THE CONDENSER GAUGE. 121 the rotatory motion of the fly-wheel shaft, into an alternate vertical motion of the valve-rod l. The advantage of an eccentric wheel is found in its smooth and unintermitting motion, producing the required changes without the perpetual recurrence of a stroke. In large engines, the pressure of the eccentric upon the shaft, is balanced by a weight. 91. Condenser Gauge. There are two circumstances which determine the effective motions of the piston: one of these is the direct pressure of the steam from the boiler; the other is the resistance of the steam in the condenser. It is important to be enabled to ascertain the relative condition of these two antagonising agents. The pressure of the steam from the boiler is denoted by the steam gauge, already described (p. 81). But a certain quantity of uncondensed vapour, arising from the hot water in the condenser, resists the action of the piston, previously to its being withdrawn by the air-pump. The force of this vapour is ascertained by the condenser gauge, an instrument which is represented in the annexed figure. It consists of a glass tube A B, upwards of thirty inches in length, and open at both ends, the upper end communicating with the condenser C, the lower end being immersed in a cistern of mercury D. The weight of the atmosphere pressing on the surface of the mercury in the cistern, forces this liquid up the tube; the length of the column thus supported in the tube indicates the difference between the pressure of the atmosphere, and that of the vapour in the condenser. On comparing this column of mercury with that of the common barometer, we are enabled to ascertain the force of the vapour in the condenser, every two inches of difference in the columns being equivalent to a force of nearly one pound on the square inch (see p. 9). Tredgold states that the condenser gauge should indicate the state of the vapour in the condenser, to be capable of sustaining from two to three inches of mercury; that, Fig. 59. while it does not exceed three inches, the condensation may. be esteemed very good, and that about two inches is the best he has seen obtained in practice. The force by which the motion of the piston is determined, is, therefore, ascertained by reference to the steam gauge, and the condenser gauge: the difference between the elastic force of the steam in the boiler, and that in the condenser, added to the height of the barometer at the time, will indicate the relative force of the steam to work the engine. From the resemblance of the condenser gauge to the common barometer, it is frequently called the barometer gauge. G 92. The Indicator.—But the force of the steam in the cylinder, and the state of exhaustion in the condenser, vary at different portions of the stroke of the engine, and these variations cannot be ascertained by means of the two gauges already described; the mercurial column would be affected by constant vibrations, corresponding with these variations, and it would be impossible to ascertain its mean height during the stroke. To obviate these difficulties, Watt employed an instrument called an indicator. It consists of a cylinder C of about an inch and three quarters in diameter, and eight inches in M length, of very uniform calibre; it terminates below in a pipe, to which a small cock D is adjusted. A solid piston is accurately fitted to the cylinder, so as to move steam-tight within, by the means of oil; the piston-rod G is about five-eighths of an inch in diameter, and sixteen inches in length; to prevent any jar or friction, the rod is made to pass through a guide, H, at a distance of about six inches from the top of the cylinder; the upper part of the piston-rod forms an index, M, which moves upon a graduated scale, K. A spiral spring, I, is attached to the piston, and to the guide H; it is about seven inches in length when at rest, and admits of being compressed an inch and a half; D I Fig. 60. THE INDICATOR. 123 its strength is such as to admit of the piston being carried down to within about an inch of the bottom of the cylinder, when loaded with fifteen pounds on the square inch. This instrument is screwed upon the steam cylinder of the engine, and a communication is thus effected between the upper part of the steam cylinder, and the lower part of the indicator cylinder. The steam on being admitted into the former cylinder, rushes into the latter, and, if its force be greater than that of the atmosphere, the indicator piston rises, and its index marks the degree of pressure on the affixed scale; as the steam piston descends, the indicator piston varies its position with the varying pressure of the steam in the steam cylinder, and these variations will be indicated on the scale. By substituting a pencil for the index, and placing a sheet of paper so as to move horizontally, these variations may be recorded on the paper in the form of curved lines. A diagram of this kind, showing the practical application of the Indicator to ascertain the effective power of a steam engine, as given by Mr. Glynn, may be seen in the appendix to Tredgold, p. 169; it is a fac-simile of that traced by the instrument. Mr. Glynn observes that the indicator shows not only the relative action of the vacuum and pressure of the steam upon the piston of an engine, but their absolute force and effect; it shows how much of that force is taken to overcome the friction of the machine, and produce the change of motion in its parts, and how much is available for useful purposes; it exhibits, if we may so say, the disposable force of the steam engine, and the perfection or imperfection of its construction or condition at the time of making the trials. RECAPITULATION. 75. By what circumstances are the proportions of a steam cylinder regulated? What relation should the length of the cylinder bear to its diameter? Explain Tredgold's plan for preventing the cooling effect of the atmosphere on cylinders? -76. What two conditions are indispensable in the construction of a piston? What is meant by a stuffing-box?— 77. Explain the construction of the hemp-packed piston. What is the amount of friction produced by the action of such a piston?-78. Describe Woolf's contrivance for tightening the packing of the piston.-79. What improvement was introduced in the construction of the piston by Cartwright? Explain the general construction of a metallic piston.—80. What is a valve? How may the valves employed in steam engines be arranged? On what does the efficiency of a valve depend ?-81. What is the construction of the common clack valve? In what parts of the steam engine is it found? In what part of the engine is the double clack valve found? At what degree of inclination should the clack valve open ?-82. How is the conical valve formed? In what part of Watt's engines are they found? What are the proper dimensions of this valve? At what angle should it fit in its seat? -83. Explain the construction of the spherical valve.-84. What is the nature of a slide? What advantage does the use of a slide possess over that of rising valves? Explain the construction and operation of Murray's slide. How are the effects of friction obviated in this form of slide ?-85. What is the objection to Murray's slide ? How is this objection obviated by Murdock's slide? Explain the construction and operation of this slide.-86. Explain generally the construction of Seaward's slides? What are the advantages of this apparatus?-87. Explain the nature of a throttle valve.-88. Explain the mechanism of the single cock.-89. What is the object of the four-passaged cock? How is it modified in expansive engines ?. 90. By what mechanism were the valves formerly opened and closed? Describe the construction and the action of the eccentric.-91. Explain the nature of the condenser gauge. -92. In what respects are the steam and condenser gauge defective? Describe the use of the indicator. MECHANICAL POWER OF STEAM. 125 CHAPTER VIII. OF THE MECHANICAL POWER OF STEAM, AND OF 93. Preliminary Remarks.—In the two foregoing chapters, the various parts of the modern steam engine have been severally examined, preparatory to the consideration of their combined action and relation to each other in the entire machine. The present chapter will be devoted to the investigation of certain laws by which the mechanical action of steam is determined or modified; to the modes of obtaining the different species of power of steam to produce useful effect, as by generation, condensation, expansion; and to the explanation of some important expressions connected with the power and duty of steam engines. These subjects will be noticed in the following order : 1. Relations existing between the 2. Temperature, and 3. Density of Steam. 2. Modes of obtaining Power from Steam. 3. Mechanical effect of Steam Engines. 4. Cornish engines. |