1839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



129 



attending it, viz., its temperature, elastic force and density, and the 

 caloric absorbed during the evaporating process. 



2. The transmission of the steam from the boiler to the cylinder, 

 and the changes of state which it undergoes during its transmission. 



,3. The pressure exerted l)y the steam against the surface of the 

 piston at every instant of its stroke. 



4. The nature and intensity of the resistance, which consists in 

 general of the useful elfect and the various resistances arising fi-om 

 tlic mechanical constniction of the engine, including the resistance 

 of (he steam during its efflux from the cylinder. 



We have already stated that M. De Pamljour, in his proposed 

 theory, entirely disregards the laws of the transmission of steam 

 through pipes, which the following quotations from his pamphlet will 

 prove. He says (page 44), " Let us, however, remark that, mathe- 

 matically speaking, the pressure P' of the steam in the cylinder can 

 never be quite equal to P, which is the pressure in the boiler ; 

 because there exist between the boiler and the cylinder conduits 

 through which the steam has to pass, and the passage of these con- 

 duits offers a certain resistance to the motion of the steam ; whence 

 results that there must exist on the side of the boiler a trifling sur- 

 plus of pressure, equivalent to the overcoming of the obstacle. But 

 as we have proved elsewhere, that with the usual dimensions of 

 engines, this difference of pressiu-e is not appreciable by I he instru- 

 ments used to measure the pressure in the boiler, the introduction of 

 it into the calculations would render the formul.-c more complicated 

 without making them more exact. For this reason we neglect that 

 difference here." 



It should be observed that the above remarks are only intended to 

 apply to the case of the minimum velocity of an engine, or its maxi- 

 mum of useful effect ; in every other case the author not only neg- 

 lects the excess of pressure necessary to force the steam through the 

 conduits, but rejects altogether the supposition that any relation 

 whatever can exist between the pressure in the boiler and that in the 

 cylinder. In page 19 of his pamphlet he uses the following 

 argument : — 



" Finally, in looking over our experiments on locomotives, it will 

 be seen that the same engine will sometimes draw a light load with 

 a very high pressure in the boiler, and sometimes a hea\'y load with 

 a very low pressure. It is then impossible to admit, as the ordinary 

 calculation supposes, that any fixed ratio whatever has existed be- 

 tween the two pressures. Moreover, the effect just cited is easy to 

 explain, for it depends simply on this, that in both cases the pressure 

 in the boiler was superior to the resistance on the piston; audit 

 needed no more, for the steam, generated at that pressure or at any 

 other, satisfying merely that condition, to pass into the cylinder and 

 assume the pressure of the resistance." 



We alluded to this opinion in our first paper, wdien we also stated 

 our acquiescence in that part which relates to the pressure in the 

 cylinder, but showed at the same time, in a general manner, that the 

 pressure in the boiler is not perfectly indifferent, the piston acting 

 in some measure as a safety valve, and thus having a greater or less 

 influence over the pressure in the boiler, according to its velocitv. 



AVith this exception, the ifluence of the above mentioned circum- 

 stances on the action of the steam engine is generally acknowledged. 

 The two first depend on the nature and properties of steam, which 

 should, therefore, be understood before we proceed any farther. 



Steam is an invisible elastic fluid, similar in its physical proper- '* 

 ties to common air, or any other permanent gas. 



It is well known that if water at 2)2 deg. fivhr. be exposed to a 

 higher temperature under a pressure of 30 inches of mercury, which 

 is about the ordinary pressure of the atmosphere at the level of the 

 sea, the caloric absorbed by the water will not raise its temperature, 

 but will convert it gradually into steam of the same temperature, 

 viz., 212 deg. The caloric thus absorbed without raising the tem- 

 perature of ihe water is called latent heat,;mi ils amount is estimated 

 at about 1000 times as much as would increase the temperature of 

 the same weight of water by one degree. The latent heat of steam 

 at 212 deg., is, therefore, said to be equal to 1000 degrees ; and from 

 experiments made by Watt and Southem, it appears, that a certain 

 quantity of water, at any given temperature, requires an addition of 

 the same quantity of caloric to convert it into steam, whatever may 

 be the temperature of the steam generated, so that the latent heat at 

 any temperature t will be 1000 + 212— «, and the sum of the sensible 

 and latent heat will be a constant quantity, viz., 1212 degrees. 



