170 president's address SECTION H. 



jacks abutting on the completed portion of the work. As soon as 

 the advance of the shield permitted it a new ring of segments was 

 put in place, the cutting and lining thus proceeding almost 

 simultaneously. The space between segments and bore was filled 

 with grout forced in by air pressure. Where much water was met 

 with, a stream of grout played on the working face greatly assisted 

 the air pressure in retarding the flow. The work proceeded at an 

 average rate of 13ft. Gin. per day. 



The tendency of modern practice is thus (when the nature of the 

 material to be pierced admits) to conduct boring opei'ations on a 

 large scale in a very similar manner to that in which they are 

 effected on a small one, namely, by the removal at one operation of 

 a core the full diameter of the finished cross section, and, where 

 lining is necessary, to supply it in the form of large segmental pieces 

 or even to mould it in place. In the other operations connected 

 with railway formation the use of machinery has greatly increased 

 the rapidity of execution. The excavation of cuttings and founda- 

 tions, formation of embankments, ditching, and even track-laying 

 and ballasting, can be much facilitated, if not entirely performed, 

 by mechanical appliances, the adoption of which is rapidly becoming 

 general. 



The production of reliable steel of great strength and moderate 

 price gave a great impetus to the construction of long- span bridges. 

 That over the Firth of Forth, with its spans of 1,661ft., height 

 above bed of Forth of 570ft., and in which 50,000 tons of steel 

 and iron were used, will probably remain vmsurpassed in dimensions 

 until a material of still higher grade is introduced. 



Turning now to inland locomotion, we find that extremely high 

 speeds have been lately attained in England and America, and 

 we are promised still greater velocities on specially constructed 

 electrical railways. Such speeds as 120 miles per hour are of 

 course possible, but would necessitate a considerable distance 

 between the tracks, and an expenditure of energy at the rate of 

 about 250 horsepower in overcoming air resistance alone. It must 

 also be remembered that it would now be difficult to locate a rail- 

 way of this kind in a district so populated as to afford reasonable 

 prospect of paying traffic without its being brought into du-ect 

 competition with some existing steam line having greater facilities 

 for the exchange of vehicles, and which has probably been con- 

 structed at a far lower capital expenditure. Though the immediate 

 future of high speed electrical railways is not promising, electricity 

 is fast displacing other methods of traction on tramways and light 

 railways. 



In America, horse traction is being superseded by the overhead 

 conductor, or, as it is there termed, the trolly system, on which 450 

 tramways, with a total of 4,000 miles of track, are now being 

 worked. An electromotive force of 500 volts is used, and geared 

 motors are universally adopted. 



