386 NATURE'S TEACHINGS. 



have already been mentioned. They are wonderfully elastic, 

 and, when the tail is extended to its utmost limit, are nearly 

 straight. When, however, the tail is contracted, the tubes 

 become self-coiled by their own elasticity, and shrink into the 

 base of the tail. 



As the tail is very transparent, it is easy to see how these 

 movements are conducted. The larvce, which may be found in 

 almost any stagnant water, should be placed in a tall and narrow 

 glass. Some mud should be placed at the bottom of the glass, 

 which should then be filled with water to the depth of three 

 inches or so. 



When the mud has quite subsided, and the water become 

 clear, the long slender tails of the larvae will be seen so elon- 

 gated that their tips reach just above the surface of the water. 

 A. magnifying-glass will easily show the two tubes within the 

 tail. 



Let the glass be but slightly tapped, and all the tail is with- 

 drawn in a moment, so as to be out of reach of external danger. 

 The magnifying-glass will then show the two tubes lying con- 

 tracted in the base of the tail, and taking astonishingly little 

 space, considering the amount of elongation which they can 

 sustain. And, on examining the various bends and curves of 

 the tubes, the value and power of the spiral spring will at once 

 be seen. True, they are very small, but in Nature all things 

 go by comparison, and our whole earth itself is as a grain of 

 sand upon the seashore among the grandeurs of the visible 

 universe. 



THE LEAPING SPRING. 



THE last of the springs which can be mentioned in this work 

 are those which are used for leaping purposes. 



The figure on the right hand represents the common Spring- 

 jack or Skip-jack with which children are always so much 

 amused. It consists of a flattened piece of wood called the 

 " tongue," which is inserted into a twisted string, so that it 

 forms a tolerably powerful spring. When twisted round, and 

 then suddenly released, it strikes against the ground with such 

 force that the whole machine is thrown into the air. 



Sometimes the Skip-jack is made of a fowl's merrythought, as 