When steam is generated in alimited space, that space will shortly 

 become saturated, provided there be a sufficiency of water present ; 

 and the quantity of water which a given space can contain in the 

 form of steam varies with the temperature. When any given space 

 is thus saturated with steam at a given temperature, the steam having 

 then attained the greatest density which it can acquire at that tem- 

 perature, is called saturated steam, which term originated in the idea 



that steam which is not in that state, if brought in contact with 

 water at the same temperature as itself, will, with the assistance of 

 heat, dissolve a certain portion of the water ; which action will cease 

 as soon as the steam has attained the maximum density correspond- 

 ing to its temperature, whence it is then said to be saturated with 

 water. 



From what precedes, it follows that if a certain space were filled 

 with saturated steam at a given temperature, and the space were 

 suddenly extended without the admission of steam or water, or any 

 gain or loss of heat by radiation, the increased space would be no 

 less filled with saturated steam, but, of course, of less density, for 

 there viould be the same quantity of water in the gaseous state and 

 the same quantity of cahu-ic contained in it. The temperature of the 

 steam would, therefore, fall to the degree corresponding to its dimi- 

 nished density. 



The most important property of steam is its elastic force, being the 

 source of all the power obtained in steam engines. We have men- 

 tioned that steam is generated at a temperature of 21-2 deg. under a 

 pressure of 30 inches of merciuy : this steam, therefore, exerts a 

 pressure eqinvalent to the weight of 30 cubic inches of mercury, or 

 about 14.7 lbs. on every square inch of surface with which it is in 

 contact. 



It would be extremely inconvenient if we only knew the elastic 

 force of steam at those temperatures at which it had been determined 

 by direct experiment, nor should we be able to judge of the accuracy 

 of experiments made for that purpose without reference to some law, 

 whether founded on reasoning, and coinciding in general wth the 

 best experiments, or deduced from the experiments alone, of which 

 they serve to correct the irregularities, and so complete the series by 

 inlerpolation. It is, however, very improbable (hat a formula con- 

 structed by the latter method should be applicable far beyond the 

 limits of the experiments from which it was drawn. 



Mr. Southern proposed the following formula, which he derived 

 from the results of his own experiments : — 



0-f513f'\ 

 ••' —87344000000"+" ' 



or by logarithms, 



log. (/— •I)=:5-13 log. (<4-51-3)-10-94123, 

 in which yis the elastic force of the steam in inches of mercurj', and 

 t its temperature by Fahrenheit's thermometer. By this formula we 

 arrive at too low an clastic force, for all temperatures except 212 deg. 

 and by Tredgold's formula, 



or 



log./= 6 [log. [i -f 100) - 2 • 24797], 

 we obtain too great an elastic force for all temperatures above 212 deg. 

 and too little for those below it. 



The following, which was adopted by M.M. Arago and Dulong, in 

 their report to the Royal Academy of Sciences at Paris, agrees very 

 well with their experiments between 4 and 24 atmospheres, above 

 which their experiments were not carried ; but gives too high a result 

 from 212 to about .300 deg., and toj low from 212 deg. downwards. 

 This formula is 



/= 30 [1 X -003974 (< — 212)]g. 



Dr. Ure remarked that the elastic force of steam at 210 deg., 

 being 28.9 inches, that of steam at 220 deg. would be found by 

 multiplying the former by 1 '23 ; that the coefficient for the next 

 interval of 10 degrees would be 1'22, and so on, diminishing the 

 coefficient by "01 for every ten degrees that the temperature is 

 raised. The equation would thus be 



/=-28-9[.23x,.22x x(l-23_':=|«)] 



This formula is in every case inconvenient, and is besides obvi- 

 ously inapplicable at high temperatures, for it gives the same elastic 

 force at 440 deg. as at 450, and would show that above the latter 

 temperature the elastic force diminishes with every rise of tempera- 

 ture, which is absurd. 



From a comparison of Dr. Ure's experiments Mr. Ivory derived 

 the following equation : — 



log. JL = -0087466 t- 

 ° 30 



- .3 



-•000013178 «--f -000000024825 t 



This is also rather difficult to apply, and becomes very erroneous 

 at high temperatures. 



