with its bottom end sufficiently immersed in a cup of mercury, the top end communicating with the condenser by means of an iron pipe. When the engine is at work, the perfection of vacuum is indicated with the rise of mercury in the tube, by the atmospheric pressure on the surface of the mercury in the cup; thirty inches in height indicating a perfect vacuum, but more than twentyeight inches is seldom in practice attained.

37. What is meant by a vacuum?

By vacuum is meant a void or empty space; hence, in a condensing engine with a perfect vacuum, steam at fifteen pounds per square inch is of equal effect to that of steam at thirty pounds per square inch in one of the non-condensing principle.

38. What is meant by condensation ?

Condensation of steam is the causing of a closer approach of its particles through the abstraction of heat. The steam being admitted into a close vessel called a "condenser," by means of a communication with the cylinder or otherwise, its superior degree of density expels through a valve

the atmospheric air which was therein retained; a jet of cold water is then opened to rush in and mix with the steam, by which its aeriform properties are destroyed, and a vacuum of more or less perfection obtained.

39. Is there any proportionate quantity of water required for the condensation of steam at a given temperature?

Yes; the proportionate quantity is as estimated by the following rule:-From 1212°, or commonly reckoned amount of the latent and sensible, or indicated temperature of steam, subtract the temperature of the condensed water, divide the remainder by the temperature of the condensed water minus the temperature of the condensing water, and. the quotient is the number of times. that the quantity for condensation must exceed that from which the steam is formed.

40. What will be the relative quantities of water between that of formation and condensation of steam, the temperature of the steam being 222.6° Fah., or three pounds above the pressure of the atmosphere, the temperature of the condensing

water being at 52°, and the required temperature of the water after condensation to be 100°?

1212°. -100°

100° 52°

23.17 times the quantity of

water for condensation to that from which the steam is formed; and as per table of the volume of steam generated under different pressures-one cubic foot of steam generated at a temperature of 222.6° Fah., requires in formation 1.226 cubic inches of water; hence 23.17 x 1.226 28.4 cubic inches of water at 52° that will be required to condense one cubic foot of steam of the given temperature to water at 100°.

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41. If steam of high density be employed in a steam-engine, is effective condensation rendered more difficult of being obtained?

No; because, when steam of high density is employed, only a small portion of the steam is allowed to enter the cylinder, the movement of the piston being continued by expansive force; hence, the power of the engine is augmented, and the steam advantageously lessened in density by expansion, or increased bulk.

42. What is the difference, when applied to the steam-engine, between the terms duty and power?

Duty implies a certain amount of work done by a certain amount of fuel without reference to time; but the power is the number of times 33,000 pounds the engine is capable of raising one foot high per minute.

43. What is the amount of advantage in working a steam-engine expansively?

The advantage of working steam expansively is the economizing of fuel, the amount of advantage being as the quantity of steam saved, minus the diminution of mechanical energy by expansion.

44. How is the amount of mechanical effect of the steam to be ascertained?

Divide the length of the piston's stroke by the distance into the cylinder the dense steam has been admitted previous to its being cut off, and the hyperbolic logarithm of the quotient is the increased efficacy due to expansion, when the force of the dense steam is 1.

Thus, suppose the stroke of an engine equal

4 feet, the dense steam being admitted through 1 feet: 4.51.5 = 3, the hyperbolic logarithm of which is 1.0986, or efficiency due to expansion, and 1.098612.0986, the ratio of elastic force.

Again, suppose the pressure of the dense steam equal 27 pounds per square inch: 4.5÷1.5=3, and 2739, logarithm of 3+1=2.0986 × 9 18.887 pounds per square inch, ratio of elastic force, and 18.887 x 4.5 84.99 units of mechanical effect produced throughout the length of the stroke.

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45. How is the steam's elastic force ascertained at any portion of the stroke after expansion?

Multiply the force of the dense steam in pounds per square inch, by the numerator of the fraction expressing that portion of the stroke passed through when the steam is cut off; divide the product by the numerator of the fraction expressing the whole amount of stroke passed through by the piston where the estimated force of the steam is required after expansion, and the quotient is the force per square inch in equal terms of unity.