22 REPORT—1 848, 
Since the above experiments were communicated to the Association a slight alte- 
ration in the form of the apparatus, calculated to give greater exactness to the results, 
occurred to Mr. Joule, and he has therefore commenced a new and extensive series 
of experiments in order to determine the equivalent of heat with all the accuracy 
which its importance to physical science demands. The result arrived at after a series 
of forty experiments was an alteration of the equivalent before stated to 771, which is 
believed to be within ;3,dth of the truth, and therefore may for the present be assumed 
as a tolerably good basis for calculation. 
The author conceives the following points to be established :—I1st. His experi- 
ments on the friction of fluids, confirming the views and experiments of Davy 
and Rumford on the friction of solids, afford another decisive proof that heat 
is simply a mechanical effect, not asubstance. 2nd. His experiments, showing that 
the thermal effects of the condensation and rarefaction of air are the equivalents of 
the mechanical force expended or gained, prove that the heat of elastic fluids con- 
sists simply in the vis viva of their particles; and 3rd. The zero of temperature, 
determined by the expansion of gases, is at 491° below the freezing-point of water. 
We may, the author thinks, employ the above propositions as a basis on which to 
calculate the specific heat of the gases. For whether we conceive the particles to 
be revolving round one ancther, according to the hypothesis of Davy, or flying about 
in every direction according to Herapath’s view, the pressure of the gas will be 
proportional to the vis viva of its particles. Thus it may be shown that the par- 
ticles of hydrogen gas at the barometrical pressure of 30 inches and temperature 
60°, must move with a velocity of 6225°54 feet per second in order to produce the 
observed pressure of 14°714 lbs, on the square inch. Now a lb. moving at that 
velocity is equivalent to 781°'45 of heat in a lb. of water, which will therefore re- 
present the absolute heat of a lb. of hydrogen at 60°. But 60° is, as already stated, 
oO. 
519° of temperature from zero, whence el = = 1°5157 will be the heat required 
to raise the temperature of a lb. of hydrogen 1°, compared with that necessary to 
give the like increase of temperature to a lb. of water, in other words, 1°5157 will 
be the specific heat of the gas. 
Further, since oxygen is 16 times as heavy as hydrogen, its particles must move 
at one-fourth the velocity in order to produce the same pressure. The specific heat 
of oxygen (as of all other gases) will be inversely as its density, or = 0°09473. 
Experiments of De la Roche and Berard 
Theory. referred to capacity at constant volume. 
Hydropenve oss :.) 195157 2°3520 
Aqueous vapour . . 0°1684 0°6050 
INTEROPCH Is cece dues ee OnLy, 071953 
Oxygen. . . . . O°0947 0°1686 
Carbonic acid . . . 0°0685 0°1579 
Notices of Aurore observed at Swansea. By Joun Jenkins, F.R.A.S, 
Dec. 3, 1845, after a general luminosity in the north, a flat elliptical arch, from 
near the vertex of which two pyramidal coruscations shot up to the zenith, grow- 
ing fainter upwards, while a third rose between the crown and the northern extre- 
mity of the arch. At half-past 8 the arch was higher, its upper edge nearly inter- 
secting the Pleiades and passing through Gemini and Taurus. Its breadth = 14 
diam. of the moon, light yellowish. 
Sept. 29, 1847.—At a quarter past 7 this fine aurora had the appearance of a 
white striated band extending from E. to W. and crossing the zenith. Jupiter was 
about 1 diam. of the moon from the upper margin of the brush near the eastern 
horizon. ; 
Oct 24, 1847.—The characteristic radiating canopy of light of this great aurora — 
was observed at Swansea ; light red and filmy, vanishing round the moon in a circle, 
whose radius was 6 lunar diameters. Apparent motion of the light upwards ; light — 
brightest in the N.W., where silvery as well as red pencils were constantly emitted, 
To the N.E. of the moon near the zenith, a partial halo or corona was formed, 
