mount of force used. Heat is a form of force. I must
urge you to realize precisely this energy of force. When you sharpen a
knife you put oil upon the hone. Why?--When the carpenter saws a piece
of wood he greases the saw. Why?--When you travel by train you see the
railway-porter running up and down the platform with a box of yellow
grease with which he greases the wheels. Why?--The answer to these
questions is not far to seek--it is because you want your knife
sharpened; it is because you want the saw to cut; it is because you want
the train to travel. The carpenter finds sawing hard work, and he does
not want the force of the muscles of his arm--his labour, in short--to
be converted into heat, and so he greases the saw, knowing that the more
completely he prevents friction, the more wood he will cut. It is the
force of steam that makes the engine travel. Steam costs money. The
engine-driver does not want that steam-force to be converted into heat,
because every degree of heat produced means diminished speed of his
train; and so the porter greases the wheels. But as you approach the
station the train must be stopped. The steam is turned off, and the
guard puts on what he calls "the brake." What is the brake? It is a
piece of wood so constructed and placed that it can be made to press
upon the wheel. Considerable friction results between the wheel and the
brake;--heat is produced;--the train gradually comes to a stop. Why? We
have now the conversion of that force into heat which a minute ago was
being used for the purpose of keeping the train a-going. Given a certain
force you can have heat _or_ motion; but you cannot have heat _and_
motion with the same force in the same amount as if you had them singly.
In every-day life, you cannot have your pudding and eat it.

[A] I need scarcely say, that whatever is of any value in the
following remarks is derived from that charming book of Professor
Tyndall's, _Heat a Mode of Motion_.

Heat then is generated by mechanical force; it is a mode of motion.
There was an old theory that heat was material. There was heat, for
instance, you were told, in this nail. Suppose I hammer it, it will get
hot, and at the same time I shall reduce by hammering the bulk of the
iron nail. A pint pot will not hold so much as a quart pot. The nail
(you were told) cannot hold so much heat when it occupies a less bulk as
it did when it occupied a larger bulk. Therefore if I reduce the bulk