811 



APPLIED MECHANIC'S. [ELASTICITY or OASES ASD TAP- 



li.lo in it lu- It.'), having an are* of 1 quare in.-li. 

 an.l that a load of 201ba. placed on the piston forced it 

 down to a position C D, an inch from A B, comi-reaaing 

 below it the air or steam occupying the part A B D C of 

 thecylm: hould then say that the elasticity or 



preasure of the air or steam U 20 lb. per square inch, 

 because it balance* or supports a load of 20 Ibs. placed 

 on a piston having an area of a square inch, exposed to 

 iU elastic force. If we suppose another inch added to 

 the length of the cylinder, the added space A E F 1 

 4 totally empty, and the former bottom A B suddenly 

 withdrawn, so that the air or steam had its space or 

 me d.ml.U-d, we should find 20 Ibs. on the piston 

 twice as much as it should be to retain the piston in its 

 place, and we should have to replace it by a load of 

 10 Ibs., because the elasticity of the air, or steam, would 

 have become half of what it was before the doubling of 

 its volume. We have already stated that the pressure or 

 elasticity of the air, near the surface of the earth, is 

 about 16 Ibs. on every square inch. If then a cylinder 

 like that which we have just described were filled with 

 air under the piston, aud placed in a vacuum, there 

 would be required on the piston a load of 15 Ibs. to 

 retain it in its place. Were the load less than 15 Ibs.. 

 the air under the piston would expand in volume and 

 raise it ; or were the load greater, the piston would be 

 pressed downwards, increasing the density of the air be- 

 low it, and proportionally its elasticity, until the load 

 became exactly balanced. But a cylinder of the kind 

 described, filled with air, and not placed in a vacuum, 

 requires no actual weight on the piston, because the sur- 

 rounding atmosphere affords a load exactly equivalent to 

 the weight that would be required in a vacuum. If the 

 cylinder were filled with steam instead of air, and with 

 no load on the piston, the steam would be said to be at 

 atmospheric pressure, because its elasticity tending to force 

 the piston upwards, is exactly balanced by the pressure of 

 the atmosphere tending to force it downwards. If the pis- 

 ton required a load of 16 Ibs. upon it, the steam would be 

 said to exert a preasure of two atmospheres, or of 15 Ibs. 

 above atmospheric pressure. So, if the steam sustained 

 loads of 30 Ibs. . 45 Ibs. , 60 Ibs. , <fcc. , placed on the piston, its 

 pressure would be called that of 3, 4, 6, <tc. , atmospheres ; 

 or of SOlbs., 45 Ibs., 60 Ibs., <fcc., 

 above atmospheric pressure. 



The pressure or elasticity of 

 fluids, such as air or steam, is 

 often expressed in terms of inches 

 of mercury, or of the height of 

 column of mercury which they 

 can sustain. It happens that 2 

 cubic inches of mercury weigh 

 very nearly 1 lb., and that 30 

 cubic inches weigh, consequently, 

 about 15 Ibs. Now, if we suppose a 

 tube, having 1 square inch of sec- 

 tional area, bent as in Fig. 143, 

 and closed at both ends A and D, 

 were filled with mercury to a 

 height of 30 inches in one limb 

 above the level in the other, the 

 part A B being a perfect vacuum, 

 aud the part 1 ) C filled with air, 

 since the weight of the column 

 30 inches high is 15 Ibs., this 

 pessure of 16 Ibs. is communicated 

 through the mercury in the bend 

 to the air in CD, which con- 

 sequently reacts with an elastic 

 force equivalent to 15 Ibs. on the 

 surface of the mercury exposed 

 t<> it That is to say, the 

 elasticity or pressure of the air 

 in C D U 15 Ibs., or 1 atmo- 

 sphere ; and the tube might )>u 

 opened at 1) to the ordinary 

 pressure of the atmosphere with- 

 out effecting any change in tho 

 equilibrium of tho mercurial column. The instrument ill 



Fig. us. 

 A 



this form would become the online vr, which 



measure* the pressure or density of the at m< unlit-re by 

 the height of a mercurial column sustained in a 

 every 2 inches of height of mercury corresponding to 

 1 lb. of pressure per square inch. 



If a beat tube (Fig. 144) were connected with a Teasel A 

 containing water, on heat being applied to the water, 

 steam would be generated in A, and press, by its elastic 

 force, the mercury downwards in one limb of the tube and 

 upwards in the other, until it attained such a posit i<m 

 that the excess of weight of mercury in the one limb, and 



Fi. HI. 



K L 



the pressure of the atmosphere on its upper surface C, 

 should exactly balance the elastic force of the steam in 

 A. If the height of C above B were 60 inches, the steam 

 would be said to exert a pressure of 60 inches of mercury, 

 or 30 Ibs. per square inch above atmospheric pressure, 

 or to have a total elasticity of 3 atmospheres. 



TEMPERATURE AXD PRESSURE We have 

 already said that the elasticity of steam is greater the 

 greater its temperature. There is no simple rule for cal- 

 culating the pressure due to any given temperature, as the 

 law which governs its variations is of rather an abstruse 

 character. The following table of the pressures and cor- 

 responding temperatures of steam, or the vapour of water, 

 is compiled from the results of numerous experiments 

 made with a view to establish some law on the subject. 



In a treatise like this we must abstain from the 

 discussion of this law, on account of the advanced 

 analysis required for its investigation. Nor need we 

 offer a formula for calculating the pressure corresponding 

 to a given temperature, as the table contains results 

 sufficiently accurate for all practical purposes. The 

 table only applies to the case of steam in a boiler, or 

 vessel, in contact with the water from which it is 

 generated. Were we to remove any portion of steam 

 into another vessel, and then subject it to heat without 

 water being in contact with it, we should simply expand 

 its volume, as we should air or any other gas, by an 

 increase of temperature ; or con fining its volume, elevate 

 its pressure according to a totally different law, already dis- 

 cussed in reference to the heated air engine. (See p. 841). 

 Or, were we to remove the steam to a separate vessel, 

 and there cool it, a portion would be condensed, and 

 the remainder would expand to fill the void, at a pressure 

 reduced according to Marriotte's law. 



In the first column of the table, the temperatures are 

 marked in degrees of Fahrenheit's thermometer. 



The second contains the pressures in atmospheres 

 corresponding to the temperatures. 



The third gives the pressure in inches of mercury, or 

 the heights in inches of mercurial columns capable of 

 balancing the elasticities. 



The fourth column gives the pressures in pounds 

 square inch above that of the atmosphere ; or the loads 

 in pounds required, in addition to that of tlie;itino.s|>here, 

 to keep down a piston having an area of 1 square inch 

 pressed upwards by the steam. 



