144 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



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 2% hours the heat necessary to boil 18^ Ibs. 

 of water. It is probable that Eumford 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 (33,000 Ibs. 

 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. 



At length, about 1845, Mr Joule of Manchester en- 

 deavoured to establish a rigorous connection between 

 Mr Joule, the mechanical effort expended and the heat gene- 

 Mechanical rated by friction ; and he appears to have satisfac- 

 effect of tori i y established (Phil. Trans., 1850) that in the 

 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 



(640.) 



heat. 



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 effect ; 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 enquy^y. 



5. SIK JOHN LESLIE. Establishment of certain Laws of Radiant Heat. Pictet Prevost. 



(641.) THE fact that heat is radiant, or passes through 

 Sir John S p ace i n the manner of light, apparently disengaged 

 progress of ^ rom an 7 material vehicle, became known at an early 

 the science period. Porta in the sixteenth century, and the Flo- 

 of radiant rentine 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 JOHN LESLIE, 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 early 

 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 and essa ; 

 connected with mathematics. Playfair, who was onmath< 

 eighteen years his senior, encouraged and directed electricit 

 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. It was 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. 



