traversed so far when the crank has descended through a given angle as it will when it has ascended through the same angle. For instance, when the crank is horizontal and descending, the piston will not be half-way down; but, when horizontal and ascending, it will be more than half-wap up; and, hence, as the crank moves from the top centre, the space traversed by the piston till the steam is cut off is less than when moving from the bottom centre. It therefore follows, that in direct-action engines the steam line is continued further in the up stroke than in the down stroke, and the reverse will be the case in beam engines; and this difference becomes more apparent as the connecting rod is shorter in comparison with the crank.

302. To calculate the mean pressure on the piston throughout the stroke from an indicator diagram.

For the purpose of ascertaining the condition of the valves, piston, &c., it will, in the hand of an experienced engineer, be quite sufficient for him to see the general outline of the diagram; but if it is to be used for ascertaining the power of the engine, its average measurement is to be ascertained. This will be be best done by

RULE CXXXII.

1. Divide the diagram in the direction of its length into ten equal parts by drawing nine ordinates, or lines perpendicular to the atmospheric line at equal distances.†

The difference in the speed of the piston at the opposite ends of the cylinder is, at the outer end of a direct-acting engine, from one-sixth to one-third greater than at the crank end-the difference varying according to the degree of angular vibration of the connectingrod. In side lever or beam engines these proportions are reversed, and the speed of the piston is greater at the upper end of the cylinder.

+ Dividing a diagram into ten equal parts is a somewhat slow process by rule and compass, and an ingenious contrivance has been introduced with Mr. RICHARDS' indicator for this purpose.

It consists of a parallel rule composed of eleven bars of thin steel, and two strips; and by bringing the edge of the first bar against a perpendicular drawn on the atmospheric line, the contrivance can be moved so that it contracts so as to be equal to the length of the diagram, whatever that may bo, and an accurate division into 10 equal parts is made with surprising facility by drawing pencil lines against each of the bars.

NOTE.-In practice 10 is the usual number of parts employed (see figure following) but any number may be made use of; and the greater the number the more correct will be the result (see Fig. 57). The advantage of taking 10 ordinates instead of 8, 9, or 11, is, that the division by 10 is accomplished by merely shifting the position of the decimal point, while 10 ordinates are enough to enable the area to be measured accurately enough for all practical purposes.

2o. With the scale to which the indicator is constructed, measure between the spaces the distance from the atmospheric line to the upper outline (or the steam side) of the diagram until this crosses the former, if it does so. Next, repeat the process for the area between the atmospheric line, or the expansion curve, after it has crossed this line and the lower outline of the diagram. (See Fig. 50 and 52 in illustration.)

3°. Take the sum of the measurements of the steam side, then of the vacuum side, and divide by 10 in each case.

4°. Add vacuum to steam which gives the total mean pressure for each part of the stroke.

In finding the mean effective pressure off the diagram it is not at all necessary to take the steam and vacuum effects separately. The usual and most expeditious mode is to proceed thus:-*

(a) Take measurements at once completely across the diagram, that is, measure between the spaces from the vacuum line to the steam line; note these scale distances down.

(b) Add the whole together and take the mean by dividing by 10, the result is the total mean pressure.

In the case of a double diagram, or diagrams from top and bottom of cylinder combined, proceed as directed in Nos. 2o, 3o, and 4°; or as directed in 4° (a) and (b), then,

5°. Lastly, add top and bottom together, and divide by 2, the result is the average of both, or the mean pressure for the whole double stroke.

NOTE.-The mode of admeasurement as given in paragraphs 2o, 3o, and 4°, of the foregoing rule is not a strictly accurate one for ascertaining the power of any condensing engine, the steam line of which expands below the atmospheric or datum line. The method, however, does not give a really wrong result when it is wished to ascertain the gross power of an engine by means of the indicator; if, however, it be desired to find separately the respective values of the steam and vacuum pressures, the method is decidedly incorrect. The gross pressure on the piston throughout the stroke is, by this method, obtained at the expense of double work—it not being at all necessary to measure separately the ordinates of the steam and vacuum curves. In any properly constructed condensing engine the steam line of the diagram expands below the atmospheric line. At the beginning of the stroke it is above the line, and further on it is below the atmospheric line. In adding up the steam pressures, only

This is the method that is always adopted by the steam department of the Admiralty, and the principal firms in the kingdom.

the plus pressures above the atmospheric line are taken into account, while the minus steam pressures below the atmospheric line are neglected. In adding up the vacuum pressures, instead of taking the complete vacuum up to the atmospheric line, the ordinates are measured between the steam line (that happens to be below the atmospheric line) and the vacuum and exhaust line. It will be at once perceived that the steam pressure is exaggerated by only taking the plus pressures into account, while the vacuum pressures are detracted from by not measuring them from the atmospheric line. The total amount comes right at last; but the separate values of the steam and vacuum pressures are wrong.

