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not of sufficient distance from the centre of the beam, and must be set out accordingly.

61. How is the proper amount of velocity determined for the piston of a steam-engine?

There are various rules used by which to obtain the most beneficial velocity to produce a maximum of useful effect; but 128 times the cube root of the length of stroke, as the velocity in feet per minute, is perhaps a rule equally simple and efficient to that of any other; thus, suppose an engine with a stroke of 5 feet:

5×128=218.86 feet velocity per minute.

62. How is the effective power of an engine properly estimated?

The power of an engine may be readily computed by the following rule:-Multiply the area of the piston in square inches, by the uniform force of the steam in pounds per square inch throughout the whole stroke, and by the velocity of the piston in feet per minute; deduct from the product 2 pounds per square inch for friction, &c., divide the remainder by 33,000, and the quotient is the actual effect of the engine expressed in terms of horse-power.

Suppose an engine with a cylinder of 40 inches diameter, the piston's velocity 240 feet per minute, and steam at 12 pounds per square inch of uniform elastic force:

402 x .78541256.64 inches area, and 12 lbs. 1256.64 x 9.5 × 240 33,000

- 2.5-9.5: then

horse-power.

= 86.52

63. How is the degree of uniform elastic force of the steam in the cylinder most readily obtained?

The most efficacious and also the most simple mode is through the application of an indicator, now an instrument so well known that comment upon its construction is unnecessary; its properties are those of describing by a pencil upon paper, a figure or diagram approximating in form that of a rectangle, subject to deviation of form with the merits or demerits of the engine's productive effect. The mean breadth or height of the figure is the elastic force of the steam in terms as the scale of the indicator, the length is the amount of revolution given to the paper on which the diagram is described during the piston's performance of its stroke.

64. How is an application of the indicator effected?

The usual mode of application is by screwing the indicator into the cylinder cover, and obtaining the required movement to the paper by means of a small line attached to one of the radius-rods; but such application cannot always be effected, particularly in marine-engines; hence, other parts of the engine having a relative movement to that of the piston must of necessity be resorted to; and, if required, because of the motions of the parts of the engine selected and that of the indicator being unequal, a pulley and axle may be introduced, so as to qualify the various movements and render them suitable to each other as compulsion may demand.

65. By what means is the diagram obtained?

The indicator being fixed and the movement of the paper properly adjusted, the pencil at the same time having its point pressed upon the paper by the spring, allów the engine to make a few revolutions previous to opening the cock, by which means a horizontal line is traced upon the paper, and distinguished as the atmospheric line, because

it is a line of distinction between the force of the steam above that of atmospheric pressure, and the force of the steam rendered available by the extent or quality of the vacuum obtained by means of the condenser. Open the cock, and if the engine be upon the descending stroke, the steam in the cylinder will instantly raise the piston of the indicator, and by the motion of the paper, the pencil still pressing thereon, the top side of the diagram will be described. In the ascending stroke, and during the time that the bottom side of the diagram is being traced, the piston of the indicator, by the surrounding atmosphere, is pressed down below the atmospheric line to some point on the scale or index, approaching the indication of a pure vacuum in proportion to the density of the resisting vacuum in the cylinder, and the degree of rarefaction of the air in the engineroom, which ought to be carefully notified. The up-stroke having terminated, the diagram is completed; hence is delineated the mean elastic action of the steam above the atmospheric line, and also the mean extent of the vacuum underneath it; or perhaps, more properly, the amount of available effect of the steam obtained through the removal

of atmospheric pressure by means of the condenser.

66. In the following diagram, how are the various deviations from that of a rectangle to be explained or accounted for?

From c to f, steam of equal density is continued. At f the steam is cut off, and from ƒ to d the piston is impelled by the steam's expansive force. At d the upper eduction-valve opens. At

g

the downward stroke is completed, and at r the valve begins to open. Any rounding of the diagram at e, or starting corner, shows a defect from want of sufficient lead being given to the valve; and rounding at g shows a defect, by the eduction passages being too small.

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