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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 