But it happens that there is a simple plan by which a person totally unconscious of the deep mysteries of plus and minus can take the separate values of the steam and vacuum pressures from an indicator diagram. It is one often employed in the steam department of the Admiralty. The atmospheric line is taken as equal to 15 lbs., and from this line, as a point of departure, fifteen other lines are marked by the scale, thus coming below the vacuum line of the engine up to the true vacuum line of a perfect exhaust. The lowest line is then taken as the zero or datum line, and the actual pressure (not the relative pressures as compared with the atmosphere) of both the steam and vacuum line can thus be obtained. The difference between the two pressures gives the gross or actual pressure on the piston.

EXAMPLES.

Ex. 1. Work out the following indicator card, and show the point where the steam is cut off, whether the slide-valve is correct or requires alteration, and if the latter, what?

In the accompanying copy of an actual diagram, the horizontal line represents the atmospheric line; a graduated scale is on the left, which measures the pounds pressure upon every square inch of the piston surface, as previously ascertained by the maker. Nine ordinates to the atmospheric line are drawn at equal distances, thus dividing the space in length into 10 equal parts, the measurements are taken at once completely across the diagram, (see 4° a), and, as an example, the length a b indicates that in the interval, while the parts of the curves at a and at b were being traced by the indicator, there was a pressure in the cylinder of 18 pounds to the square inch; the sum of all these areas divided by 10 gives the average mean pressure of steam and vacuum in the cylinder.

Atmospheric line

9.6

8.6

10/ 153.5

This is a diagram showing the mode of determining the effective pressure of the steam,

and the mean vacuum effect, these combined are the "total effective pressure." To obtain these proceed as follows:-Divide the diagram into 10 equal spaces or points, draw lines at right-angles to the atmospheric line across the diagram at these points, and with the wooden scale corresponding to the spring used, measure between the spaces from the atmospheric line upwards to the diagram line, note the number on the scale corresponding to this

YY

15 lbs

10

distance, and write it down in this space; repeat the measuring and noting of the 10 spaces above the atmospheric line; add the whole of these measurements together, and take the mean—that is, as ten measurements have been made, the amount must be divided by 10; the quotient is the mean effective pressure of the steam upon the piston.

Next, measure between the spaces from the atmospheric line downwards to the vacuum line, write down the amounts, add the whole together, and take the mean; this quotient is the vacuum effect upon the piston.

Ex. 2. Work out the following indicator card, and show the point where the steam is cut off, whether the slide-valve is correct or requires alteration, and if the latter, what?

The foregoing diagram is taken from a condensing engine (not a marine one, however),

which engine may be considered a good example of expanding steam in the cylinder, and also of the setting of the valves. The pressure of steam in the boiler is 30 lbs. The steam in the engine having expanded to 6 lbs. below the pressure of the atmosphere is better used up than in most cases. The steam is cut off at one-seventh the length of the stroke, the pressure of the steam decreasing as the piston advances the other six parts of the stroke.

In this diagram measurements are separately made from the atmospheric line to the steam and vacuum curves respectively, but after the steam line expands below the atmospheric line, the ordinates are measured between the steam line and the vacuum and exhaust line. This, it will be seen, is the method alluded to in the note at page 343, No. 5°, as being a mode of admeasurement not strictly accurate for ascertaining the power of a condensing engine, but, as before remarked, it gives the gross power correctly, but in finding separately the steam and vacuum pressures it is decidedly incorrect.

Ex. 3. Work out an indicator card, and show the point where the steam is cut off, whether the slide-valve is correct or requires alteration, and if the latter, what?

Indicator Diagram of S.S. Havelock, November 9th, 1858.

Steam 20 lbs.; vacuum 13 lbs.; revolutions 26.