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mechanic, David H. Mason, by whom, or under whose direction, the nicer parts of the work were executed, and who assisted, also, in the experiments.
The committee propose first to give a general description of the apparatus used, followed by details in the more complex parts; next to report the results of their examination upon each of the questions proposed for investigation.
GENERAL DESCRIPTION OF THE APPARATUS. The boiler used by the committee, and represented in Plates 1, 2, and 3, was twelve inches in interior diameter, two feet ten and a quarter inches in length within, and one-fourth of an inch thick; of rolled iron, with the heads rivetted in the usual manner. Plate 1 is a side view, and 2 and 3 are end views of the boiler, and of the apparatus connected therewith. The boiler was placed horizontally in a furnace, the fire surface extending about half way round the cylinder.
The furnace was arranged for a charcoal fire, the grate bars extending the whole length of the boiler, and the fire being applied through nearly the whole length. The draught entered by an opening, closed in the usual manner, and left the furnace through a flue placed at one end and side of the boiler. It will be convenient to use the terms fire end or front of the boiler, in reference to the proximity to the furnace door, and back end of the boiler. In Plate 2, A is the ash-pit door, B the furnace door, and in 1 and 3, C is the furnace chimney.
In order to examine, readily, the interior of the boiler during the progress of the experiments, each head was provided with a glass window, (D, Plates 2 and 3.) The glass used was three-eighths of an inch thick. The openings in the ends, which were rectangular, were wo and a half by one and threequarter inches wide. At first a glass plate, rather larger than the rectangle just mentioned, was applied to the opening, and kept in its place by four strips of brass secured to the heads, on which a rectangular frame, having the surface next to the glass accurately ground, was secured; the pressure of the steam keeping the glass against this frame, closed the boiler. Fractures occurring frequently from the rapidly varying, and often considerable, pressures within the boiler, and taking place by pressing the middle part outwards, as was proved by examining the fractures, frames with cross bars, see Plates 2 and 3, having the interior surface carefully ground, were used. The difficulty of properly adapting the surface of these frames to the glass having been re. moved, they were used in the later experiments, and were found to afford a sufficiently good view of the interior of the boiler, notwithstanding the obstruction by the cross bars.
Three gauge cocks were placed in the front head of the boiler; their positions will be particularly stated hereafter; they are shown in Plates 1 and 2, at a, b, and c.
To the same head and by the side of the gauge cocks, a glass water gauge (w, x, Plates 1 and 2) was attached, a particular description of which will be given in the detail of experiments made to compare its performance with that of the gauge cocks.
To supply the boiler with water, a forcing pump E E F G, Plates 1 and 3, was placed near the back end. This pump was of the ordinary construction, with a solid plunger and conical valves; the diameter of the pump was one inch, and the play of the piston one inch and three-quarters. The diameter of the pipe, F G, by which the water was conveyed from the pump to the boiler, was three hundredths of an inch. By a coupling screw, this pipe could be connected with either of the stop cocks, d, e, Plate 3, in the back end of the boiler: the opening of these cocks was two hundredths of an inch in diameter.
To ascertain the elasticity of the steam within the boiler, a closed steam gauge (H, Plates 1 and 3) was used, a particular description of the construc. tion, &c. of which will be given. This instrument was placed upon the same stand (I, Plates 1 and 3) which supported the pump, so that the same experimenter could observe its indications and attend to the working of the pump. The cistern of the gauge was connected by a flexible pipe, f, g, with the upper part of the boiler.
The safety valve is shown on the top of the boiler, (K, Plate 1,) midway between the heads. The graduation of it required much pains, and will receive a separate discussion.
Near the safety valve is represented at L, Plates 1 and 3, the fusible plate apparatus, consisting of a sliding plate of iron, moved by a lever. On the other side of the safety valve are the thermometers, M and N, Plate 1, plunged into iron tubes to give the temperature of the steam and water within the boiler. Above this appears the reservoir, O, containing the water intended to maintain the scales of the thermometers at a constant temperature. All these parts require a more detailed description.
DETAILS OF THE APPARATUS.
Of the Steam Gauge. The steam gauge consisted of a glass tube closed at the upper, and open at the lower end, which passed steam-tight into a reservoir for mercury: when this reservoir was connected wth the boiler the pressure of the steam raised the mercury into the gauge tube, compressing the air which the tube contained. The first mercurial gauge which was made was broken by a sudden access of surcharged steam, in the experiments upon that subject, and was replaced by a second one. The method of graduation, and in general the description of the second gauge, will serve also for the first; the details, only, varied slightly.
The glass gauge tube was 26.43 inches in length. To the lower end was connected an iron ferule, terminated above by a projecting ring. This ring was pressed upon the upper end of the pipe, h, Plate 1, by a coupling screw, which served to form a tight juncture between the gauge and the cistern. The cistern i was a cylindrical vessel of cast iron, having the two projecting tubes h and k upon which screws were cut; the first of them has been alluded to as giving a passage to the glass tube of the gauge; the second was coupled by the pipe f g, Plates 1 and 3, to the boiler.
