Let steam be admitted at 25 pounds per square inch, and cut off when the piston has performed ths of its stroke; to find the amount of elastic force that the steam will exert at ths of the stroke: 25 × 3 = - 3.125 lbs. per square inch. Or, if steam at 25 pounds per square inch be cut off at half the stroke; to find the amount of elastic force exerted at three-fourths: 25 x .5 = 16.66 lbs. per square inch. 46. Suppose steam at a given density is to be cut off at a certain point, so as to maintain a required degree of elastic force at the termination of the stroke, how is that point to be determined? Multiply the required elastic force of the steam in pounds per square inch, at the termination of the stroke, by the length of the stroke in inches, and divide the product by the given force of the dense steam in pounds; the quotient is the distance from the commencement of the stroke at which the steam must be cut off. Thus, suppose steam at 20 pounds per square inch, length of stroke 24 inches, and, at the termination of the stroke, the steam to have an elastic force of 7 pounds per square inch : 7 x 24 20 the stroke. = 8.4 inches from the commencement of Again, suppose the above application to be in that of a condensing-engine; it might be required to determine how far below the line of atmospheric pressure the steam pressure would indicate, the dense steam being at 20 pounds per square inch, or 5 pounds above the pressure of the atmosphere. 47. By what means is the steam cut off, whereby to effect any required amount of expansion in the cylinder of a steam- engine? By the lap on the steam-slide or slide-valve, the steam may be cut off at half, and by an expansion-valve, at any point of the stroke required. 48. What is meant by lap on the steam-slide or slide-valve? By the lap of the slide-valve is meant a certain additional breadth of its face on the steam-side, more than sufficient to cover the aperture in the cylinder when the valve is at half-stroke. 49. When the point of stroke is given at which the steam is required to be cut off by the slide-valve, how is the necessary additional extent of valve-face to be ascertained? Divide by the length of the piston's stroke in inches the distance the piston has to travel when the steam is cut off; multiply the square root of the quotient by half the travel of the valve, also in inches, and from the product subtract half the valve's lead; the remainder is the lap or cover required. Thus, suppose an engine with a stroke of 36 inches, the steam being required to be cut off when the piston has passed through 23.4 inches, the travel of the valve being 5 inches, and the lead one-fourth of an inch. = 36 23.4 12.6 inches the piston has to travel. valve 12.6 36 = = .5916 × 2.5, or half the travel of the 1.479.125 or half the lead= 1.354 inches, the lap or cover required. 50. Suppose the lap, lead, travel of the valve, and stroke of the piston to be given, and also the required point at which the steam is to be cut off; how then? Reverse the former rule; thus, to the lap of the valve on the steam side in inches, add half the lead, divide the sum by half the travel of the valve, multiply the square of the quotient by the piston's stroke in inches, and the length of the stroke minus the last product equal the distance the piston will have travelled when the steam is cut off. Suppose, as before, that the lap of the valve equal 1.354 inches, half the lead ⚫125 inches, and stroke of piston 36 inches: 1.354 + .125 12.6 = 2.5 = = 1.4792 × 36 = 12.6 and 36 23.4 inches the piston will have travelled when the steam is cut off. 51. What is meant by the lead of the valve? The lead is what the steam aperture is open, for admission of the steam when the piston is at the top or bottom of the cylinder, and about to commence the returning stroke. 52. What is the travel of the valve? The travel of the valve is an amount of movement caused by the eccentric through the intervention of levers or otherwise; and in total extent equal the width of the two steam openings, plus the lap of the valve over each opening. But in direct-action engines, standing vertically, the lead is necessarily greatest for the up stroke, in order to counteract, in some measure, accelerated motion, which the weight of the piston, in falling, tends to obtain. 53. And what is an eccentric? An eccentric is a circular pulley whose centre of revolution is more or less distant from its centre of formation; and by means of a surrounding hoop, in which it revolves, an alternate reciprocating movement is obtained to the valve through |