ENGINEERING. 



319 



the tunneling of the Straits of Dover and Nor- 

 thumberland, and for the more perfect water- 



adjacent property and by engineers, who pro- under rivers and estuaries, but for drawing 

 nounced it impracticable on account of tbe na- supplies of fresh water for cities from large 

 ture of the soil. Permission was at last grant- lakes. The invention here described ofiers a 

 ed, and work begun at both sides of the ridge possible solution for many of the most impor- 

 in 1884. No difficulties of an unusual nature tant projects of modern engineering, such as 

 were encountered until the western heading 

 ing had advanced about forty feet, when a sub- 

 sidence occurred of such a serious 

 character that work was suspend- 

 ed, and Capt. Lindmark decided 

 to freeze the earth before exca- 

 vating further. A Lightfoot dry- 

 air refrigerator was procured from 

 London and placed in the advanced 

 heading which was temporarily 

 converted into a freezing - cham- 

 ber by means of board partitions 

 packed with charcoal. In Septem- 

 ber, 1885, the machine was set in 

 motion, and in sixty hours the 

 gravel was a mass of ice varying 

 from five feet in thickness at the 

 bottom to one foot at the crown 

 of the tunnel. At the floor of the 

 tunnel the temperature was readily 

 kept at 40, but barely touched 

 + 32 at the crown of the tunnel. 

 This, however, was hardly an in- 

 convenience, for the roof must 

 have been planked in any event, in 

 preparation for brick-work. The 

 precautionary measure of erecting 

 an iron safety- wall was soon dispensed with, 

 and the gravel was so solidly frozen that it had 

 to be quarried with special tools. During aver- 

 age weather the freezing-machine had to be 

 run ten or twelve hours every night. In very 

 rainy weather a longer time was necessary. 

 The machine was capable of delivering 25,000 

 cubic feet of air per hour at a temperature of 

 67. About eighty feet of the tunnel were 

 driven with the aid of the refrigerating process, 

 at the rate of about one foot a day. During 

 the progress of the work one house settled a 

 little, but no permanent damage resulted. The 

 tunnel was opened for public traffic on June 1. 



ELEVATION AND PLAN OF HEADWORKS, SUBMARINE TUNNEL. 



supply of cities like Chicago, Racine, Milwau- 

 kee, and other towns situated near large bodies 

 of fresh water. The illustrations sufficiently 

 indicate the general principles that govern the 

 construction. The head - works consist of a 

 water-tight, plow-shaped caisson connected by 

 a stuffing-box with the tube to be laid, and 

 pushed forward by hydraulic jacks set against 

 the end of the tube. As the tube is finished, 

 a track is laid within it, and the segments are 

 carried forward as required and bolted togeth- 

 er in the caisson, where there is plenty of room 

 to work the usual appliances for handling heavy 

 weights. As each segment is bolted on, the 



Submarine Tunnels. Within a few years the jacks are set against its forward edge, and, 



demand for tunnels of this character has large- 

 ly increased, not only for carrying railways 



when the ring is complete, the caisson is 

 shoved forward in a trench, dredged out in 



SUBMARINE TUNNEL IN COURSE OF CONSTRUCTION. 



