pencil-lever is pivoted to the radius link. The slot-curve is that peculiar curve which would be described by the guide-roller as a scribing point while the pencil is being moved in a true line; this, it is claimed, insures a correct parallel motion to the pencil. The guide-roller is journalled in a free collar held in the cylinder

cap, which allows all the moving parts

to revolve freely, as

the pencil is brought in contact with the

Tabor's Indicator.

paper.

The paper drum revolves on a steel spindle, upon which the bottom nut is screwed; the nut inside the drum is simply a milled head firmly screwed on the upper end of the spindle. The recoilspring is seated in a cup on the bracket, the outer end being fixed to it, while the

inner end is hooked

by the hub on the drum-base. A stop

block on the cup, engaging with a lug in the drum-base, forms the stop for the recoil motion. If the spindle be slacked somewhat, the drum-base may be revolved over the stop-block, and more or less tension given to the recoil-spring. By simply unscrewing the cylinder-cap, the whole motion work may be removed in one piece. The pencil-lever, piston-rod, and radius-link, are all of steel,

spring tempered; the

small number of

moving parts, and their lightness, reduce the error of momentum that exists in instruments of heavier parts, which is frequently a source of uncertainty. The whole instrument is very light, the design simple, and the workmanship neatly done. It is claimed that the diagrams produced are very good.

Functions of the Indicator.

The function of the indicator is to automatically trace out on paper a diagram that will graphically represent the pressure of the steam in the cylinder of the engine to which it is attached, with all its variations during both forward and return strokes of the piston. It enables those who use or have charge of steam-engines, to ascertain the condition of the parts of the engine subject to the direct action of the steam, and to what advantage the steam is applied; whether the valves are properly designed and accurately set, and if the steam-passages or ports are of the proper size to receive and discharge the steam in time to produce the best effect; what pressure of steam there is upon the piston at every position in the cylinder, as well as its average during the stroke; what is the value of the vacuum acting upon the piston of a condensing engine in all its positions in the cylinder, and what is its

average; whether the exhaust passages from the cylinder are sufficiently large to give free exit to the steam, and, if not, what percentage of power is lost in forcibly expelling it; the actual consumption of steam in giving motion to the engine, and also what additional steam is used in giving motion to the shafting and millwark, the paddle-wheel or screw-propeller; and also what power is required to move the machinery, or any part of it.

In manufacturing establishments where power is let to tenants it will show how much is consumed by each, and it will also demonstrate the degree of economy in using steam at different pressures, the benefits of expansion, and the relative efficiency of different kinds of expansion-gear.

Indicator cards are of great value, as they demonstrate the initial, mean effective, and terminal pressures, the back pressure, the cushion, whether by compression or lead; the point of cut-off, and the relative economy of different engines, aside from leakage and condensation. It may be applied not only to steam-engines, but to those driven by compressed air, or any vapor or fluid, as well as to the cylinders of air-pumps, air-compressors, blast-engines, etc. The diagram produced is the joint production of two movements, viz., a vertical movement of the marking point due to the pressure of the steam acting on the piston of the instrument, in opposition to the force of a spring of known strength, and a horizontal movement of the paper, as the drum, on which it is placed, makes partial rotations to and fro coincident with the movement of the piston. Hence, when the pencil is held in contact with the paper during one revolution of the engine, both will arrive at the point from which they started at the same moment, and a closed figure will be the result, except when a great change in the load and pressure occurs during the stroke in which the diagram was taken.

The value of indicator diagrams is that they show what proportion of the boiler pressure is contained in the cylinder; how early in the stroke the highest pressure is reached; how well it is maintained; at what point and at what pressure the steam is cut off;

whether it is cut off sharply, or in what degree it is wire-drawn; at what point, and at what pressure it is released; whether it is freely discharged, or what proportion of it (in excess of the atmosphere or the vacuum in the condenser, according as the engine is condensing or non-condensing) remains to exert a counter or back pressure; whether, before the commencement of the stroke, there is any compression of the vapor remaining in the cylinder, and if so, at what point in the stroke it commences, and to how high a pressure it rises. The foregoing particulars can only be learned by observation, though a scale, corresponding with the spring used, is needed to measure the pressures, and to locate the exact events in the stroke. The points to be observed in estimating diagrams are, the mean or average pressure; the total mean, or the mean effective pressure; the indicated horse-power, I. H. P., and the theoretical water consumption. The indicator shows the pressure at each and every point in the stroke; to represent this faithfully is its sole office. The causes which determine the form of the figure must be determined by the engineer.

Technical Terms Used in Connection with the Employment of the Indicator.

The term Adiabatic literally means no transmission. As applied to an expansion curve, it means that it correctly represents at all points the pressure due both to the volume and the temperature, just as if no transmission of heat to or from it had taken place.

Admission. This term is applied to the induction of the steam into the cylinder when the valve opens at the commencement of the stroke.

The term Asymptote means a line which approaches nearer and nearer to some curve, but which, though infinitely extended, would never meet it. The clearance and vacuum lines of a diagram are asymptotes of a true expansion curve.

The letter B at the end of a diagram means that that end was taken from the bottom end of the cylinder.

A. B. or Aba. is understood to stand for above atmosphere, and B. A. or Bla. below atmosphere.

The term Compression is a term used to express the distance through which the piston moves in the cylinder after the exhaust has closed. Compression takes place between the piston and the cylinder-head at the end of each stroke; and the distance from the end of the cylinder at which it takes place depends on the amount of lap on the valve.

The term Cushion means the resistance offered on the opposite side of the piston induced by the steam shut up in the cylinder.

Cylinder efficiency. This term is used to designate the amount of work performed in the cylinder of a steam-engine for a given pressure.

The term Clearance is used to express the extent of the space which exists between the piston, the cylinder-head, and the valveface at each end of the stroke. See page 122.

Displacement. This term is applied to the cubic contents, or the volume of water, steam, or air displaced by the piston during one stroke. It may be found by multiplying the area of the piston in inches by its stroke in inches. The product will be its displacement in cubic inches.

Duty. This term is understood by engineers to mean the efficiency of steam-engines, or the number of pounds that an engine is capable of raising one foot high per second with an expenditure or consumption of one hundred pounds of coal.

The term Flexure means bending or curving. The point of flexure in a diagram is the point at which the cut-off closes and the expansion curve begins, as shown at C, explanatory diagram