Principle on which the Indicator works.-Putting aside for the present the mechanical arrangements as the Indicator is now made, and considering it in its simplest form, it is merely a small piston working in a cylinder with considerable clearance, carrying a pencil at the end of its piston-rod. One end of this small cylinder is placed at pleasure in connection with either end of the main cylinder (that is the cylinder of which it is desired to know the horse-power), by means of a cock and pipes, and the other end of the Indicator cylinder is in free communication with the air by means of the loose stuffing-box through which the piston-rod of the indicator moves, and further, by means of a hole drilled in the cylinder cover of the indicator, so that if steam goes into the main cylinder, a portion of it may be admitted directly to the bottom side of the indicator piston, while upon the other side the air presses continually with whatever the barometric pressure may be at the time. A spiral spring is attached to the cover of the indicator cylinder at one end, and to the indicator piston itself at the other. This spring regulates the movements of the piston, and as the steam is of a greater or less pressure, so the spring is more or less compressed.

Suppose now that the piston of the engine is at one end of the stroke, and about to begin a fresh stroke, and that the steam is admitted to the cylinder for this purpose, the indicator spring will be compressed by the steam pressure under it, and the amount to which the indicator piston rises is a measure of the steam pressure; for example, supposing that the spring compresses th inch for every pound on it, then, if the steam pressure is 20 lbs., the piston will rise 2 inches. As the piston of the engine travels forward on its stroke, the steam pressure begins to diminish, and becomes less and less able to compress the indicator spring, and consequently the indicator spring continually falls. In order to register these continually varying pressures, a piece of paper is held on a small cylinder or barrel* in front of the pencil on the indicator piston, and as the main piston moves backwards and forwards, the barrel of the indicator partially rotates backwards and forwards, and the curved line traced by the pencil moving up and down, on the paper moving at right-angles to the up and down movement of the pencil, is called the indicator card or diagram. The diagram is nothing more than a register of the varying pressures in the cylinder as the piston moves to and fro.

289. Richards' Indicator.-Figure 8 is a drawing of Richards' Indicator, which is the best form of indicator now in use, and is becoming generally adopted, as it is in some respects better fitted for obtaining accurate results from engines working with steam of high pressure, and having a high speed of piston, and great range of expansion. Its peculiarity, as distinguishing it

* In the original form of the indicator invented by JAMES WATT, a flat reciprocating plate carries the sheet of paper, and modern indicators differ chiefly from the original construction by the substitution of a reciprocating cylinder for the flat plate,

from older forms (as McNaught's, for example), consists in the arrangement by which the motion of the piston is transmitted to the pencil, the object being to enable stronger springs to be used, so as to lessen the tendency to vibration of the pencil. The drum or brass barrel A, having the split graduated scale B attached, is called the paper cylinder, and on it the paper for the diagram is stretched; this paper cylinder can be detached from the instrument by pulling it upwards; on being detached another barrel is seen, having the horizontal pulley L on its bottom end, and a small coiled spring contained in a drum at the top. It will be observed that on pulling the cord, the pulley barrel will revolve until it comes against a stop, on slackening the cord the coiled spring will bring it back; at the bottom of this pulley barrel will be seen a small projection C. When the paper cylinder is replaced, and the notch in it fitting this projection, the revolving and return motions of the paper cylinder will be clearly understood.* On the upper end of the barrel D the lever arm M is carried; these levers EE are connected by the link F-this link is the pencil carrier; † this metallic pencil is held by a screw which regulates its distance from the paper cylinder; the lever E1 is attached by a swivel at J to the piston-rod G. This arrangement constitutes a simple straight line motion, and moves just four times the travel of the indicator piston. On unscrewing the top milled cap H the lever arm can be detached from the barrel D, bringing with it the piston-rod and also the spring used. Within the barrel D is contained the small cylinder in which the piston I works; to shift the spring, unscrew the rounded milled nut J on the end of the piston-rod, then unscrew the milled cap H from the nut on the end of the spring, the piston-rod is then free, and the spring can then be unscrewed from the piston I. It will be observed that the springs have a brass nut on each end, on one nut is marked the limit pressures, thus, 17-47 lbs. steam, 15 lbs. vacuum-on the other the scale strength of the spring, thus, On putting the small piston into the cylinder it will be seen that it is an accurate fit, great care must therefore be taken to keep the cylinder perfectly clean. On the bottom end of the barrel D is the conical stem having the tightening nut K attached, this nut has two small arms on it; the conical stem is a ground fit into the top end of the cock supplied with each indicator, this cock has its

All the RICHARDS' Indicators now supplied by the makers are fitted with DARKE'S "Patent Detent" and "Cord Adjuster," which is an arrangement for instantaneously stopping and starting the paper-drum, thus avoiding the inconvenience and delay incedental to the present method of connecting and disconnecting the cord which gives motion to the paper-drum. It is exceedingly simple; a pawl is made to fall or rise by the movement of a spring, so as to engage or liberate the paper-drum by means of a small segment of a ratchet placed at its base. It will be found most convenient in all cases; especially useful in trial trips, and where it is desirous to take a number of diagrams in rapid succession. It is certain and smooth in its action. The CORD ADJUSTER will be found of great service as a means of adjusting and re-adjusting the length of the cord of the paperdrum without the use of knots, which are generally difficult (and indeed sometimes through the presence of grease almost impossible) to untie.

