942 
great, an operation with which, as we have seen, he 
was professionally connected. In one experiment, a 
steel-borer pressed with a force of 10,000 Ibs. against 
gun metal, and revolving 32 times in a minute, gene- 
rated in 24 hours the heat necessary to boil 18% Ibs. 
of water. It is probable that Rumford carried his 
views so far as to infer a necessary and constant 
relation between the quantity of heat generated and 
mechanical action expended ; and if we take an esti- 
mate of horse-power more conformable to reality 
than the nominal horse-power of Watt (23,000 lbs. 
raised 1 foot in the minute, which is too great), we 
shall find a tolerable approximation between his re- 
sults and those now generally admitted. Davy fa- 
voured Rumford’s theory, but the mechanical ques- 
tion remained for 40 years almost unconsidered. 
(640.) At length, about 1845, MrJoule of Manchester en- 
pant re- deavoured to establish a rigorous connection between 
Mr ae the mechanical effort expended and the heat gene- 
Mechanical rated by friction; and he appears to have satisfac- 
effect of | torily established (Phil. Trans., 1850) that in the 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
Fahrenheit.1_ Mr Joule’s experiments and inferences, 
however, go much farther than this, namely, that in 
all circumstances where heat is generated, it is at 
the expense of a precisely similar equivalent of me- 
chanical effeet ; and conversely, that mechanical effect 
is never used up, without a corresponding evolution 
of heat, and that this is the case whatever be the 
fluids or other substances employed. Thus in the 
steam-engine the possible efficiency of the engine is 
only limited by the mechanical effort due to the heat’ 
given out by the condensed steam. So the heat given 
out by compressed air represents the force expended 
in compression; and even the heat produced by voltaic 
or magnetic electricity is that which corresponds to 
the work it might do. A step farther leads to the 
equivalence of heating effects by chemical combina- 
tion to the amount of energy which, differently di- 
rected, might have been realized in the shape of 
work ; and though a larger induction is still required 
to justify all the conclusions which the zealous pro- 
mulgators of this comparatively new “mechanical 
theory of heat” have advanced, it cannot be doubted 
that there is a basis of important truth in the matter 
which well deserves farther enquiry. 
§ 5. Sir Joun Lesiin.—EZstablishment of certain Laws of Radiant Heat.—Pictet—Prevost. 
te case of water agitated by beaters, the work expended 
by the fall of 772 pounds through 1 foot is capable 
of raising the temperature of a pound of water by 1° of 
(641.) Tue fact that heat is radiant, or passes through 
ee space in the manner of light, apparently disengaged 
peas of fom any material vehicle, became known at an early 
the science period. Porta in the sixteenth century, and the Flo- 
of radiant yentine academicians in the seventeenth, had reflected 
~ heat by mirrors. Marriotte and Newton respectively 
assigned some of the laws which characterize it. 
Lambert, in the middle of the last century, made 
some real advances, but it was not until the very 
close of that period that heat in the radiant form was 
carefully and systematically studied. The group of 
philosophers simultaneously engaged on it consisted 
of Leslie, Rumford, Herschel, Pictet, and Prevost. 
The two last named were earliest in point of date ; 
but as we owe to Leslie by far the ablest series of 
experiments, and which for many years, and even to 
the present time, have formed part of the body of 
science, we shall connect his name principally with 
this section. 
Sir Joun Lesziz, born in 1766, completed his 
studies at a very early age in the University of St 
Andrews, From boyhood he was remarked for a 
decided and independent turn of character; and as 
his favourite studies were mathematical, he for some 
time pursued them to the exclusion of the classics. 
Ultimately, however, he attained also to a respect- 
able knowledge of these, and by his strong natural 
(642.) 
His earl 
studies ; 
talents, and his love of reading, he acquired an im- 
mense stock of information on all sorts of subjects. 
This he displayed not only in his conversation, but 
also in his writings on technical and purely scienti- 
fic matters, in which he frequently introduced with- 
out much apology illustrations from his miscella- 
neous reading, and even metaphysical disquisitions. 
As is frequently the case in persons addicted to 643.) 
natural philosophy, his first original researches were @"4 essays 
connected with mathematics, Playfair, who was oo ae 
eighteen years his senior, encouraged and directed electricity. 
him; Ivory, who was almost his contemporary, and 
also his fellow-student at St Andrews, was per- 
haps no less influential in confirming his geometrical 
tastes. The former communicated Leslie’s first ori- 
ginal paper to the Royal Society of Edinburgh in 
1788. Itwas on Indeterminate Equations, and was 
printed in their Transactions. Down to this period 
we have no record of his being engaged in original 
experiments ; but it is probable that such was the 
case, for in 1790, and the following years, we have 
evidence not only of his having speculated on subjects 
of natural philosophy, but also that he had made 
experiments intended to confirm or refute prevailing 
theories, A paper.on Electrical Theories was read 
to the Royal Society of Edinburgh; which, finding 
them reluctant to print, he withdrew, and he only 
1 To Rumford, I believe, is due the attempt (in conformity with this view) to ascertain the heat developed by the friction of 
fluids, for instance in churning (which, I think, was one experiment proposed by him), but I have not been able to find a 
reference to it amongst his scattered writings. 
i 
