. Canadian engineer. ally replace the present standard structural steel,just as the latter has superseded wrought iron. This, inturn, may require some different types of metal bridges tobe designed, so as to take advantage of the greaterstrength of the metal without sacrificing stiffness in thestructure as a whole; or perhaps the present quality ofstiffness in metal structures will give place to more elas-ticity. .Another tendency is toward the use of reinforcedconcrete construction in preference to steel work whereverthe conditions are suitable for this type of bridge, and to-ward continuous
. Canadian engineer. ally replace the present standard structural steel,just as the latter has superseded wrought iron. This, inturn, may require some different types of metal bridges tobe designed, so as to take advantage of the greaterstrength of the metal without sacrificing stiffness in thestructure as a whole; or perhaps the present quality ofstiffness in metal structures will give place to more elas-ticity. .Another tendency is toward the use of reinforcedconcrete construction in preference to steel work whereverthe conditions are suitable for this type of bridge, and to-ward continuous ballasted roadbeds over all is also a tendency toward the construction ofmovable bridges re\olving in vertical planes instead of inhorizontal planes. These tendencies indicate that the evolution of railroad bridges has not as yet reached its finalstage, but is still in progress under influences which willcontinue to direct its trend toward meeting changes inphysical and economic Fig. 4.—Viaduct with Steel Towers. Typical of I9I5Construction. In conclusion, it may be stated that the recent im-provements in .American railway bridge practice embracethe substitution of solid-ballasted decks for open decks onshort-span, beam-girder bridges and for trestles; of plategirders for short-span, riveted trusses ; of riveted trussesfor short-span, pin-connected trusses; of stiff bracing andcounters for adjustable members; and of concrete andreinforced concrete masonrv for cut stone work. \\hile afew years ago the dominating characteristics of .Vmericanrailroad bridge construction were lightness, flimsiness andcheapness, the characteristics of the present-day standardsare stability, solidity and permanence. ^84 THE CANADIAN ENGINEER Volume 30. PRECISK HYSUHVHV. THi; GEODETIC b) 1-. U. Heid, , Supervisor of Levelling. fConcluded from last issue.) ?On all siciions upon which the for\vard_ancl back-ward measures dilTer by more than
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