stern, and the shaft brackets are incapable of taking the thrust, so that in such cases both ahead and astern thrusts must be wholly taken inboard, by means of a thrust-block on the shafting.

This system of taking the whole of the thrust both ahead and astern by means of a thrust-block fitted inside the vessel is a satisfactory method, the thrust-blocks being very efficient. They can be always under observation, and can be easily adjusted and kept in proper order. Besides being adopted for twinscrew ships, its convenience has led to its adoption in single-screw ships, so that the single-screw mercantile marine vessels are also generally without any outboard thrust arrangements, and depend on the inboard thrustblock to take the whole thrust. In Fig. 304, showing the stem tube and propeller of a modern single-screw mercantile vessel, there is no outer thrust bearing.

Shaft Brackets. - In twin-screw ships, the propellers on either side of the ship usually work outwards when driving the ship ahead; but in many recent war-vessels, as the result of experiments made in Admiralty experimental tank at Haslar, they are made to turn inwards. These experiments, made on models, indicated a

the

slight increase in efficiency, so that, as it is more convenient from the engineer's point of view for the screws to turn inwards, the starting platforms being then both in the front of the engines at the middle line, the plan has been adopted in many ships for the Royal Navy. Experience with engines so arranged has, however, caused some adverse criticism as regards the power of the screws to turn the ship when working one ahead and one astern, which appears to be reduced.

The after parts of the propeller shafting pass outside the ship and work in bearings close to the propellers, carried by brackets secured rigidly to the hull. In the older high-speed ships, in which the after run is very fine, the length of the shafting outside the ship is so great that intermediate bearings, between the stern-tube and the afterbracket, have been fitted, but the resistance of these additional bracketbearings is great, and they have not often been fitted to modern vessels. In modern ships, hollow steel shafts, of enlarged diameter between the bearings to give increased stiffness, are fitted, to dispense with the intermediate bearings and their resistance.

Figs. 309 and 310 show the stern and propeller fittings of modern high-speed twin-screw warships. The bush carrying the lignum-vitæ in the after bracket is in these sketches withdrawn by removing the propeller and drawing the bush aft, but as this involves considerable work, these bushes in the more modern vessels are made in halves, and arranged so that it can be withdrawn forward so that the propeller need not be disturbed.

CHAPTER XXVI.

THE INDICATOR AND INDICATOR DIAGRAMS.

WE will now proceed to describe the apparatus which enables the engineer to ascertain many facts of the greatest importance as to the action of the steam inside the cylinder. This instrument is called the steam-engine indicator, or, shortly, the 'indicator.' Unfortunately, even when the indicator has told us all it can, regarding the interior economy of the steam-engine, there remains much respecting which our knowledge is very imperfect.

The steam engine indicator is an instrument which shows the pressure of steam in the cylinder at each point of the stroke of the piston. This pressure varies considerably, and is shown for both the outward and inward strokes, which enables the effective pressure at any point of the stroke of the piston to be ascertained, and the mean effective pressure on the piston during the stroke to be calculated.

General features of indicators.-The general features of the instrument are as follows :-A pencil is attached by means of a system of levers to the piston of a small cylinder of known area, open to the atmosphere at the top, and connected by means of stopcocks and pipes to either end of the engine cylinder as required. When the stopcock is open, so as to place the bottom of the indicator cylinder in connection with one end of the engine cylinder, the indicator piston, carrying the pencil, is moved up and down by the varying pressures of the steam, the motion of the indicator piston being resisted by the action of a spiral spring of known elastic force. A sheet of paper is fixed on a barrel, which is caused to revolve backward and forward in a manner coincident with the motion of the engine piston, and on this moving paper the pencil traces a curve or diagram, from which, at any given part of the stroke of the engine, the corresponding pressures of steam in the engine cylinder may be measured. From the mean effective pressure ascertained from this diagram the I.H.P. of the engines is ascertained. This determination of the horsepower is the principal use of the indicator. By means of the indicator diagram, however, many other particulars relative to the action of the steam in the cylinders, and the adjustment and condition of the slidevalves and pistons, may be ascertained; and many improvements that have been made in the performance and efficiency of steam-engines have been largely assisted by the application of this instrument.

The following important particulars may be seen by inspection of the diagrams:

(1) Whether the admission of steam is early or late, the amount that the initial pressure in the cylinder is below the boiler or receiver

pressure, and whether the pressure is well maintained up to the point of cut-off or not.

(2) The part of the stroke of the piston at which the admission of steam to the cylinder is cut off, and whether the cut-off is sharp or gradual.

(3) At what point and pressure the steam is admitted to the condenser.

(4) The amount of back pressure or vacuum, whether the reduction is obtained quickly or not, and the amount of compression at the end of the stroke.

It must be borne in mind, however, that as the indicator shows only the pressure at each point of the stroke, the engineer has to account for peculiarities in the form of the diagram by reasoning, and errors are here often committed.

The indicator in a crude form was invented by James Watt. Since his time its construction has been simplified and perfected, McNaught being one of the earliest to effect improvements.

McNaught's indicator. In the McNaught indicator, which did excellent service with engines fitted with low steam pressures and moving at slow speeds, the pencil is attached directly to the indicator piston, so that their extent of motion is the same. Consequently, they are unsuitable for quick-moving, high-pressure engines, as the necessarily long springs used in them have to be instantly compressed to a considerable extent on the admission of steam, and in quick moving engines this causes violent oscillation of the pencil and a series of undulations resulting in a serrated diagram which is almost useless as an indication of the action of the steam in the cylinder.

Modern indicators.-To obtain satisfactory diagrams it has been found necessary to fit springs of high tension so as to permit of only a small motion of the piston. This reduces vibration, but to obtain a sufficient height of diagram it necessitates that the motion of the pencil be much greater than that of the indicator piston. The various types of modern indicators differ principally in the means of producing this multiplication, while still keeping the pencil moving in a straight line, and preserving a constant ratio (4 to 6) between the motions of piston and pencil. The difficulty with such motions is not so much to make the pencil move in a straight line, as to insure that it also moves, throughout its range, exactly the same number of times faster than the indicator piston.

The following features are common to all modern indicators, sketches of two of which are given in Figs. 312 and 313. At the lower end of a cylindrical case, A B, is the small steam cylinder in which a piston works practically steamtight, with as little friction as possible. To the lower end of this cylinder a straightway cock, c, is fitted, shown only in Fig. 312, with its end screwed to enable it to be attached to the nozzle of a right-angled three-way cock, called the indicator cock, which is connected by pipes to the two ends of the engine cylinder, so that the indicator may be placed in communication with either side of the piston as desired, enabling the two diagrams showing the pressures of the steam on both sides of the piston to be taken on one card. Sometimes the indicator is connected directly to each end of the cylinder, so as to get separate diagrams, which is often desirable in very quick-moving engines, especially