motor cycle. They are more simple, and it is easy to find in our bodies
examples of all the three orders of levers. The joints at which bony
levers meet and move on each other are very different from those we find
in motor cycles. Indeed, I must confess they are not nearly so simple.
And, lastly, I must not forget to mention another difference. These
levers we are going to study are living--at least, are so densely
inhabited by myriads of minute bone builders that we must speak of them
as living. I want to lay emphasis on that fact because I did not insist
enough on the living nature of muscular engines.

[Illustration: Fig. 1.--Showing a chisel 10 inches long used as a lever
of the first order.]

We are all well acquainted with levers. We apply them every day. A box
arrives with its lid nailed down; we take a chisel, use it as a lever,
pry the lid open, and see no marvel in what we have done (Fig. 1). And
yet we thereby did with ease what would have been impossible for us even
if we had put out the whole of our unaided strength. The use of levers
is an old discovery; more than 1500 years before Christ, Englishmen,
living on Salisbury Plain, applied the invention when they raised the
great stones at Stonehenge and at Avebury; more than 2000 years earlier
still, Egyptians employed it in raising the pyramids. Even at that time
men had made great progress; they were already reaping the rewards of
discoveries and inventions. But none, I am sure, surprised them more
than the discovery of the lever; by its use one man could exert the
strength of a hundred men. They soon observed that levers could be used
in three different ways. The instance already given, the prying open of
a lid by using a chisel as a lever, is an example of one way (Fig. 1);
it is then used as a lever of the first order. Now in the first order,
one end of the lever is applied to the point of resistance, which in the