200 NATURE'S TEACHINGS. 



thought a most wonderful feat to drive a tunnel under the 

 Thames, while in these days the tunnel through Mont Cenis 

 has been completed, and we are hoping to make a submarine 

 tunnel from England to France. 



IN Nature we can find many examples of Tunnels used for 

 similar purposes. The silken tunnel of the Wax-moth larva 

 has already been mentioned, and we now come to Tunnels 

 where earth in some form, and not silk, is the material of which 

 they are constructed. 



The lowermost figure on the left-hand side of the illustration 

 represents that well-known and most destructive burrower, the 

 Ship- worm (Teredo], which, by the way, in spite of its popular 

 name, is not a worm, but a mollusc. This creature has a pecu- 

 liar interest for engineering, inasmuch as its mode of working 

 gave Brunei the first idea of subaquatic tunnelling in loose, 

 sandy soil, just as the Victoria Regia leaf gave to Paxton the 

 idea y/hich afterwards developed into the Crystal Palace. 



The plan adopted by the Ship-worm is at the same time 

 simple and effective. It feeds upon wood, and gradually eats 

 its way through almost any timber that may be submerged. It 

 does not, however, merely bore its way through the timber, but 

 lines its burrow with a coating of hard, shelly material. Taking 

 this hint, Brunei proceeded in the same fashion to drive his 

 tunnel through the very ungrateful soils which form the bed of 

 the Thames. 



Ho built a " shield," as he called it, of iron, exactly fitting 

 the tunnel, and divided into a number of compartments, each 

 of which could be pushed forwards independently of the others. 

 In each compartment was a single workman, and, as he 

 excavated the earth in front of him, he pushed forward his 

 portion of the shield, while the interior was cased with brick- 

 work, just as a Teredo tunnel is cased with shell. 



ABOVE the Teredo is represented another marine tunnel- 

 maker, as it appears in its burrow. 



This is the mollusc popularly known as the Piddock, and 

 scientifically as Pholas dactylus. It may be found abundantly 

 in all our chalk cliffs, boring its tunnels deeply into the stone, 

 and aiding the sea in its slow, but never-ending task of 



