70
to say, immediately over its vertical lines. The force thus communicated wasapplied by two pistons, forced upwards by a modification of Bramah ’s hydraulicpress; the principal of which, it will be remembered, depends upon the powergained by forcing water (by means of a small piston) into a strong cylinder inwhich a larger piston works; the power being increased in the proportion borneby the area of the piston to be raised to the area of the small piston. A registeringapparatus affixed to the pipe leading from the force-pump to the testing-machine,afforded the means of adjusting the pressure exercised by the hydraulic press. Acareful observation of this apparatus conveyed the assurance, that every girder,according to its ultimate destination, was proved to a strain of either 9, 15, or 22tons. After testing, the girder was released from its confinement, again raised bythe crane, and stacked in a convenient place ready for removal. So admirably werethe various arrangements made for conducting these operations, that it was possiblefor a girder to be lifted from its waggon, weighed, secured in the testing-machine,proved, released, again raised, and finally deposited, in less than four minutes,riie columns and In order to elevate the columns to their places, what is known in technicalrated, b ’ llo ' v language as a pair of shear-legs was employed. This simple apparatus consists of
two poles lashed together at their heads, and maintained in a steady position byropes extending from the apex of the triangle formed by the base-line of theground, and the inclination of the poles to one another, to stakes driven into theground at a considerable distance. From the apex of the triangle a series of ropespassing over pulleys were suspended perpendicularly ; and, by means of this “ fall,”the majority of the columns, girders, and other heavy portions of the construction,were elevated to their places. The operation of raising girders is shown in the
view, fig. 18, but on sosmall a scale as to conveyonly an imperfect idea ofits detail. Modificationsof the simple apparatusdescribed sufficed to hoistalmost every part of thenecessary iron-work. Aconnecting-piece was at-tached to each columnprevious to its elevation ;and so soon as two columnswith their connecting-pieces were fixed, a girderwas run up, slipped be-tween the projections ofthe connecting-pieces, andsecured in its place. Anopposite pair of columnshaving been similarly ele-vated, another girder wasattached to them; andthus two sides of a squarewere formed, and main-tained in a vertical posi-
Fig. 18 .
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