MECHANICS AND USEFUL ARTS. 27 



of conception and engineering skill can compai-e with the proudest 

 achievements of any age or country. The growth of the city of 

 Chicago has been marvellous, even for America, and its water 

 supply, always insufficient, and of late years unwholesome from 

 the filth poured into the lake near the shore by the sewers, had 

 become a source of great anxiety to its citizens, when it was pro- 

 posed to take water from the lake two miles from shore, and con- 

 duct it to the city through a tunnel under the bed of the lake. 

 Many engineers doubted the practicability of the undertaking, and 

 the estimates of its probable cost varied from $250,000 to $6,000,- 

 000. Surveys of the lake bed by means of an auger inclosed 

 in a tube, revealed the favorable circumstance of a continuous 

 underlying stratum of hard blue clay. The contract was awarded 

 to Messrs. Dall and Gowan in October 1863, for $315,139. They 

 have expended, it is said, more than double that amount, and the 

 total cost will probably be not far from a million dollars. Work 

 was commenced on the shore end of the tunnel, March 17, 1864 ; 

 and its completion in so short a time is due principally to the sldll 

 and energy of the City Engineer, Mr. E. S. Chesbrough, formerly 

 connected with the Cochituate Water Works at Boston. 



The shore-end shaft consists of sections of great cast-iron tub- 

 ing, about 36 feet long and 9 in diameter, let into the earth by 

 simply excavating beneath them, and allowing them to sink as 

 the earth was removed. Having in this way worked through the 

 sand and into the blue clay, which forms the bed of the lake, the 

 shaft was narrowed to 8 feet, and carried down over 40 feet 

 lower, with brick walls a foot thick. This shaft was sunk four 

 feet Ipwer than the lake shaft, causing a descent of two feet per 

 mile in the tunnel to facihtate emptying when required. From 

 the shore end the tunnel extends two miles in a straight line, at 

 right angles to the shore. 



At the lake end of the tunnel the greatest engineering difficulty 

 and triumph occurred. Many engineers believed that it would be 

 impossible to make a permanent structure at this point, on ac- 

 count of the violent storms on the lake. It was, however, effected 

 by a huge wooden crib or coffer-dam, built, like a ship, on the 

 shore, launched, and towed to its destined location. 



This immense crib was launched July 26, 1865 ; it is 40 feet 6 

 inches high, pentagonal, in a circumscribing circle of 98 feet 6 

 inches in diameter. It is built of logs one foot square, and con- 

 sists of three walls, at a distance of 11 feet from each other, leav- 

 ing a central pentagonal space having an inscribed circle of 25 

 feet, within which is fixed the iron cylinder, 9 feet in diameter, 

 to run from the water line to the tunnel, 64 feet below the surface 

 and 31 feet below the bed of the lake. The crib is very strongly 

 built, containing 750,000 feet of lumber, board measure, and 150 

 tons of iron bolts, and weighs about 1,800 tons. It was towed to 

 its position, two miles from the shore, on the same day, and the 

 process of sinking began by opening sluices and placing some 600 

 tons of stone in the bulkheads. The crib will hold 4,500 tons of 

 stone when filled, giving an extra weight of 3,900 tons for steady- 

 ing against the waves. As built it will stand about seven feet 



