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ment. No notice is taken of the temperature of the scales of the thermome ters, it having varied but 10° Fah. namely, from 86 to 96°.
The first experiment is introduced, to show the temperature of the water within the boiler, before the long continued heat had sensibly affected the indications of the thermometer.
At the close of these experiments, the metal was, in many places, but little short of a red heat, visible in daylight.
The precise state of things in a boiler, of which parts are unduly heated, was represented in these experiments; there was the surcharged steam, and heated metal ready to give up its heat to replace what might be absorbed in the conversion of the water injected, into steam. This latter circumstance renders the case different from that contemplated in the deductions of theory which have been brought to bear upon the question. The greater or less intensity of the heat afforded by the top and sides of the boiler would necessarily modify the effects observed, by the injection of any given quantity of water; this is observable in the numbers given in the table, where although the greater quantity of water injected does not fail in two consecutive experiments to show a greater depression of the gauge, yet in distant experiments the same is not the case. We see that in no case was an increase of elasticity produced by injecting water into hot and unsaturated steam, but the reverse, and in general that the greater the quantity of water thus introduced, the more considerable was the diminution in the elasticity of the steam. The quantity of water injected was from 3.5 to 24.3 cubic inches. The immediate rise of the gauge after each experiment, shows how rapidly heat was supplied by the sides of the boiler to the steam within.
That the steam was highly surcharged with heat, appears by comparing the pressures corresponding to the temperatures, with those given by Dulong
and Arago, for saturated steam. For example, the pressure shown by the gauge when the steam was at 506° Fah. was 6.15 atmospheres, while the table just referred to gives for this temperature a pressure of forty-eight atmospheres. The temperature was carried to 533° Fah. when the pressure shown by the gauge was 6.82 atmospheres, while saturated steam at that temperature would have had a pressure of more than sixty atmospheres.
In order to ascertain whether the thermometer relied upon to give the temperature of the steam was affected, if at all, in excess or defect by the conducting power of the metal; the temperature of the boiler just beyond the tubes was taken, as nearly as was practicable, by a thermometer, R, Plate 1, dipping into a clay receptacle, upon the top of the boiler. This thermometer did not rise above 405° Fah.; its distance from this source of heat was ten inches, and that of the iron tube inclosing the thermometer, six and a half inches. Supposing the temperature stationary on top, the temperature of the metal of the top of the boiler near the tube of the thermometer would have been 479°,* showing that it tended to carry off heat from the thermometer, which, if at all affected by the metal above it, showed too low a temperature for the steam in contact with it. The temperature of the source of heat would have been from these data, 582° at the extreme end of the part covered with fuel, which was of course at a lower temperature than the middle portion.
On examining the apparatus after the conclusion of the last day's experiments, it was found that some of the putty used in tightening the lower joint of the thermometer in the water, had been softened by the heat, and had flowed into the tube, thus affording a direct communication between the steam and the bulb of this thermometer: this circumstance accounts for the instrument being affected in this day's experiments and not in the preceding ones.
* If we suppose the heat of a small bar of metal, cut from the top of the boiler, to have been derived by the conducting power of the metal alone, the heating effect of the steam within being neglected, and further, that the temperatures of the bar had become constant, then the ratio of the excess of the temperature (y) of any point at a distance (x) above the temperature of the air, to that (y') of any point at a distance, (x',) is given by the proportion,
Log. x: Log. x'.
In the case before us, y = 405 · 80= 325°, x = 10.0 inches, and r′ = 6.5 inches, whence y=399°, and the temperature at that point is y' + 80 = 479°. To find the temperature of the source of heat, we have the equations
in which y' and y are the excesses of the temperatures at the distances and z over that of the air. A is the temperature of the source, e the base of the Naperian logarithms, 2 the thickness of the bar, and the ratio of its radiating to its conducting pow
in the case before us,
and therefore A = 582°.
The boiler must have been hotter at the furthest point than it would have been if not in contact with the surcharged steam.
(To be continued.)