1853] 



MEETING OF THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, 



51 



year. It is the first American nautical almanac and is consi- 

 dered to reflect great credit on the astronomers of that country. 

 It is under the superintendence of Lieut. C. H. Davis, assisted in 

 the physical department by professor Pierce. 



No one has contributed more to the knowledge of Terrestial 

 Magnetism, during the last few years, than my distinguished pre- 

 decessor in this chair. Formerly we owed theories on this subject 

 much more to the boldness of ignorance than to the just confidence 

 of knowledge, but from the commencement of the s3 T stematic 

 observations which Col. Sabine has been so active in promoting, 

 this vague and useless theorizing ceased, — to be succeeded, pro- 

 bably ere long, by the souud speculative theories of those who 

 may be capable of grappling with the real difficulties of the sub- 

 ject, when the true laws of the phenomena shall have been deter- 

 mined. Those laws are springing forth with beautiful precision 

 from the reductions which Colonel Sabine is now making of the 

 numerous observations taken at the different magnetic stations. 

 In his address of last year, he stated to us that the secular change 

 of the magnetic forces were confirmed by these recent observa- 

 tions, — and also that periodical variations depending on the solar 

 day, and on the time of the year had been distinctly made out, 

 indicating the sun as the cause of these variations. During the 

 present year the results of the reduction of the observations made 

 at Toronto, have brought out, with equal perspicuity, a variation 

 in the direction of the magnetic needle going through all its 

 changes exactly in each lunar day. These results with reference 

 to the sun prove, as Colonel Sabine has remarked, the immediate 

 and direct exercise of a magnetic influence emanating from that 

 luminary; and the additional results now obtained establish the. 

 same conclusion with regard to the influence of the moon. It 

 would seem, therefore, that some of the curious phenomena of 

 magnetism which have hitherto been regarded as strictly terres- 

 trial, are really due to solar and lunar, as much as to terrestrial 

 magnetism. It is beautiful to trace with such precision these 

 delicate influences of bodies so distant, producing phenomena 

 scarcely less striking either to the imagination or to the philoso- 

 phic mind than more obvious phenomena which originate in the 

 great luminary of our system. 



New views which have recently sprung up respecting the nature 

 of heat have been mentioned, though not in detail, by my two 

 immediate predecessors in the chair of the Association. They 

 are highly interesting theoretically, and important in their practi- 

 cal application, inasmuch as they modify in a considerable degree 

 the theory of the steam-engine, the air-engine, or any other in 

 which the motive power is derived immediately from heat; and 

 it is correct theory alone which can point out to the practical 

 engineer the degree of perfection at which he may aim in the 

 construction of such machines, and which can enable him to com- 

 pare accurately their merits when the best construction is arrived at. 



A theory which proposes to explain the thermal agency by 

 which motive power is produced, and to determine the numerical 

 relations between the quantity of heat and the quantity of me- 

 chanical effect produced by it, may be termed a dynamical theo- 

 ry of heat. Carnot was the first to give to such a theory a 

 mathematical form. His theory rested on two propositions which 

 were regarded as axiomatic. The first embodied the abstract 

 conception of a perfect thermo-dynamic engine, and has been 

 equally adopted by the advocate of the new theory of heat. — 

 Again, suppose a given quantity of heat to enter a body by any 

 process, and thereby to change its temperature and general phy- 

 sical state, and then, by a second process, suppose the body to 

 be restored exactly to its primitive temperature and condition, — 

 Carnot's second fundamental proposition asserts that the quantity 

 of heat which passes out of the body into surrounding space, or 



into other bodies, in the form of heat, during the second opera- 

 tion, is precisely the same as that which passed into the body 

 during the first operation. This view does not recognize the 

 possibility of heat being lost by conversion into something else, 

 — and in this particular it is at variance with the new theory, 

 wh c'a asserts that heat may be lost by conversion into mechanical 

 effect. To elucidate this distinction, suppose a quantity of water 

 to be poured into an empty vessel. It might then be asserted 

 that, in emptying the vessel again, we must pour ont just as much 

 water as we had previously poured in. This would equivalent, 

 to Carnot's proposition with respect to heat. But suppose a part 

 of the water while in the vessel to be converted into vapour; 

 then it would not be true that in emptying the vessel the same 

 quantity of water in the form L of water, must pass out of the 

 vessel as had before passed into it, since a portion would have 

 passed out in the form of vapour. This is analagous to the as- 

 sertion of the new theory with regard to heat, — -which may be 

 lost according to that theory, by conversion into mechanical 

 effect, in a manner analagous to tliat in which water may be 

 said to be lost by conversion into vapour. But the new theory 

 not only asserts generally the convertibility of heat into mechan- 

 ical effect, and the converse, — but also more definitely, that, 

 whatever be the mode of converting the one into the other — and 

 whether the heat be employed to produce mechanical effect, or 

 mechanical force be employed to produce heat, — the same quan- 

 tity of the one is always the equivalent of the same quantity of 

 the other. The proposition can only be established by experi- 

 ment, Rumford, who was one of the first to adopt the funda- 

 mental notion of this theory as regards the nature of heat, made 

 a rough attempt to determine the relation between the force pro- 

 ducing friction and the heat generated by it; but it was reserved 

 for Mr. Joule to lay the true foundation of this theory by a se- 

 ries of experiments which, in the philosophical discernment with 

 which they were conceived and the ingenuity with which they 

 were executed, have not often, perhaps been surpassed. In 

 whatever way he employed mechanical force to produce heat, he 

 found, approximately, the same quantity of heat produced by 

 the same amount of force; the force being estimated in foot- 

 pounds according to the usual mode in practical mechanics, — 

 ?, e, by the motive power employed in raising a weight of 1 lb, 

 through the space of 1 foot. The conclusion adopted by Mr. 

 Joule is, that 1 ° Fahr. is equivalent to 772 foot-pounds. 



These results are unquestionably among the. most curious and 

 interesting of those which experimental research has recently 

 brought before us. When first announced some ten or twelve 

 years ago, they did not attract the attention which they deserved;. 

 but more recently their importance has been fully recognized by 

 all those who cultivate the department of science to which they 

 belong. Of this Mr. Joule received last year one of the most 

 gratifying proofs, in the award made to him by the Council of 

 the Royal Society of one of the medals placed annually at their 

 disposal. It may be known to many of you that we ha\e in 

 Mr. Joule a pupil, a friend, and fellow-townsman of Dalton. 



This theory is in perfect harmony with the opinions now very 

 generally entertained respecting radiant heal. Formerly light 

 and heat were regarded as consisting of mateiial particles contin- 

 ually radiating from luminous and heated bodies respectively ; but 

 it may now be considered as established beyond controversy that 

 light is propagated through space by the vibrations of an ex- 

 ceedingly refined ethereal medium, in a manner exactly analo- 

 gous to that in which sound is propagated by the vibrations of 

 the air, — and it is now supposed that radiant heat is propagated 

 in a similar manner. This theory of radiant heat, in accord- 

 ance with the dynamical theory of which I have been speaking 

 involves the hypothesis that the particles of a heated body, or a 



