THE WORLD'S ADVANCE 



317 



In Boston, for instance, during the con- 

 struction of the subway in 1897, a seri- 

 ous explosion occurred as a result of 

 leaking gas. Several people were killed 

 and much property was damaged. Phila- 

 delphia had a similar experience in 1906, 

 during the construction of the Market 

 street subway. In neither Boston nor 



SHOOTING CONCRETE IN PLACE 

 WITH COMPRESSED AIR 



The old method of laying concrete 

 tunnels for sewers or railroad work in- 

 volved the slow and laborious task of 

 packing the concrete into tunnel forms 

 by hand. Today, however, a new and 

 clever patented method re- 

 duces the cost of such work 

 by more than one-half and 

 makes far greater speed 

 possible. 



The new method consists 

 merely in using compressed 

 air for both mixing the 

 concrete as well as for 

 blowing it in place in the 

 forms. As an example of 

 the marked efficiency of this 

 method, it is interesting to 

 note the results of work be- 



Two Views of the Wooden 

 TYestles Which Carry the 

 Gas Pipes Over the Streets 

 During the Construction of 

 New York's Subways. 

 These Overhead Pipes Have 

 Eliminated the Danger of 

 Explosions Due to Leaking 

 Pipes. 



Philadelphia was the gas supply trans- 

 ferred from the underground mains to 

 pipes laid above the ground in the open 

 air. 



In New York these gutter mains, or 

 "by-passes," as they are called, are car- 

 ried around the curb corners and back 

 into the crosstown streets far enough 

 to insure safety against a leakage of gas 

 through the intervening earth and into 

 the excavation. They are then carried 

 across the street imderground and con- 

 nected to the existing mains. Where 

 trunk mains run in the street, parallel to 

 the excavation, they are carried on tres- 

 tles built over the sidewalk. 



ing undertaken at Memphis, Tenn., where 

 two great concrete tunnels which will be 

 employed to drain a disease-breeding 

 bayou are now under construction. The 

 tunnels are sixteen feet in diameter and 

 dug by forcing steel shields through the 

 soft dirt. As the shields move along inch 

 by inch, the dirt is carted back. Wooden 

 rings prevent the earth from caving in, 

 while steel tunnel forms sustain the con- 

 crete in place until it sets. An eight-inch 

 pipe connected with the pneumatic mixer 

 and conveyer delivers into the form the 

 proper proportions of sand, gravel, ce- 

 ment and water. A thirty-foot section is 

 filled in three or four hours and, in view 



