PRINCIPLE OF ELASTICITY. 37 



mens of the thread. In one of these, part was stretched, and 

 part quite loose, the latter appearing flat when seen through a 

 microscope. The thread, which was highly elastic, was 

 increased about three inches in a minute. The slug was white, 

 and about an inch and a half in length." 



Now we come to the elastic system of the Chain Cable, and 

 find it anticipated in Nature in various ways. 



One curious example was that of a Spider, which found its 

 wheel- like net in danger from a tempestuous wind. The Spider 

 descended to the ground, a depth of about seven feet, and, 

 instead of attaching its thread to a stone or plant, fastened it to 

 a piece of loose stick, hauled it up a few feet clear of the ground, 

 and then went back to its web. The piece of stick thus left 

 suspended acted in a most admirable manner, giving strength 

 and support, and at the same time yielding partly to the wind. 



By accident the thread became broken, and the stick, which 

 was about as thick as an ordinary pencil, and not quite three 

 inches in length, fell to the ground. The Spider immediately 

 descended, attached another thread, and hauled it up as 

 before. In a day or two, when the tempestuous weather had 

 ceased, the Spider voluntarily cut the thread, and allowed the 

 then useless stick to drop. 



A CURIOUS example of the elastic cable is seen in the egg-case 

 of the Dog-fish, which is given on page 35. The egg-case is 

 formed like that of the common skate, and has a projection 

 from each of its angles. But the projections, instead of being 

 mere flattened horns, are lengthened into long elastic strings, 

 tapering towards the ends, and twisted spirally, like the 

 tendrils of a grape-vine. 



These tendril-like appendages twist themselves round sea- 

 weeds and other objects, and, on account of their spiral form, 

 can hardly ever be torn from their attachments. Sometimes 

 after a storm the egg is thrown on the shore, still clinging to 

 the seaweed, but to find an egg detached is very rarely done. 



I have already mentioned the tendrils of the vine, and their 

 great strength. The reader may remember the corresponding 

 cases of the Pea and the Bryony, the latter being a most 

 remarkable example of the strength gained by the spiral form. 



