arranged in a way which is quite different from that which we find in the human machine.  In the motor cycle all the levers are of that complex kind which are called wheels, and the joints at which these levers work are also circular, for the joints of a motor cycle are the surfaces between the axle and the bushes, which have to be kept constantly oiled.  No, we freely admit that the systems of levers in the human machine are quite unlike those of a 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 case just mentioned was the lid of the box.  At the other end we apply our strength, force, or power.  The edge of the box against which the chisel is worked serves as a fulcrum and lies between the handle where the power is applied and the bevelled edge which moves the resistance or weight.  A pair of ordinary weighing scales also exemplifies the first order of levers.  The knife edge on which the beam is balanced serves as a fulcrum; it is placed exactly in the middle of the beam, which we shall suppose to be 10 inches long.  If we place a 1-lb. weight in one scale to represent the resistance to be overcome, the weight will be