142 ANNUAL OF SCIENTIFIC DISCOVERY, 



produced by it, may be termed a dynamical theory of heat. Carnot 

 was the first to give to such a theory a mathematical form. His theory 

 rested on propositions which were regarded as axiomatic. The first 

 embodied the conception of a perfect thermo-dynamic engine, and has 

 been equally adopted by the advocates of the new theory of heat. 

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

 cess, and thereby to change its temperature and general physical state ; 

 and then, by a second process, suppose the body to be restored exactly 

 to its primitive temperature and condition, Carnot's second funda- 

 mental 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 operation, 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, which 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 out just as much 

 water as we had previously poured in. This would be equivalent to 

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

 water while in the vessel to be converted into vapor ; then it would 

 not be true that in emptying the vessel the same quantity of water, in 

 the form of water, must pass out of the vessel as had before passed in- 

 to it, since a portion would have passed out in the form of vapor. This 

 is analogous to the assertion of the new theory with regard to heat, 

 which may be lost, according to that theory, by conversion into me- 

 chanical effect, in a manner analogous to that in which water may be 

 said to be lost by conversion into vapor. But the new theory not 

 only asserts generally the convertibility of heat into mechanical 

 effect, and the'converse, --but also more definitely, that whatever be 

 the mode of converting the one into the other and whether heat be 

 employed to produce mechanical effect, or mechanical force be em- 

 ployed to produce heat, the same quantity of the one is always the 

 equivalent of the same quantity of the other. This proposition can 

 only be established by experiment, liumford, who was one of the first 

 to adopt the fundamental notion of this theory as regards the nature of 

 heat, made a rough attempt to determine the relation between the 

 force producing friction and the heat generated by it; but it was re- 

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

 series 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, approxi- 

 mately, 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, L e., by the motive power employed in 

 raising a weight of 1 Ib. through the space of 1 foot. The conclusion 

 adopted by Mr. Joule is, that 1 Fahr. is equivalent to 772 foot-pounds. 



