Mr. J. P. Joule on the Mechanical Theory of Heat. 3S5 



been able to convince myself. This atmospheric wire was about 



,400 feet in length, suspended between the top of my own house 

 and the turret of a distant chapel, and insulated at the two ex- 

 tremities by silk strings which were under cover. A second 

 insulated wire proceeded from the first into my laboratory, and 

 was there in connexion with a battery of three Leyden jars, 

 containing together about 10 square feet of surface; these 



, were connected with a discharging electrometer, terminated by 

 balls, and which was capable of being adjusted by silk strings 

 and cranks, carried like bell-wires to a place of safety. During 

 the continuance of a thunder-storm, I have frequently noticed 

 the extraordinary inductive action produced upon the wire at the 

 instant of a flash of lightning, though at a distance of two, and 

 even three miles, as evidenced by the number of seconds elapsing 

 before hearing the thunder. This action was fteqaently so great 



; as to cause an instantaneous torrent of discharges from this 

 battery, over an interval of three-tenths of an inch, of precisely 

 the same character as those produced by the induction coil, but 

 very much louder. Never having employed any test of the 

 quantity thus discharged, I could scarcely imagine that the jars 

 were really charged and discharged at so rapid a rate from so 



, small a wire; but the experiments with the coil now perfectly 

 satisfy me as to the identity of the character of these atmospheric 

 discharges, and I merely allude to them as the only ones ap- 

 proximating in effect to those developed by the induction coil 



.t hat I have ever witnessed., ogru,^ 'a, [I y^n^^^n i>iio 'fuJaiJ-ifiib 



1 XLVIII. Note on Prof. Q\di\x.^\\Ji^^^ Application of the Mechanical 

 ti'iioTheory of Heat to the Steam-engine. By J. P. Joule, Esq.^ 

 ^'^ni S the motives which induced Professor Thomson and myself 

 -^jL to undertake experiments on the thermal effects of fluids 

 in motion appear to me to be somewhat misunderstood by 

 Professor R. Clausius in his paper published in the October 

 Number of this Magazine, I may be allowed to refer to Prof. 

 Thomson's paper "On the Dynamical Theory of Heat" (quoted by 

 Clausius), of which Part IV. is devoted to the discussion of a 

 method of discovering experimentally the relation between the 

 ■ mechanical work spent and the heat produced by the compression 

 of a gaseous fluid. In it, referring to the hypothesis first assumed 

 by Mayer, that the work spent in compressing a gas is exactly 

 equivalent to the heat evolved, he notices my paper " On the 

 ' Changes of Temperature produced by the Rarefaction and Con- 

 densation of Air,'' in which the hypothesis is verified as far as the 

 limits of experimental accuracy permitted. Prof. Thomson then 

 ' proceeds, § 72-77 f, to point out a method whereby an excess- 

 • ively delicate test of Mayer's hypothesis may be obtained for any 

 Vi-rmif Communicated by the Author. f Phil. Mag. vol. iv. p. 429. 



