368 NATURE'S TEACHINGS. 



gun," the spring driving a needle through the explosive mix- 

 ture, 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. 



On the extreme right is a corresponding spiral cell from the 

 common Lily, in which the spring power is given by two fibres 

 twisted in opposite directions. The reader will now under- 

 stand and admire the mechanism by which these plants attain 

 their great strength and elasticity, the stems being made of 

 myriads of these spiral fibres. 



The oval body on the upper part of the illustration is a 

 poison-cell of a marine polyp, and is given here as an example 

 of an animal spiral spring, the others all belonging to the 

 vegetable world. 



We shall see more of its structure a little further on, and 

 will not now examine it in detail. 



The two remaining figures represent the remarkable objects 

 called Antherozoids, i.e. the living creatures of anthers. They 

 exist in vast numbers in the non-flowering plants, and inhabit 



