A good example of the value of this structure of bone may be found in young children. Before they are old enough to take care of themselves they are perpetually falling down, and never hurting themselves. I have seen a little girl of five years old roll from top to bottom of a lofty staircase. It looked as if the child must be killed, but she was only giddy with her many revolutions, and a little bruised about the elbows. The reason of this curious immunity from injury is, that the bones, especially those of the skull, are not completely united, and so act on the principle of the compound spring. THE SPIRAL SPRING. This subject is so large, and there are so many examples, both in Art and Nature, that it is not very easy to make selections which will sufficiently answer the purpose. [Illustration: WATER-LILY. LILY. POISON-CELL OF HELIANTHOID. ANTHEROZOIDS.] [Illustration: SPIRAL SPRING. BED SPRING. WATCH SPRING.] The upper left-hand figure of the illustration represents the ordinary Spiral Spring made of wire, and used for its power of resuming its shape when compressed. In early childhood most boys have had practical experience of this spring in the toy guns and cannons with which they are supplied. The spring is compressed by the ramrod, and held in its place by a catch. If a pellet be placed in the gun, and the catch released by pulling the trigger, the spring flies back to its former shape, and drives the pellet. An exactly similar spring is used in the well-known “Needle-gun,” the spring driving a needle through the explosive mixture, and so igniting the charge. Our brass instruments would be very badly off without the spiral spring, which is placed under the pistons. The elasticity allows the pistons to be pressed down, and when the fingers are raised the pistons spring up again. Another form of this instrument is seen on the right of the ordinary spring. This is used in the manufacture of spring mattresses and couches, and is made thinner in the centre, so as to allow of greater elasticity. Below them is the spring which is used for watches and clocks, one end being fastened to the rim of the barrel, and the other to the pivot. When the latter is turned the spring becomes “wound up,” and, when released, keeps the works going by pressing against them. Of the “pall-and-ratchet” wheel, by which the movements are retarded, we shall treat in another place. * * * * * ON the left hand of the illustration are a few figures of the Spiral Spring as seen in Nature. On the extreme left of the group is a spiral cell taken from the flower-stem of the Water-lily. As the reader will see, it is composed of a number of fibres laid parallel to each other, and twisted into a hollow spiral. In order to exhibit its shape the better, the spiral has been partially uncoiled.