THE MERCURIAL BAROMETER. 181 at the present time for the construction of the Glasgow Central Railway), or the caissons of a great bridge while they are being sunk (as in the case of the Forth Bridge), or go down into the sea in a diving-dress or diving-bell, then we do feel a most uncomfortable sensation in our ears, eyes, &c. Or, if we climb a very high mountain, or rise far into the air in a balloon, we have a somewhat similar sensation, but due to an opposite effect-viz., a diminution from the normal pressure to which we are accustomed. The Mercurial Barometer.-The pressure of the atmosphere is usually measured by a mercurial barometer, which consists of a vertical tube of glass about 33 inches long, of uniform calibre, hermetically sealed at the top end, into which has been carefully introduced mercury freed from air. The lower end dips into an open dish containing a quantity of that liquid metal. Consequently the pressure of the atmosphere acting on the mercury in the open dish forces it up inside the tube to a height directly proportional to its pressure, since there is supposed to be a perfect vacuum between the upper surface of the mercury and the closed end of the glass tube. EXAMPLE VI.-Suppose the height of mercury as registered by a mercurial barometer is 30 inches, and that the specific gravity of mercury be taken as 13.6, what would be the height in feet of a water column which would support the same atmospheric pressure? ANSWER. I: 136: 30 inches: x .. x = 30 × 13.6 = 408" = 34 feet. Low Pressure and Vacuum Water Gauges.*-It is often necessary for the engineer to measure low pressures or vacuums of gases. For example, in the supply of illuminating gas to a town, or in the pressure of air feeding a boiler furnace by natural or forced draught, or the vacuum produced by a chimneystalk; or, in the case of the vacuum in a coal mine produced by a furnace below the earth, or by a guibal fan situated near the upcast shaft, &c. In such cases, as well as in many others where low pressures have to be observed, the force is not reckoned by pounds per square inch, or by inches of mercury sustained in a vertical column, but by the number of inches of water which the pressure will support or which the vacuum will detract from the atmospheric pressure. The accompanying figure illustrates the apparatus usually employed in determining such low pressures. It consists of a *For a description of mercurial pressure and vacuum gauges, as well as Bourdon's high-pressure and vacuum gauges, refer to the Author's Elementary and Advanced Books on "Steam and Steam Engines." simple bent U glass tube with a scale between the vertical legs of the U, divided into inches and tenths of an inch, so that either the pressure or the vacuum may be read off in inches of water pressure, according as the forward pressure from the point of supply is positive or negative in respect to the pressure of the W GT -W GAS PRESSURE GAUGE. If atmosphere. For example, let the leg of the U tube next the cock be connected to the gas pipe of a house, then the pressure of the gas supply acts on the water in the right-hand leg of the tube, and forces it downwards, whilst the water in the other leg rises correspondingly. The reading observed on the scale S, below or above the zero or equilibrium line, has of course to be doubled in order to ascertain the exact total pressure in inches of water. the U tube be connected to a vacuum or negative pressure, then the water rises in the inner leg of the U tube, owing to the greater pressure of the atmosphere on the outer limb, and the inches of water representing the amount of the vacuum are accordingly read off in the same way. For example, if the apparatus be connected to the base of a steam boiler chimney, or to the inlet of a guibal fan creating a draught in a coal mine, then the suction produced forms a vacuum which requires the supply of atmospheric air, and consequently the air presses on the open water of the outer limb of the U tube, and forces it downwards. The vacuum is therefore observed and recorded by adding the inches of water below and above the zero line. INDEX TO PARTS. GT S W EXAMPLE VII.-A difference of level is observed of 4 inches between the outer and inner limbs of a U tube water-gauge. What is the pressure of the gas supply in lbs. per square inch? ANSWER. A vertical column of 34 feet of water corresponds to 15 lbs. pressure on the square inch. Consequently, The Siphon is simply a bent tube for withdrawing liquids. from a higher to a lower level by aid of the atmospheric pressure. It is used in chemical laboratories and works for emptying acids from carboys, in breweries and distilleries for extracting beer from vats and spirits from casks, in the crinal glass tube of Sir Wm. Thomson's recorder for conveying ink from the ink-pot to the telegraph message-paper; and on a large scale for draining low-lying districts, such as the fens of Lincolnshire. The conditions for the successful working of a siphon are, that- 1. The liquid shall be carried by the outer limb of the tube to THE SIPHON. a lower level than the surface of the supply. 2. The vertical height from the free surface of the liquid being drained to the top of the bend of the siphon shall not be greater than the height of the water barometer at the time-say only 30 feet -on account of the necessary deduction of 3 or 4 feet to be made from the full height of 34 feet, due to having to overcome the friction of the pipe. 3. The end of the siphon dipping into the liquid to be drained, shall not become uncovered. To start the siphon, either the tube must be filled with liquid, the ends closed, and the siphon inverted, with the shorter limb under the fluid to be drained, before uncovering the ends; or, whilst the shorter limb is in position a vacuum must be formed in the siphon tube by extracting the air from the end of the longer leg. The principle upon which the siphon acts is as follows: A vacuum having been formed in the tube, the pressure of the atmosphere acting on the free surface of the liquid to be drained, forces it up the shorter limb, and having turned the highest point of the it naturally descends the longer limb by the action of gravity with a velocity proportionate to the difference of level between the outlet and the free surface of the source of supply. The outflowing liquid is always acting as a water-tight piston at the bend of the П, and in this way keeping up the vacuum there, until either the inlet and the outlet free surfaces come to a level (when the siphon stops for want of "head"), or, when the difference of level between the free surface of the supply and the top of the bend exceeds the height supportable by the atmosphere, when it stops for want of breath or atmospheric pressure. LECTURE XVIII.