172 DA VIES— TUNNEL CONSTRUCTION. [February i8, 



habit of working under pressure to, say, not over thirty years; and 

 careful medical examination of every employee for physical and 

 organic condition, and the exclusion of highly strung nervous sub- 

 jects; (2) the honest cooperation of employer with the medical offi- 

 cer, so that no man is employed who has not been passed as sound ; 

 (3) the rule that no man should work when sick; (4) the rule that, 

 while the time interval for compression should be reasonably slow, 

 the all-important thing is to limit the time rate for decompression. 

 This is the reason for the use of varying stages of pressure. 



In the tunnel work by the Hudson companies, from 1902 to 

 1907, there were 40,000 men examined for work under pressure up 

 to 42 pounds above normal. In that time, in the tunnel work (not 

 including caisson construction) only three men lost their lives owing 

 to caisson disease. Work in caisson is in this respect more hazard- 

 ous than in tunnel, even at the same pressures, owing largely to the 

 small capacity possible within the working chamber and the higher 

 temperatures under which work has to be carried on. 



It was before mentioned that the original idea of using air 

 pressure was simply to exclude water. Later, when about 1869 

 Colonel Haskins conceived the idea of our Hudson tunnels, he also 

 evolved the idea that air pressure balancing the external earth pres- 

 sure would retain it, neglecting the fact that the elastic properties of 

 air allow it to take any form, and that consequently rigid support to 

 maintain the shape of the chamber is essential to the application of 

 air pressure for balancing earth pressures. This oversight was the 

 direct cause of a great disaster which occurred in the early days of 

 the Hudson River Tunnel. The invention of the " shield " by Sir 

 Marc Isambard Brunei, in 1818, marks the most interesting step in 

 the art. He had been asked if it was possible to build a tunnel 

 under the Neva at St. Petersburg, and working out a scheme, took 

 pattern from nature. The Toredo navalis destroys wooden piles by 

 boring holes through them just at the mud line. The worm attaches 

 itself to the wood and then bores the hole as it moves forward, lin- 

 ing its hole with a calcareous shell and discharging the borings 

 through its body. Its body also closes the hole against water com- 

 ing in; for if the worm penetrates the wood, or admits water, it dies, 



