354 
REPORT—1854. 
This speculation is founded, like the former, on the data furnished by Pou* 
illel's researches on the solar radiation, and Joule’s theory of the mechanical 
energy equivalent to the effect of heat produced: the calculation turns on 
the assumption that the velocity of vibration can only be a small fraction 
(probably not one-fiftieth) of the velocity of the propagation of warn, and 
from the velocity of vibration we may calculate the density or conversely. 
Hence the author conceives that we may assign a limit, ami that a cubic 
fool of luminiferous tether, at the distance of the earth from the sun, cannot 
contain less than ( , - - * of one pound of matter. 
\ 15o0 X 10 / 
With regard to the results of Prof. Thomson, especially when the now 
character of some of the reasonings is taken into account, some difference o 
opinion may reasonably be expected. There are certainly many const er- 
ations involved which might suggest important topics of discussion; on t 
it is not my purpose to enter: I will merely remark, that in all thesem 
gations the essential point is the expenditure of mechanical energy m pr 
vibrations, of whatever kind. The whole question then assumes a more** J 
mechanical aspect j—the sure indication that this entire branch of science is 
a state of approximation at least towards that stage v^ich characterizes 
perfection of any branch of jthysical knowledge, when all its varied p 
mens shall be shown to he susceptible of analysis up to simple coiubina 
of the elementary laws of force and motion. , 
I would merely add, that in speaking of the effect of the evolution oj « > 
there is nothing in Professor Thomson's conclusion which restricts • 
any one species of heat; the essential point is the production oi wwa ■ 
and his results am thus In entire accordance with the theory vvluch 
kinds of heating effect to the stoppage or absorption of rays, in fl “ ier * 0 [ 
the extinction or destruction of the vibratory motions, constituting r 
different wave-lengths, some of which are also within those linnMl* _ 
to that part of the scale, which renders them capable of affecting,ou^ 
with the sensation of vision;—which (as already remarked)consi e . 
on all hands seem now tending to show is the most probable hype ie '' 5 
the subject. 
Radiation of Heat from the Zodiacal Light andfrom the Comet of 18^ 
Some interesting observations on these points are given by M* Matth'® 
in the * Comptes ft end Us,' April 184*3 (vol. xvi. p. 68“)- . .,, r . 
On the Siylh of March, AT. Matthicsseti placed at the focus of a 
mirror of 1 metre diameter, an air-thermometer, which showed a • 
dKauu (la ..it_ .. . •*» i.i * .r ilia mirror 
above Its indication in other positions, when the axis of the mirror 
rected to the zodiacal light. , a 
He next substituted fur the thermometer, a thermo-electric pile with S* 
precautions; which having its condensing cone 
Directed to the nucleus of the comet, gave a deviation of 
„ the zodiacal light near its summit 
„ the zodiacal light at base . . . 
» the part of sky over the sun . . 
In other directions 
On removing the condensing cone to try whether the effect was doe .. 
atmospheric causes, he still found towards the base of the zodiacal W 
or 3 . 
instead of the mirror he next used a flint-glass lens , 56 centim* diau>eter- 
16 centim. focal length. 
t 
10 ° 
12 ° 
3 s 
0 ° 
J 