-QUESTIONS. 1. What are the respective specific gravities and the weights per cubic foot and per gallon of fresh and of salt water? 2. A cylindrical vessel, 120 inches long and 10 inches in diameter, is sunk vertically in water, so that the base, which is horizontal, is at a depth of 100 inches below the surface of the water. Find the upward pressure in pounds on the base of the vessel. Ans. 284.2 lbs. 3. A cubical box or tank with a closed lid, the length of a side of which is 5 feet, rests with its base horizontal, and an open vertical pipe enters one of its sides by an elbow. The tank is full of fresh water, and the pipe contains water to the height of 10 feet above the top of the tank. What are the pressures of water on the top, bottom, and sides of the tank? Ans. 15,625 lbs. ; 23,437.5 lbs.; 19,531 25 lbs. 4. A water tank 10' long, 10' wide, with an inclined base 10' deep at one end and 5' at the other end, is filled with fresh water. Find the pressure in pounds on each of the four sides and on the base. Ans. 31,250 lbs.; 7,812.5 lbs.; 18, 229 16 lbs.; 52,500 lbs. 5. A lock gate is 12 feet wide, and the water rises to a height of 8 feet from the bottom of the gate. What pressure in pounds does it sustain? The weight of a cubic foot of water is 624 lbs. Ans. 24,000 lbs. 6. A vertical rectangular sluice gate, measuring 2 feet horizontal by 3 feet vertical, is immersed so that its upper side is 4 feet below the surface of the water pressing on it. Find the pressure on the gate: you are required to explain the reasoning on which your calculation is founded. (S. and A. Exam. 1891.) Ans. 2062.5 lbs. 7. What is meant by the "centre of pressure" in the case of a plane surface immersed in a liquid? If the plane be a horizontal circle, where does the centre of pressure act? If it be a vertical rectangle 10 feet wide and 6 feet deep, immersed in water so that the upper edge of the rectangle is flush with the surface of the water, where does the "centre of pressure' act? Ans. at the centre of the circle; 4 feet below surface of water. 8. State the law discovered by Archimedes, and the conditions for a body in equilibrium floating in a liquid. A cylinder 10 feet long and 2 feet in diameter floats in fresh water, with 2 feet projecting from the surface; find the weight of the cylinder. Ans. 1,571.42 lbs. 9. A rectangular tank, 5 feet square, is filled with water to a height of 7 feet. A rectangular block of wood, weighing 312·5 lbs., and having a sectional area of 5 square feet, is placed in the tank, and floats with its sides vertical and with its section horizontal. How much does the water rise in the tank, and what is now the pressure on one vertical side of the tank? Ans. 3 inches; 10,000 lbs. 10. The mercurial barometer registers 31"; calculate the height of columns of fresh and of salt water that will balance the corresponding pressure. II. Sketch and describe a mercurial barometer. State how it is made, and how it acts as a register of the pressure of the atmosphere. 12. Describe some simple form of gauge which would enable you to measure the pressure at which gas is supplied, and explain the principle on which it is constructed. 13. Sketch and explain the action of the siphon, and give a few practical examples of its use. Also state under what circumstances it fails to work. ( 185 ) LECTURE XIX. CONTENTS.-Hydraulic Machines-The Common Suction Pump-Example I. The Plunger, or Single-acting Force Pump-Example II.-Force Pump with Air Vessel-Continuous-delivery Single-acting Force Pump without Air Vessel-Combined Plunger and Bucket PumpDouble-acting Force Pump-Example III.-Questions. Hydraulic Machines.-The Common Suction Pump consists of a bored cast-iron barrel PB, terminating in a suction pipe, SP, fitted with a perforated end or rose R, which dips into the well from which the water is to be drawn. The object of the rose is to prevent leaves or other matter getting into the pump, that might clog and spoil the action of the valves. At the junction between the barrel and suction pipe there is fitted a suction valve SV, of the hinged clack type faced with leather. The piston or bucket B is worked up and down in the barrel of the pump by the force P, applied to the end of the handle H, being communicated to it through the connecting link of the hinged pistonrod PR. In the centre and at the top of the bucket is fixed the clack delivery valve DV, which is also faced with leather in order to make it water-tight. The bucket is sometimes packed with leather; but, as shown by the figure, a coil of tightly woven flax rope wrapped round the packing groove would be more suitable in the present instance. Action of the Suction Pump.—(1) Let the barrel and the suction pipe be filled with air down to the water-line, and let the bucket be at the end of the down stroke. Now raise the bucket to the end of the up-stroke by depressing the pump handle. This creates a vacuum below DV; therefore the air which filled the suction pipe only, expands, opens SV, and fills the additional volume of the barrel. Consequently, according to Boyle's law, its pressure must be diminished in the inverse ratio to the enlargement of its volume. This enables the pressure of the atmosphere * The student may refer to Lecture XII. of the Author's Elementary Manual on "Steam and the Steam Engine," for an explanation and demonstration of Boyle's law; where it is shown that if p=the pressure of a gas and vits volume, then at a uniform temperature pva constant, or p varies as |