216 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS, temperature of the scale of the thermometer, with a view to show, that it was not allowed to vary too considerably; the observed height of the mercury in the gauge, reduced to its mean height; the temperature of the air in the gauge; its volume at the observed temperature; the volume reduced to 48°, the temperature of graduation of the gauge at which the column of mercury, equivalent to an atmosphere, is very nearly 30 inches; the elasticity of the compressed air, in inches of mercury; the correction in the height of the column of mercury, for the depression pro duced in the cistern below; the height thus corrected; the height after subtracting the sensibly constant number for the column of water between the level of the steam-pipe from the boiler and the cistern of the gauge; the total elasticity in inches of mercury; the elasticity in atmospheres. The first number in the table is merely introduced for the convenience of presenting certain data required for subsequent calculation, it gives the height of the mercury in the gauge bêfore beginning the observations, after correeting for the height of the barometer. TABLE, No. I.-Of the Elastic Force of Steam at different Temperatures. t A curve traced to represent these observations, the ordinates representing the pres sures, and the abseissæ the temperatures, is quite regular, until the temperature corresponding to eight atmospheres is attained, when it rises abruptly. This fact was explained by examining the gauge; it was found that the cement used in attaching the glass tube to its ferule had become softened, and had permitted the tube to rise. This defect was remediet and its recurrence prevented. It was then determined to repeat the entire series of observations, and to carry them as high as could be done, with reasonable convenience, aiming particularly to embrace the range of working pressures of the American engines. The results are contained in the following table, in which the observed data, and calcu lated numbers, arranged as in the last table. This table extends to 9.91 atmospheres, and to the temperature of 352° Fah. Care was taken that the elasticities were increased not too rapidly, and the last numbers obtained, were verified by keeping the temperature sensibly constant for a considerable time. There is one observation, namely, that at 3293, which is certainly recorded erroneously; but omitting this one, the rest which are given, present a very tolerable regularity in the curve traced to represent them. This observation shows the height of the gange before the experiment, corrected for the height of the barometer. 1 atmospheres of 30 inches: REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. TABLE, No. III.-Of the Elastic Force of Steam at different Temperatures. 23 23.29 22.00 248.92 8.50 ⚫23 23 47 22 18 267.62 8.92 23 23 57 22.28 278.33 9.28 290 35 9.68 9.91 This observation shows the corrected height of the gange before the experiments. 217 218 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. ✨✨ A curve which would be traced by the following table, which may be considered to represent the mean of the foregoing, woulddiffer little more than one-tenth of an atmosphere in any part of the range, from the ob servations, omitting one noticed in the first, and another noticed in the second table; the pressures in general differing less than onetenth of an atmosphere from the observed pressures. Table of the Elastic Force of Steam, from One to Ten Almospheres. To compare our results with those given by the Committee of the French Academy, we have traced, on Plate 5, a curve, from the above table, and another from those of the thirty observations, selected by the Committee of the Academy, from their experi ments, below ten atmospheres. The curve of our observations, passes at low pressures nearer to the line AB than that of the French experiments, and after coinciding at the medium pressures of the table, crosses the latter, different at ten atmospheres 5 degrees, or at 352 degrees .65 of an atmosphere. The curve of our observations is traced in a full line that of the French Academy is dotted. The difference here noticed is too considerable to be admitted, as within the limits of errors in the apparatus or in observation. Having an authority of so much weight against them, the Committee have been driven to examine their results very closely. The care employed in the graduation of the gauge seems to exclude the idea of error from it; the upper portion of the scale was di vided to 05 of an inch, and could easily be read to half of that distance, making about 1 of an atmosphere at the highest pressure attained. A specific correction for capillarity was ascertained and employed. In one point of manipulation, namely. the method employed to dry the air, the Committee differed from what was usual, and though they think there is reason to confide in that method, they have examined what effect would be produced if the air were saturated with mois ture. Recent experiments, on the passage of gases, out and into vessels placed over mer. cury, and observations connected with them, warrant, moreover, a suspicion that dry air standing in a glass vessel over mereury, the surface of which is covered by water, may become impregnated with vapour. The ef fect of such a source of error they have cal culated* in the highest and lowest results of