Chai*. VIII.
505
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Fig. 005.
PLAN OP P1F.it,
Fig. 506.
8BCTION OF PIER,
pose, where greater economy was necessary. The masonry of the abutments was wellexecuted, and great care taken to allow for the expansion and contraction of the iron,
Fig. 507.
r r §
£
which was found considerable; it is even reported that the heat of the sun affected thelength of some of the outer ribs, during the progress of hoisting them to their situation :and that the dimensions of one, taken by the workmen when
on the ground, was increased so much, that it would not j| jj |_j
answer the position for which it was designed until its tern- -pd,
perature had been lowered. One of the first considerationsof the engineer, when he employs iron for construction,should be its properties of increasing and diminishing; andhe should always provide a means by which the metal canaccommodate itself, without thrusting out the works onwhich it rests or may be in contact.
At the Kildare Canal, in Ireland , the first skew bridgeswere introduced by Mr. Chapman, in the year 1787; beforethat time their advantages do not appear to have been muchnoticed in England. One executed by that engineer atFinlay Bridge, near the town of Naas, in Ireland , deviated51 degrees from a rectangle with the canal, and formed anacute angle of 39 degrees with its abutments : its span in anoblique direction was 25 feet, and its rise 5 feet 6 inches.
The only difference of construction is, that in common bridges the courses are all parallelwith the abutments, and in the skewed arches they run obliquely to them. The face ofthe abutments making an oblique angle to the sides of the bridge, it became necessary
5
3r Y
Fig. 50S SECTION.