+ In McNaught's instrument the pencil is attached to the piston-rod.

conical seat protected by a brass cap; unscrew this cap and insert the conical stem, screw down the tightening nut, and as this nut has its threads of different pitches, it firmly secures the indicator to the cock. The communication between the top and bottom of the engine cylinder is obtained by means of a pipe attached to each end, and coupled in the centre to a cock, this cock has a projection on it which is tapped to suit the indicator cock.

NOTE.-In order to reduce the weight of the moving parts as far as possible, with a view of lessening the vibration, Mr. THOMPSON exhibited an indicator at the Philadelphia Exhibition. A single lever only is used, one end being caused to move in a straight line by a parallel motion. The lever is pivoted to a bracket which has its lower end fixed so that its upper end is free to move. A light and short radius bar is attached to a rigid standard, and also to the lever aforesaid, at such a point that its effect is to counteract the arc which the free end of the lever would describe. The line produced is practically straight, though beyond certain limits the lines produced by this and Mr. RICHARDS' indicator, begin to curve, and the length of the straight portion is the same in each. The joint is brought down near the piston, and is a modified form of ball and socket, which allows both the lateral movement required by the parallel motion and the rotative movement involved, in swinging the lever to and from the drum. This joint is made compensating, so that any looseness may be taken up.

There are thus two classes of indicators: those where the piston travels with the pencil, and those where the pencil moves a greater distance than the piston of the indicator.

The reason adduced for giving preference to the latter system consists, as before stated, in the lessened momentum of the moving parts, owing to the small space through which they have to pass in a given period, and thus for quick speed engines the indicator diagram produced has a more steady and reliable form. But it does not yet appear that any indicator has been constructed which shall, under all circumstances, give a reliable diagram for engines which run at very high rates of speed.

The defect of irregular movement is generally most perceptible at the beginning of a stroke, and this is commonly supposed to be due to causes in the instrument itself, though some consideration of the subject leads to a doubt as to whether such is always the case. A sudden admission of steam, often at a high pressure, may probably cause a movement in the piston and its connections which sets the indicator piston into a synchronous vibration; and the yet only half-known effects of condensation in the cylinder may give an impulse through the indicator cocks, which results in a more or less rapid blow delivered immediately underneath the indicator piston; and it may so happen that the instrument forms a convenient thing to blame for a vibrating appearance as the stroke begins, in cases where a current of water actually causes the evil. Such a defect might perhaps be obviated by having a crooked passage or a guard under the indicator piston, so that steam could pass easily underneath it while water is intercepted.

of

Care must always be taken to have the indicator cleaned after every time using; and the best easy test to discover that its piston works frictionless,

is to draw it up and let it gently descend to a fixed point, which ought to bring the pencil to the atmospheric line, and then to press the piston down and let it slowly ascend. If the pencil reaches the same position with both tests, an easy, frictionless motion can be relied upon.

When applied to purposes where the temperature is not more than that of the atmosphere, a corrector is necessary, because springs are stronger when cold than when hot; and there will have to be 1 lb. added to the apparent indication for every 40 lbs. shown by the diagram-the springs being all set for a temperature of 212° Fahrenheit.

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289. How to affix and use the Indicator.-It must be borne in mind that it is the action of the steam inside the cylinder that we wish to portray; therefore, the motion for the indicating paper, or card, must be derived from the engine piston-rod. Starting then with this rule, without exception, we must next note the correct position of the indicator on the engine cylinder. The indicator should be secured to the end of the cylinder (front and back ends of the cylinders), never on the side; for this reason, 66 the steam that is impelling the piston is also rushing along with it, and when exhaustion ensues, the volume has a retrograde motion, but still in a line with the cylinder's length." The position of the indicator beyond the nozzle must be close to it to ensure a truthful indication of the steam in the engine cylinder. If a branch pipe is requisite, let it be as straight as possible.

A common plan of fixing indicators, especially for marine purposes, is to have pipes from the top to the bottom of the cylinder, leading to the same indicator cock, so that both may be indicated without loss of time, and both figures be produced on the same indicator card. If such pipes are employed it is necessary to have them continuous from the cylinder to the indicator without any length being open past the indicator cock itself. Such a cul-desac has the effect of distorting the apparent pressure altogether. All bends and turns in such pipes should be gradual, and of the largest convenient radius, and narrow crooked passages avoided. Having supposed the indicator to be fixed, and in connection with the cylinder, the next point for consideration is the best mode to give motion to the paper or card. In the case of a marine engine, there is more trouble and difficulty in applying the indicator than in that of a land engine, as the proper amount of travel for the string is not so easily to be obtained. The most convenient arrangement would seem to be to add a small wheel and axle, or wheel and pinion, to the indicator, so that a string attached to the piston-rod and wound on the wheel will give the right motion to the cylinder on which the paper is fixed; and, the string being tied on the top of the piston-rod, the motion will be reduced, at once, to that for the paper cylinder, by means of the wheel and axle. It will do, however, when the proposed wheel and axle is not in readiness, to attach the string to any part of the parallel motion, or any part of the side