table No. II., and find it to be as follows: For 2484 the tension of the vapour is 1.96 instead of 197, and 991. - I Differing from the numbers given in table No. II. by 01 and 13 of an atmosphere. This supposition is thus shown to be inadequate to explain the discordance, and must, in fact, be deemed, to a certain extent, gratuitous. The Committee have next compared the results furnished by the safety-valves graduated independently of the gauge, and these, as has already been shown, gave calculated pressures 4 per cent. and 10 per cent. higher than the pressures indicated by the gauge. From these independent experimental data we have, then, an evidence that our results are, probably, not too high. The question of the elastic force of steam has been examined by many experimenters, and with very various results. The Com mittee propose to show the state of knowledge on the subject by comparing the prin cipal series of experiments referring to temperatures above 212o, with their own, which are now under examination. In the first table, below, they have compared their results with those of Robison,* of Ure,t and of Taylor. The first two experimenters named used an open mercury-gauge in their experiments, and the thermometers were exposed to the pressure of the steam. This latter circumstance would tend to render the observed temperature slightly too high or the observed pressure, relatively to the temperature, too low, as far as it produced any effect. 2120 240 250 260 270 280 290 3 89 4.00 3.82 +·07 300 4.60 4.66 4.46 +14 The experiments of Watt are not referred to, as he states himself that he has doubts of their accuracy, and defers to the results of Mr. Southern, which will be given presently, The results of the Committee as to pressure corresponding 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 even more nearly with the experiments of Mr. Taylor, tending, generally, to gain upon them; thus at 2600 the difference is 01 of an atmosphere, and at 3200 is 42. The temperature 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, 3204, the latter two agreeing very closely. In the following table are given a comparison of the experiments of the Committee, with those of Mr. Southern,§ Professor Arzberger, of Vienna, and the Commission of the Academy of Paris. The pressures were obtained in the experiments of Mr. Southern by a piston-valve, which is stated to have been checked, in part. by a mercury gauge; in the experiments of Professor Arzberger by a spherical valve of steel; and in those of the French Commission by a closed gauge, containing air. The numbers for these lastnamed results are those deduced from the empyrical formula adopted as representing, most closely, the experiments. •Robison's Mech. Philos. vol. ii. + Phil. Trans. 1818. Phil. Mag. vol, ix. Robison's Mech. Philos, vol. it. Tredgold on Steam-Engine, quoted from Bull, Sc. Tech. vol. i. ¶ Annales de Chim, êt de Phys. vol. xliii. 220 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. Fah. Fah. From these comparisons it appears, that for given temperatures the pressures determined by the Committee are lower than those found by Professor Robison, between 1 and 3 atmospheres; lower than those of Dr. Ure, from 1 to 5 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 Professor Arzberger; higher than those of the French Commission. The temperature given by the Committee for the pressure of 8 atmospheres differs about 3 from that inferred from the temperature given by Christiant for 7-8 atmospheres; viz. 337° Fah. The empyrical formula, adopted by the Committee of the French Academy, as representing the law of relation between the pressure and temperature of steam, is of the form, e = (a + nt) 5 Where e represents the elastic force of the steam, the temperature, and a and n are constants, determined, as well as the index 5, from observation. Tredgold had previously adopted a formula similar to this in form, as agreeing nearly with the best experiments to which he had access, and which have already been compared with the results obtained by this 1.97 atmospheres. 2.96 atmospheres. ↑ Quoted by Mr. Ivory, from Mech. Indust. vol. ii. Committee. Of this formula the French Commission remark, that the numbers which it gives accord, at the lower temperatures of their series, better with their experiments than those furnished by their own formula. Besides the differences in the numerical coefficients between the two formulæ now in question, Tredgold's formula has the numer 6 instead of 5 for an index. In other words, the elasticity increases more rapidly with the temperature than would be shown by the formula written on the preceding column.. With this law the experiments of the Committee coincide; the index 6 applying much more nearly to their results than 5. The empyrical formula adopted to represent their results is, 6 e= ('00333 t + 1) “ A where e is the elasticity of the steam in atmospheres, and t the excess of temperature above the boiling-point of water in degrees of Fahrenheit's scale. This formula will be found to accord very well at the higher pressures with the experiments of this Committee, and its variations from them at other pressures to be sometimes in excess, and at others in defect. This will appear by calculating the values of t for the different pressures recorded in the table of the Committee, on p. 218, and comparing them with the numbers found by experiment. Difference. |