Like many other things in the world, there is nothing new in the idea of boring a hole through the earth and lining it with brick or iron. As Pope suggests, mankind doubtless learnt the art from Nature, though the correctness of the poet’s zoological knowledge is hardly shown in the examples heading this chapter. For ages past--before London existed--that skilful excavator, the mole, tunnelled through the earth, making roads and galleries, the friction of his fur, set perpendicularly on his skin, lining his tube so that the soil did not fall in. The larvæ of the humble caddis-fly covered the inside of their cases with fine silk; and the trap-door spider lined its 12-inch long shaft with similar material to prevent the tumbling in of loose particles and to afford itself a foothold in climbing up; while the ant constructed her galleries and stuccoed them with the finest grains of soil, so that the inner walls presented a smooth, unbroken surface. With the advent of man and his civilisation came the extensive use of furs, and in these the grubs of the moth--in the abstract the most engaging of creatures--made galleries whenever they got a chance, lining them with their own silk, wherein to undergo their transformation into the pupa stage. Well-experienced engineers, such as the vine, beech, pine, and bark-boring beetles, are all tube-makers; but it is the pholas, or _teredo navalis_, who is the arch-borer, so skilled an expert in lining, that, though only the size of a quill and “soft in body,” he pierces the hard timbers of ships and quay-piles, lining the tubes as he proceeds with a saliceous substance as hard as china. The body of the Teredo is like a long white worm, varying from a foot to two inches and a half in length, and about the width of a finger. From him, it is said, the elder Brunel took his idea of the shield which he employed in constructing the tunnel beneath the Thames after the shaft had been excavated. [Illustration: FIG. 12. A 3,000 H.P. TRIPLE EXPANSION CENTRAL VALVE ELECTRICAL ENGINE _By permission of_ _Willans and Robinson, Rugby_] But his clever system was crude, and not calculated to cope with porous or aqueous soil; therefore, when the stratum of clay, through which the work was being carried forward, broke off abruptly, a serious influx of water took place. The work had to be abandoned, and was only completed after much delay and ruinous expense. In a commercial sense, it was an utter failure. Since Brunel’s time, engineering has developed its resources _pari passu_ with the development of science. Hydraulic force displaces the primitive screw power, and steel plates the cumbersome timber works used in the Thames tunnel.