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replies, “Quite true; but this only shows that one of the 
laws of thermodynamics is inconsistent with the doctrine of 
the mechanical equivalence of heat.” Now the first law of 
thermodynamics asserts nothing else than that there is a 
mechanical equivalent, constant in all cases; whilst the 
second law, as usually stated, involves the first law, and 
involves nothing else but Carnot’s axiom and the principle 
that in conduction heat flows from the hot to the cold body, 
both of which no one will doubt. Mr. Highton’s reply is 
very similar to stating that one of Kepler’s laws is incon- 
sistent with the planets moving in ellipses. What Mr. 
High ton proposes as a paradox is then a necessary conse- 
quence of the principle he attacks. 
Though the doctrine of the mechanical equivalent of heat 
finds its firmest basis in the immortal experiments of Dr. 
Joule, the fact, that assuming it we can explain many phe- 
nomena, is a valuable supplementary proof. 
“Examples of the Performance of the Electro-Magnetic 
Engine,” by J. P. Joule, D.C.L., F.R.S., &c., V\P. 
Some experiments and conclusions I arrived at a quarter 
of a century ago having been recently criticised, I have 
thought it might be useful to place the subject of work in 
connexion with electro-magnetism in a different and I hope 
clearer form than that in which I have hitherto placed it. The 
numbers given below are derived from recent experiments. 
Suppose an electro-magnetic engine to be furnished with 
fixed permanent steel magnets, and a bar of iron made to 
revolve between the poles of the steel magnets by reversing 
the current in its coil of wire. Such an arrangement is 
perhaps the most efficient, as it is the most simple form of 
the apparatus. In considering it, we will first suppose the 
battery to consist of 5 large Daniell’s cells in series, so large 
that their resistance may be neglected. We will also sup- 
pose that the coil of wire on the revolving bar is made of a 
