To what are the temperature and the elastic force of steam equal? What is the difference in the pressure of steam when the mercury is in a vacuum, or when exposed to the atmosphere? If the proper relation of the temperature between the steam and the water from which it is formed be disturbed, what will be the effect? What is the total heat of steam at 212° Fah.? How is the latent heat of steam found? When does heat in steam become latent? How would you ascertain what amount of water is necessary to condense a given quantity of steam? What is low-pressure steam? Why is the steam of salt water fresh? What is the most extraordinary property of steam? What are the two modes of applying the power of steam to the cylinders of steam-engines? State the rule for finding the mean or average pressure in a cylinder. Is the effluent velocity with which steam of different pressures flows into the atmosphere uniform? State the difference between the latent and sensible heat of steam at different pressures. State the total heat and relative volume of steam at different pressures. As the sensible heat in steam increases, does the latent heat decrease? In what way does the change affect the economy of the steamengine? What is meant by the volume of steam? What is the difference in volume between water and steam at atmospheric pressure? How much does one cubic foot of steam at atmospheric pressure weigh? State the velocity with which steam at different pressures flows into the atmosphere or into steam of a lower pressure. If steam, at a given pressure, be cut off in the cylinder at a certain point of the stroke, what will be the pressure for the whole length of the stroke? Give the rule for finding the amount of benefit to be derived from working steam expansively. Give the rule for finding the average pressure of the steam in the cylinder for different points of cut-off. Is surcharged steam indicated by the steam-gauge? Or will it affect the vacuum? What is superheated steam? What is the steam-jacket? What is the difference in effect between superheated and saturated steam? Which is capable of producing the most economical results? 9* PART SECOND. Steam-Engines in General. Steam-engines embrace a great variety of designs and names; such as the beam, side-lever, inclined, oscillating, trunk, horizontal, vertical, and steeple, which are in turn termed single-acting, doubleacting, reciprocating, rotary, semi-rotary, compound duplex, inverted, and geared, each of which was probably designed to meet some peculiar requirement- either economy of space, fuel, or ef ficiency in speed. (Judging from the appearance of things at present, the horizontal and vertical are destined to supersede all other designs for land and marine purposes.) All steam-engines, of whatever design, or for whatever purpose employed, are embraced under two heads, commonly called high- and low-pressure, but more properly termed condensing and non-condensing. In the non-condensing engine, the steam, after acting on the piston, escapes into the open air; therefore the pressure of the outgoing steam must exceed atmospheric pressure, or 14.7 lbs. to the square inch. Thus, if steam at 45 lbs. average pressure above vacuum be admitted to the piston of a high-pressure engine, it will exert a force equal to its pressure; but 14·7 lbs. per square inch of that pressure will not be converted into work, as it will be lost in overcoming the pressure of the atmosphere, which may be illustrated by the following example: Diameter of cylinder, 12 in.; area, 113.09 in. As before, area, 113.09 sq. in. Atmospheric pressure, 147 lbs. Total atmospheric pressure, 1662-423 lbs. 5089-050 Loss due to atmospheric pressure, 1662-423 Effective steam pressure on piston, 3426-627 lbs. The foregoing example shows the resistance to be overcome at each stroke of the piston before the steam acting against it can produce any useful effect. Thus it will be seen that the piston of a high-pressure steam-engine is exposed to the action of two pressures, namely, the pressure of the steam from the boiler on one side, and that due to the atmosphere and the steam remaining in the cylinder after exhaust takes place on the other. The pressure utilized or converted into work will be the difference between the two. In the low-pressure or condensing engine, the steam, after acting against the piston, escapes into a condenser, where it is condensed into water and a vacuum is formed; thus rendering not only a considerable portion of the steam pressure in the boiler, but also the 147 lbs. per square inch required in the non-condensing engine to overcome the pressure of the air, available as an effective force against the piston, which may be explained as follows: Diameter of cylinder, 12 in.; area, 113·09 sq. in. Average steam pressure per square inch, 45 lbs. Total effective steam pressure, 5089-05 lbs. As before, area, 113.09 sq. in. Power due to vacuum, 1470-17 lbs. 3958.15 Total effective pressure due to steam and vacuum, 5428-32 lbs. The back pressure in the condenser, which represents the difference between the indications of the vacuum-gauge and a perfect vacuum, must be deducted; but, as a perfect vacuum is not attainable, the back pressure varies from 2 to 5 lbs., according to the condition of the engine and the quantity of uncondensed steam remaining in the condenser. Waste in the high-pressure engine. In the best types of modern high-pressure engines, the useful effect obtained from the work stored up in good fuel may be calculated as follows: Loss through bad firing and incomplete combustion, 10 per cent. Carried off by draught through chimney, 30 66 66 The foregoing may seem incredible, and yet any one wishing to do so may demonstrate its truthfulness to his own satisfaction by placing a thermometer in the steam-pipe and noting its temperature during its escape from the boiler to the cylinder; then placing it in the exhaust-pipe, close to the engine, and noting the temperature at this point, when it will be discovered that the steam has lost very little of its heat in passing through the cylinder. Consequently the difference in temperature between the steam when it escapes from the boiler and from the exhaust-pipe, constitutes all of the heat that was contained in the fuel that was utilized. Waste in the low-pressure or condensing engine. According to the dynamic theory of heat, as shown on page 105, a certain weight of coal contains within itself a certain amount of work stored up, and ready to rush out under the necessary surroundings, as in the case of a compressed spring set free. The supply of a given weight of coal to the furnace of a steam-boiler represents the application of a definite amount of force at one end of a series of transformations, a part of which force at length appears as useful work at the other, the balance having been wasted in the various processes through which it has passed. Take, for example, a modern marine-engine of the best construction and design. The force supplied to the furnace in the combustible is first developed as heat by the burning of the coal; a portion of this heat is utilized in changing the water into steam, the balance being wasted either in radiation, or by being carried off in the hot gases through the chimney. A part of the steam formed is applied to move the piston, the remainder being wasted by condensation against the sides of the pipes and cylin |