The gauge tube was not of precisely equal diameter throughout, and it was judged more accurate to graduate small portions of it into equal volumes. This was done by introducing equal measures of air from the point of a sliding-rod gas measure (Hare's); this operation was performed repeatedly, and by multiple measures to verify the results, until the marks made for the equal volumes, on a paper scale attached to the tube, coincided, in the various trials. The lengths of the spaces occupied by the equal volumes were then carefully measured upon the brass scale to be used with the gauge. The slight differences between the lengths given by adjacent parts of the tube, showed that it might be considered as divided into so many small portions of uniform diame. ter. The mercury rising into the gauge tube from the cistern when pressure is applied, the level of the cistern is necessarily depressed; the amount of the correction for this depends upon the relation between the areas of the cistern and tube, supposed uniform. The areas of the cistern were found to be, within the limits of its use, sensibly the same; those of the tube might be so assumed for such a purpose: the ratio was therefore found by filling the gauge tube with mercury, and pouring this into the cistern, noting the rise produced ; comparing this with the mean length of the tube, the ratio of depression in the gauge for elevation in the tube was found to be as .01 to 1. The air within
the tube was next carefully dried by the introduction of a receptacle of chloride of calcium, of the same length with the tube ;* the air having been in contact with this substance for a sufficient time, the receptacle was withdrawn through the mercury over which the drying had been effected; the tube was next placed over a dish of mercury, in the receiver of an air pump, and the air withdrawn until on re-admitting air to the receiver, the mercury rose in the tube above the iron ferule.
The gauge tube was next introduced into the cistern, the level of which corresponding to the zero of the brass scale was then arranged, and the point of the scale at which the mercury stood was ascertained, the barometer and thermometer being noted.
It was intended in the experiments to keep the pipe from the gauge to the boiler cool, so that it might contain water, and thus give a nearly constant pressure upon the mercury of the cistern,t besides preventing the exposure of the apparatus to heat; the height of this column, above the level of the cistern, was therefore ascertained, after the gauge was put in place by screwing the cistern i to the stand.
All the elements for calculating the elasticity of the steam within the boiler, from the height of the mercury of the gauge, were thus known; the temperature of the apparatus being supposed constant.
The elastic force of the steam within the boiler, together with the column of water in the steam pipe, balances the elasticity of the compressed air within the gauge, together with the column of mercury above the level of that in the cistern. This level is not the original zero, but lower than that by the depression produced by the rise of mercury in the gauge tube. The depression of the mercury changes the level above which the pressure of the column of water in the steam-pipe, is measured, but the change in the pressure, by the column of water, is altogether inconsiderable. The law of the elastic force of dry air, which has been recently shown, by Dulong and Arago, to be accurate, at pressures from one to fifty atmospheres, was made use of in determining the elasticity of the air in the gauge: this elasticity is inversely as the space occupied by the air. From the data already obtained and upon the principles just stated, a table was calculated by which the observed heights of the gauge were converted into the corresponding pressures in inches of mercury or in atmospheres. The calculations were rendered rather tedious by the unequal diameter of the bore of the tube, on account of which equal lengths did not correspond to equal volumes. The usual method of calculation was resorted to, namely, to determine, by rigid calculation, the pressures, for points sufficiently near each other, and then to interpolate for intermediate heights.
The foregoing remarks take for granted that the temperature of the air in the gauge, as well as that of the mercury, remains constant; to secure this, an arrangement was adopted similar to that employed by Dulong and Arago for the same purpose. The gauge and scale were surrounded by a glass tube, 1, Plates 1 and 3, cemented below into a brass cap, m, Plate 1, which had an opening in the side, communicating with a discharge pipe, n, Plates 1 and 3. The tube was attached above by an air-tight juncture to a tin vessel, P, of con. siderable capacity, compared with the tube. Water being introduced into the glass tube surrounding the gauge, the flow through this tube was regulated by
* By this method, each volume of air in the tube was in contact with nearly a twelfth of its bulk of the chloride.
+ This and very many of the other precautions to insure accuracy, are borrowed from the able memoir of Dulong and Arago on the elastic force of steam at different tempera. tures; the result of their labours, as members of a Committee of the French Academy. Those who have engaged in questions of research will know that too great care cannot be taken to prevent the introduction of error, even in researches where great nicety may not be considered essential.
a stop-cock, o, placed at the end of the discharge pipe, the cistern above being filled with water.
To ascertain the temperature of the column of water surrounding the gauge, a thermometer, p, Plate 3, with a very small bulb was attached to the scale at the middle of its height: by this instrument, the flow of water through the casing of the gauge was regulated so as to keep the temperature nearly constant, and any deviations from a constant temperature were ascertained and noted, that the proper correction might be applied. The correction for the expansion of the air in the gauge, by a rise in its temperature during the progress of the experiments, was made according to the rules furnished by ihe rate of expansion of the gases, as determined by Gay Lussac, extended to compressed air by the experiments of Davy.* The correction for the changes of height of the mercurial column, within the range to which the temperature was suffered to increase, could not have been appreciable if acting entirely, and the counteracting effect of the expansion of the glass, further justified its being neglected. For similar reasons no reference was made to the effects of heat on the mercury in the cistern, i, on the cistern itself, and on the water within the pipe communicating with the boiler.
