USEFUL AETS. 



CHAPTER Y. 



THE PRINCIPLE OF THE SPRING-. THE ELASTIC SPRING-. 

 ACCUMULATORS. THE SPIRAL SPRING. 



Springs and their various Structure. The Elastic Spring. The Boy's Catapult 

 and its Powers. The Pistolograph, its Principle, and Uses to which it can he 

 put. Leaf-rolling Caterpillars, and their Way of Work. The Carriage 

 Spring. The Horse's Hoof and its complex Structure. Fungi and their 

 united Power. The Chinese Cross-bow. The ancient Balista. Skull of 

 the Crocodile. Bones of young Children. The Spiral Spring and its many 

 Uses. The Toy-gun. The Needle-gun. Valved Brass Instruments. 

 Watch and Clock Springs. The Bed Spring. Parallels in Nature and Art. 

 Buffers of Railway Carriages. Spring Solitaires. The Bell Spring. 

 Spiral Springs in Vegetable Tissues. Poison Cells of various Marine 

 Animals. Effects of the Spiral Springs. 



ELASTIC SPRINGS. 



HERE we come upon a subject so large, that it is difficult 

 to define its exact requisite limits. The principle of the 

 elastic spring pervades all Nature, and the numerous adapta- 

 tions in Art are closely, though perhaps not directly, attri- 

 butable to the wide distribution of the spring in Nature. 



There is, for example, the simple elasticity which enables a 

 tree, when bowed by the wind, to spring back so soon as the 

 pressure is removed, and which, indeed, is the power which 

 enables a bow to propel an arrow. Then there are spiral 

 springs innumerable, many of them so minute that they can 

 only be seen by the aid of the microscope, and there are many 

 springs which exhibit their elasticity by their power of exten- 

 sion and shortening, just as is done with the elastic fabrics 

 which are so much in vogue at the present day, and which 

 seem so necessary to ordinary comfort that we feel disposed to 

 wonder how our forefathers managed without them. 



