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5. No doubt the coincidence between the mechanical 
equivalent of heat, found by Dr. J oule from friction, and that 
by M. Favre from working a magnetic engine, seems very 
striking ; but 
A. The value of Favre’s experiment disappears on exami- 
nation. It was but a single experiment, either never 
repeated, or never repeated with the same results; in a 
very delicate experiment there was only the difference of 
300 units out of 18,000 ; and even the permanent* enlarge- 
ment which always takes place in magnets which are in 
use might account for these ; and 
B. Numerous and long-continued experiments by M. 
Soret show results entirely discordant with the single one of 
M. Favre. 
6. It seems incredible, that with the imperfectly con- 
structed engine used by Joule and Scoresby, they should at 
the very first trial have succeeded in utilizing 2-3rds of the 
magnetism evolved, or capable of being evolved, by their 
battery; and Dr. Joule now tells us that according to his 
latest calculations of the mechanical equivalence of heat, 
they utilized 6-7ths of the power of the battery. The only 
conclusion we can arrive at is, that the real power of the 
battery, and therefore of a grain of zinc, must have been 
much greater than he calculated. 
7. For consider the disadvantages under which the 
engine acted: 
A. The temporary and permanent magnets were never 
nearer than J of an inch apart. Though Dr. Joule assures 
us this does not affect the power of the engine, it certainly 
produces a waste of zinc, as the near approach of the mag- 
nets creates counter-currents which check materially the 
consumption of zinc. 
B. The copper wire was not tested for conductivity; a 
subject little thought of at that time, and it is found that 
a very small impurity in copper wire will very, very, largely 
diminish the power of an electro-magnet. 
