122 
Hence the authors draw the conclusion : — ‘ Therefore when 
b vanishes, or becomes infinitely small, the economical 
duty is a maximum.’ Certainly this is a most startling 
result; that the maximum of work should be done when 
no zinc at all is consumed.” The last sentence is a mis- 
statement of the conclusions of Joule and Scoresby’s paper, 
in which (Philosophical Magazine, vol. 28, p. 451) it is 
stated that “ the economical duty will be a maximum when 
b vanishes or becomes infinitely small in comparison with a. 
In this case sc =158, while the power of the engine will 
become infinitely small with regard to work performed in a 
given time.” Comparing the phrases ‘ economical duty’ and 
‘ maximum of work,’ as he uses them, he evidently confuses 
the duty of an engine with the whole work done by it. 
A little further on we have — “ They calculate the maxi- 
mum theoretical power of a grain of zinc to be 158 foot 
pounds, and yet using permanent magnets, which, by their 
own statement, were so badly constructed as to have only a 
quarter the power they ought to have had, with the poles 
of the electromagnets never approaching the permanent 
magnets nearer than \ of an inch (and what an enormous 
loss is incurred here!); with an engine constructed almost 
at haphazard, and with scarcely a consideration of the best 
principles or of the most advantageous construction of such 
engines, they actually obtained a result of 1029 foot pounds 
out of a calculated theoretical maximum of 158. With a 
little care and consideration, I do not hesitate to say the 
duty per grain of zinc might easily have been increased 
tenfold.” It is hardly credible, but the above looks very 
like a confusion between Force and Work ! The author 
seems to assume that if the forces in operation in an engine 
are greater, that the engine will necessarily produce more 
work from the same quantity of fuel. In these experiments 
the quantity of zinc (a — b) used to produce work W is 
observed; if the engine was made more powerful, if the 
