384 



STEAM. 



pressures, but the discrepancies, even at working 

 pressures, of the different tables of the elastic force 

 of steani, made it important to push those trials as 

 far as could be done without material changes. 

 They succeeded, without much difficulty, in reaching 

 ten atmospheres, which is but one atmosphere less 

 than the reputed working pressure of our high-pres- 

 sure engines, and as the experiments on the safety 

 valves have rendered probable, is very near the true 

 working pressure. 



Table, of the Elastic Force of Steam, from One to 

 Ten Atmospheres. 



The results of the committee as to pressure cor- 

 responding to temperature, all fall below those of 

 professor Robison, the extremes being 14 and -40 

 of an atmosphere ; they approach nearer to those of 

 Dr Ure, differing in the extremes -06 and + -12 

 of an atmosphere. They agree e ven pretty nearly with 

 the experiments of Mr Taylor, tending, generally, to 

 gain upon them; thus at 260 the difference is -01 

 of an atmosphere, and at 320 is -42. The tem- 

 perature corresponding to six atmospheres, in the 

 table of the committee, is 315, to the same 

 (5-98) in that of Mr Taylor, 320, and to the same 

 in that of the French commission, 320-4, the latter 

 two agreeing very closely. 



From these comparisons it appears, that for given 

 temperatures the pressures determined by the com- 

 mittee are lower than those found by professor 

 Robison, between 1 and 3 atmospheres; lower 

 than those of Dr Ure, from 1 to 5 j atmospheres, 

 except at the highest pressure, differing, however, 

 but little from them ; nearly the same from 1 to 2| 

 atmospheres with those of Mr Taylor, and higher 

 from 2| to 6 atmospheres; higher than those of 

 Mr Southern; much higher than those of profes- 

 sor Arzberger; higher than those of the French 

 commission. 



In a work like the present, it would be out of 

 place to investigate the formulae used for the cal- 

 culations where experiments were rendered im- 

 practicable; but we will give a very convenient 

 rule, which is near enough the truth for all com- 

 mon purposes. 



Add 100 to the temperature expressed in degrees 

 of Fahrenheit, divide the sum by H7, and raise 

 the quotient to the sixth power, the result will be 

 the elastic force of the steam in inches of mercury, 

 which, being divided by 2, will give the pressure 

 upon the square inch. Thus if the temperature be 

 307 we have 307 + 100 = 407 -f- 177 = 2-3, 

 which raised to the sixth power is 2-3 x 2-3 x 

 2-3 x 2-3 x 2-3 x 2-3 x 2-3 = 148-035 inches 

 of mercury, to which the pressure of the steam is 

 equal, or 148-035 -j- 2 = 74 pounds upon the 

 square inch nearly. 



When water is converted into steam, the vapour, 

 as before observed, will occupy a much greater space 

 than the water from which it was formed. It was 

 found by Watt that a cubic inch of water, when 

 converted into steam at a temperature of 212, will 

 occupy the space of a cubic foot, or rather more 

 than 1700 cubic inches, this steam having an elastic 



force of one atmosphere, or being capable of excit- 

 ing a pressure of 15 Ibs. upon the square inch. If 

 the pressure of the atmosphere be diminished or 

 taken away, the steam formed from the same quan- 

 tity of water will occupy a much greater space than 

 one cubic foot, but its pressure will be diminished 

 in a proportionate degree. On the other hand, 

 when the water is made to boil under a greater 

 pressure than that of the atmosphere, the steam 

 formed will occupy a less space than one cubic foot, 

 but will be possessed of a proportionately greater 

 elastic force. Thus suppose the pressure on the 

 surface of the boiling water to be two atmospheres, 

 or 30 Ibs. upon the square inch, then will the 

 steam formed have a pressure of two atmospheres, 

 but it will only occupy half a cubic foot. If it be 

 allowed more free space, it will expand and fill that 

 space, but as it expands its pressure will be lessened. 

 From what has just been said, the reader will 

 readily conclude that as the steam from a cubic 

 inch of water may be made to occupy a larger or 

 smaller space, and to have a greater or lesser elastic 

 force accordingly, that the density or specific gravity 

 of the steam will increase with its elastic force. This 

 law will be seen exemplified in the following table : 



At the commencement of this article the attention 

 of the reader was drawn to the quantity of latent 

 heat contained in steam, and the results of different 

 experiments were given. We stated that the num- 

 ber 1000 was the most convenient and could not 

 be far from the truth, but this is only the latent 

 heat of steam at 212. It was not one of the least 

 important of the discoveries of Watt, that, what- 

 ever be the temperature of steam, the 'amount of 

 its latent and sensible heat will be the same. 

 From this law, it is easy to find the latent heat of 

 steam at any temperature ; for since 1000 added to 

 212 gives a sum of 1212, we have only to subtract 

 the sensible heat from this constant number to ob- 

 tain the latent heat. Thus if the temperature of 

 the steam be 307, its latent heat will be 1212 307 

 = 905 ; and if the temperature be 400, the latent 

 heat, in the same way, will be found to be 1212 

 400 = 812 ; hence the latent heat decreases while 

 the sensible heat increases. 



It is a singular fact that though low pressure 

 steam will scald most dreadfully, yet high pressure 

 steam will not. Dr Thomson accounts for this as 

 follows : 



" When the steam of boiling water comes in 

 contact with any part of the living body, it occa- 

 sions a most severe scald, but when steam of a 

 higher temperature than boiling water, or high 

 pressure steam, as it is called, issues into the atmos- 

 phere, the finger, or any part of the body, may be 

 passed through it with impunity. It has not the 

 property of scalding. And if a thermometer be 

 placed into it, we find the temperature greatly be- 