On the Thermometers. In most of the researches of the committee, refinements in the mode of using the common thermometer would have been out of place. Results which might be obtained with little additional labour, and which would be interesting in both a practical and scientific point of view, were not to be neglected, and
Let e represent the elastic force of the air within the gauge tube, expressed in inches of mercury; let h be the height of the mercurial column above the original zero; X, the height of the column above the new level; a, the height of the column of water in the steam pipe above the zero; s, the specific gravity of mercury; t, the tension of the steam within the boiler, in inches of mercury. Then k-h is the depression in the cistern caused by the rise of mercury in the gauge, and ath-h, the height of the column of water in the steam pipe above the new level in the cistern. We have then,
eth th' - h-
17.5 .01 h
1.29 - .0007 h = t;
13.6 the term .0007 h may be neglected as inconsiderable, since for k=24 inches, this term is only 0163. The equation then stands,
et 1.01 - 1.29 = t. At the temperature of 48°, and at a mean pressure, the observed value of h was 3.23; of course, c = 26.77. The volume of the air in the gauge was 8.63.
To find the elasticity for any other height, k', find from the data relating to the volume of the air in the gauge, the new volume; call this Ó', and the elasticity due to it d'; then:
0 : 8.63 : : 26.77 : d'; and
é + 1.01 k' - 1.29 = t. To introduce the correction for temperature, since the elasticity produced by an increase of temperature corresponds with the expansion produced, and since the expansion of condensed air follows the same law as that of air of ordinary density, expanding oth of its bulk at 32°, for each additional degree of Fahr. above this point, or both of its bulk at 480; calling é' the elastic force of the heated air, é that of the same air at 480, x being the number of degrees of heat above 48o.
nel d =dt
=é (1 + .002 n) 8.63 X 26.77 whence, since é' =
.(1 + .002 n) + 1.01 k – 1.29 = t.
to some of them great accuracy was essential. In the questions of the first class the thermometers were provided with wooden scales, and were graduated by immersion up to the point at which the scale commenced, the scale and upper part of the tube being exposed to the air; this was proper, as they were intended to be immersed in mercury nearly up to the scale. These instruments were examined after coming from the makers' hands, and the instrumental error ascertained. The tubes in which the thermometers were placed, and which contained mercury, were at first placed horizontally in one of the heads of the boiler; this had the advantage of rendering the tube for indicating the temperature of the water entirely independent of the steam, and thus any difference between the temperature of one and the other might be more effectually ascertained, than when the tube giving the temperature of the water passed through the steam. The position of these instruments interfered so much with other parts of the apparatus, and so much inconvenience and danger of error was experienced from the separation of the column of mercury in the thermometer, that these tubes were not used after the first weeks of experiment, and two vertical tubes, placed as already shown, were substituted for them.
The thermometers used, when the relation between the temperature of the steam and water, and the elasticity of the steam were to be observed in conjunction with some of the subjects more directly under the cognizance of the committee, had much pains bestowed upon them.
The scales (M and N, Plate 1,*) were metallic, and surrounded by glass tubes, fitting into a cup, a', through the bottom of which the stem of the ther. mometer passed water tight; a pipe, b' c', Plate 2, from the side of each cup, and provided with a stop-cock, d', regulated the flow of water through the enveloping tubes: a tight connexion above, with a reservoir, (0, Plates 1 and 3,) served, as in the case of the gauge, to supply the tubes with water. Small thermometers on the back of the scale of the large ones, showed the temperature of the water which surrounded them. The enveloping tubes being filled with water at 60°, the position of the boiling point of water and of the fusing point of tin, were used to verify the accuracy of graduation. The latter point, which is high upon the scale of the thermometer, having been very accurately determined, and being easily and with certainty ascertainable, serves as an excellent check upon the graduation. The greatest error within the limits just stated, was, in one instrument, three-fourths of a degree, and in the other one degree of Fahrenheit. The scales were graduated from two to two degrees, one quarter of a degree being readily estimated upon them. The corrections required by this examination were made through the medium of a table prepared for the purpose. In order to call the attention to the temperature of the water surrounding the scales, this temperature was recorded from time to time, when the height of the thermometers was observed. At no time did the rise of temperature, permitted in the water, make it necessary to apply a correction for the expansion of the scale.f None was required for the cooling effect of the water around the stem upon the mercury, owing to the method of verifying the scale.
The other parts of the apparatus, less general in their use, as the water gauge, safety valve, fusible plate apparatus, &c., will be more conveniently described in connexion with the experiments for which they were devised.
* In Plate 2, thermometer N, to render it conspicuous, is shown, as if the scale were turned to the front of the boiler.
+ Upon the scale of one of these instruments there were 314° in 6 inches. Brass expands tāds of its length, from 32° to 2129. These 6 inches, at 32°, would become 6.0113 at 212e. Ten degrees upon the scale would become 9.99 by a variation of temperature from 32 to 212", a diminution of only .01 of a degree for a variation of 180° in the temperature of the scale. In practice, the variation never exceeded thirty degrees.