If it were possible to observe and record the varying position of the index on the piston-rod of the indicator, and to refer each of these varying positions to the corresponding point of the descending stroke, we should then be able to declare the actual pressure of the steam at every point of the stroke. But it is evident that such an observation would not be practicable. A method, however, was contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt, by which this is perfectly effected. A square piece of paper, or card, is stretched upon a board, which slides in grooves formed in a frame. This frame is placed in a vertical position near the indicator, so that the paper may be moved in a horizontal direction backwards and forwards, through a space of fourteen or fifteen inches. Instead of an index a pencil is attached to the indicator of the piston-rod: this pencil is lightly pressed by a spring against the paper above mentioned, and as the paper is moved in a horizontal direction [Pg276] under the pencil, would trace upon the paper a line. If the pencil were stationary this line would be straight and horizontal, but if the pencil were subject to a vertical motion, the line traced on the paper moved under the pencil horizontally would be a curve, the form of which would depend on the vertical motion of the pencil. The board thus supporting the paper is put into connexion by a light cord carried over pulleys with some part of the parallel motion, by which it is alternately moved to the right and to the left. As the piston ascends or descends, the whole play of the board in the horizontal direction will therefore represent the length of the stroke, and every fractional part of that play will correspond to a proportional part of the stroke of the steam-piston.
Fig. 81.
The apparatus being thus arranged, let us suppose the steam-piston at the top of the cylinder commencing its descent. As it descends, the pencil attached to the indicator piston-rod varies its height according to the varying pressure of the steam in the cylinder. At the same time the paper is moved uniformly under the pencil, and a curved line is traced upon it from right to left. When the piston has reached the bottom of the cylinder, the upper exhausting-valve is opened, and the steam drawn off to the condenser. The indicator-piston being immediately relieved from a part of the pressure acting upon it descends, and with it the pencil also descends; but at the same time the steam-piston has begun to ascend, and the paper to return from left to right under the pencil. While the steam-piston continues to ascend, the condensation becomes more and more perfect, and the vacuum in the cylinder, and therefore also in the indicator, being gradually increased in power, the atmospheric pressure above the indicator-piston presses it downwards and stretches the spring. The pencil meanwhile, with the paper moving under it from right to left, traces a second curve. As the former curve showed the actual pressure of the steam impelling the piston in its descent, this latter will show the pressure of the uncondensed steam raising the piston in its ascent, and a comparison of the two will exhibit the effective force on the piston. [Fig. 81.] represents such a diagram as would be [Pg277] produced by this instrument. A B C is the curve traced by the pencil during the descent of the piston, and C D E that during its ascent. A is the position of the pencil at the moment the piston commences its descent, B is its position at the middle of the stroke, and C at the termination of the stroke. On closing the upper steam-valve and closing the exhausting-valve, the indicator-piston being gradually relieved from the pressure of the steam the pencil descends, and at the same time the paper moving from left to right, the pencil traces the curve C D E, the gradual descent of this curve showing the progressive increase of the vacuum. As the atmospheric pressure constantly acts above the piston of the indicator, its position will be determined by the difference between the atmospheric pressure and the pressure of the steam below it; and therefore the difference between the heights of the pencil at corresponding points in the ascending and descending stroke, will express the difference between the pressure of the steam impelling the piston in the ascent and resisting it in the descent at these points. Thus at the middle of the stroke, the line B D will express the extent to which the spring governing the indicator-piston would be stretched by the difference between the force of steam impelling the piston at the middle of the descending stroke, and the force of steam resisting it at the middle of the ascending stroke. The force therefore measured by the line B D will be the effective force on the piston at that point; and the same may be said of every part of the diagram produced by the indicator.
The whole mechanical effect produced by the stroke of the piston being composed of the aggregate of all its varying effects throughout the stroke, the determination of its amount [Pg278] is a matter of easy calculation by the measurement of the diagram supplied by the indicator. Let the horizontal play of the pencil from A to C be divided into any proposed number of equal parts, say ten: at the middle of the stroke, B D expresses the effective force on the piston, and if this be considered to be uniform through the tenth part of the stroke, as from f to g, then the number of pounds expressed by B D multiplied by the tenth part of the stroke expressed in parts of a foot, will be the mechanical effect through that part of the stroke expressed in pounds' weight raised one foot. In like manner m n will express the effective force on the piston after three fourths of the stroke have been performed, and if this be multiplied by a tenth part of the stroke as before, the mechanical effect similarly expressed will be obtained; and the same process being applied to any successive tenth part of the stroke, and the numerical results thus obtained being added together, the whole effect of the stroke will be obtained, expressed in pounds' weight raised one foot.
(162.)
(163.)
With a view to the economy of heat, this waste steam tube is sometimes conducted into the feeding cistern, where the steam carried off by it is condensed, and heats the feeding water.
The magnitude of the safety-valve should be such that, when open, steam should be capable of passing through it as rapidly as it is generated in the boiler. The superficial magnitude, therefore, of such valves must be proportional to the evaporating power of the boiler. In low pressure boilers the steam is generally limited to five or six pounds' pressure per square inch, and consequently the load over the safety-valve in pounds would be found by multiplying the superficial magnitude of its smallest part by these numbers. In boilers in which the steam is maintained at a higher pressure, it would be inconvenient to place upon the safety-valve the necessary weight. In such cases a lever is used, the shorter arm of which presses down the valve, and the longer arm is held down by a weight capable of adjustment, so that the pressure on the valve may be regulated at discretion. Two safety-valves should be provided on all boilers, one of which should be locked up, so that the persons in care of the engine should have no power to increase the load upon it. In such case, however, it is necessary that a handle connected with the valve should project outside the box containing it, so that it may always be possible for the engineer to ascertain that the valve is not locked in its seat, a circumstance which is liable to happen.