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277 
the century which followed the publication of his Principles of Natu- 
ral Philosophy, is connected mainly with the establishment of the law 
of universal gravitation, and with the deduction of its chief conse- 
quences; so are the mathematical and physical researches of the 
present age likely to be associated, for the most part, with the study 
of light and heat and electricity, and of their causes, effects, and con- 
nexions. Whatever, then, whether on the practical or on the the- 
oretical side, in the inductive or the deductive way, may serve to 
extend or to improve the knowledge of these powerful and subtle 
agents or states of body, which are always and everywhere present, 
but always and everywhere varying, and which seem to be concerned 
in all the phenomena of the whole material world, must be received 
by scientific men as a welcome and valuable acquisition. 
Among researches upon heat, the highest rank is, (I suppose, ) 
by common assent, assigned to such works as those of Fourier and 
Poisson, which bring this part of physics within the domain of ma- 
thematical analysis. That such reduction, and to such extent, is pos- 
sible, is itself a high fact in the intellectual history of man; and from 
the contemplation of this fact, combined with that of the analogous 
success which it was allowed to Newton to attain in the study of 
universal gravitation, we derive a new encouragement to adopt the 
sublime belief, that all physical phenomena could be contemplated 
by a sufficiently high intelligence as consequences of one harmo- 
nious system of intelligible laws, ordained by the Author and Up- 
holder of the universe; perhaps as the manifold results of one such 
mathematical law. 
But if those profound and abstract works, in which so large a part 
is occupied by purely mathematical reasoning, suggest more imme- 
diately the thought of that great intellectual consummation, we must 
not therefore overlook the claims of experimental and practical inqui- 
rers, nor forget that they also have an important office to perform in 
the progress of human knowledge ; and that the materials must be 
supplied by them, though others may arrange and refine them. Es- 
pecially does it become important to call in the aid of experimental 
research, when facts of a primary and (so to speak) a central charac- 
ter require to be established ; above all, if the establishment of such 
facts has been attempted in vain, or with only doubtful success, by 
