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length with the tube;* the air having been in contact with this substance for a sufficient time, the receptacle was withdrawn through the mercury over which the drying had been effected; the tube was next placed over a dish of mercury, in the receiver of an air pump, and the air withdrawn until on re-admitting air to the receiver, the mercury rose in the tube above the iron ferule.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 considerable 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 temperatures; 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 the 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; k', 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 '-h is the depression in the eistern caused by the rise of mercury in the gauge, and a+h'-h, the height of the column of water in the steam pipe above the new level in the cistern. We have then, a + h h

S

=t

· h = .01 h, a = 17.5 inches, also s = 13.6; then

e+h+h' — h

-

For the gauge in question, k'

the term .0007 k may be neglected as inconsiderable, since for h=24 inches, this term is only .0163. The equation then stands,

e+1.01 h-1.29=t.

At the temperature of 48°, and at a mean pressure, the observed value of h was 3.23; of course, e = 26.77. The volume of the air in the gauge was 8.63.

To find the elasticity for any other height, h', find from the data relating to the volume of the air in the gauge, the new volume; call this ', and the elasticity due to it e'; then :

8.63 26.77 e'; and

e' 1.01 h - 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

th of its bulk at 32°, for each additional degree of Fahr. above this point, or th of its bulk at 48°; calling e" the elastic force of the heated air, e that of the same air at 48°, n being the number of degrees of heat above 48°.

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 thermometer 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, (O, 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.† 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 Dandsds of its length, from 32° to 2129. These 6 inches, at 32°, would become 13 at 212. Ten degrees upon the scale would become 9.99 by a variation of temiture from 32 to 212, a diminution of only .01 of a degree for a variation of 180° in temperature of the scale. In practice, the variation never exceeded thirty degrees.

SUBJECTS OF INVESTIGATION.

The queries originally proposed, together with the new matters, which were made the subjects of experiment, will be treated in the following order.

I. To ascertain whether, on relieving water heated to, or above, the boiling point from pressure, any commotion is produced in the fluid.

Including the examination of the efficacy of the common gauge-cocks, of the glass gauge, and of Ewbank's proposed gauge-cocks.

The investigation of the question whether the elasticity of steam within a boiler may be increased by the projection of foam upon the heated sides, more than it is diminished by the opening made.

II. To repeat the experiments of Klaproth on the conversion of water into steam by highly heated metal, and to make others, calculated to show whe-ther, under any circumstances, intensely heated metal can produce, suddenly, great quantities of highly elastic steam.

First, The direct experiment in relation to the production of highly elastic steam in a boiler heated to a high temperature.

Not to interrupt the general train of investigation which follows a well known theory of explosions of steam boilers, the results of the experiments on the former part of this query, are inserted in another place.

III. To ascertain whether intensely heated and unsaturated steam can, by the projection of water into it, produce highly elastic vapour.

IV. When steam, surcharged with heat is produced in a boiler, and is in contact with water, does it remain surcharged, or change its density and temperature?

V. To test, by experiment, the efficacy of plates, &c., of fusible metal, as a means of preventing the undue heating of a boiler, or its contents.

1. Ordinary fusible plates and plugs.

2. Fusible metal, inclosed in tubes.

3. Tables of the fusing points of certain alloys.

VI. To repeat the experiments of Klaproth, &c.

1. Temperature of maximum vaporization for copper and iron under different circumstances.

2. The extension to practice, by the introduction of different quantities of water, under different circumstances of the metals.

VII. To determine, by actual experiment, whether any permanently elastic fluids are produced within a boiler when the metal becomes intensely heated.

VIII. To observe accurately the sort of bursting produced by a gradual increase of pressure, within cylinders of iron and copper.

IX. To repeat Perkins' experiment, and ascertain whether the repulsion stated by him to exist between the particles of intensely heated iron and steam be general, and to measure, if possible, the extent of this repulsion, with a view to determine the influence it may have on safety valves.

X. To ascertain whether cases may really occur when the safety valve. loaded with a certain weight. remains stationary, while the confined steam ac

quires a higher elastic force than that which would, from calculation, appear necessary to overcome the weight on the valve.

XI. To ascertain by experiment the effect of deposits in boilers.

XII. Investigation of the relation of the temperature and pressure of steam, at ordinary working pressures.

Table from 1 to 10 atmospheres.

I. To ascertain, by direct experimemt, whether on relieving water heated to, or above, the boiling point, from pressure, any commotion is produced in the fluid.

The first experiments on the effect of relieving water in ebullition from pressure, were made in a glass boiler; here the fire was under the whole length of the boiler, which was a cylinder of fourteen and a quarter inches in length, and seven and a half inches in diameter. The steam within, being at a pressure of less than two atmospheres, by opening a cock at the end of the boiler, or the safety valve, also at the end, large bubbles were formed through the whole extent of the boiler.

The inquiry was prosecuted in the iron boiler already described, a distinct view of the interior being had through the glass windows placed in the heads. The greatest intensity of the fire was in front of the middle of the boiler, and extended through about one-third of its length: the part immediately near the flue, was next to this band in temperature. With this boiler experiments were made, which showed, that on making an opening in the boiler, even when the pressure did not exceed two atmospheres, a local foaming commenced at the point of escape, followed soon by a general foaming throughout the boiler, the more violent in proportion as the opening was increased. This small boiler was completely filled with foam by opening the safety valve, (nearly twotenths of an inch in area) which was placed on the middle of the top, and the water violently discharged through the opening of the valve. The area of the valve bears to the horizontal section of the boiler, at the water line, the ratio of one to two thousand and fifty-five nearly.

The boiler was half full of water in these experiments. The gauge fell always on making the opening.

The foaming, which was so repeatedly observed, must be produced in a greater or less degree every time that steam is drawn from a boiler to supply the engine; every time that a gauge-cock is opened, or the safety valve raised. It is interesting in two points of view; first, in its effects upon the apparatus designed to show the level of the water within the boiler; second, by its throwing the water against the heated sides of the boiler.

Gauge-cocks and Glass Water-gauge.

The apparatus most commonly used in our country for determining the level of the water within a boiler, consists of three gauge-cocks attached to the boiler head, one of them being at the water level, and the others equally distant above and below that level.

These cocks in the experimental boiler, a, b, c, Plates 1 and 2, were 1.95 inches and 1.8 inches apart, measuring from the centre of the opening of the middle one, to the one above and to the one below.

The steam in the boiler being not higher than two atmospheres, the following experiment was made. The level of the water was reduced until it stood just below the lowest gauge-cock. On opening this cock, steam at first flowed out, then water and steam; on opening the second cock, in addition, water flowed freely from the lowest, which was above the hydrostatic level; the