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are so placed that each piece is subject to noother strain but what thrusts or draws it in thedirection of it’s length.

To explain how this principle is applied tobridges, it will be necessary to point out thenatures of the strains upon the different partsof a truss-frame.

If two beams A B, CD, fig. 11, plate 10,support an upright loaded beam E F, abuttingupon them, the pressure they sustain is evi-dently in the direction of their length. To findthe strain on the beams or rafters A B, CD,produce F E, and let E G represent the pres-sure upon the post E F. Draw G H, G I, pa-rallel to A B, C D, and resolving the force E Ginto E I, E H, these lines will show the strainupon the beams A B, C I).

The weight or force, E G, remaining the same,the strains upon A B, C D, will increase in pro-portion as the angle BED becomes more ob-tuse. For let the beams slant so as to form theangle b E d, complete the parallelogram, and,resolving the force E G into E m , E n, we findthat the strains on E b, E d, are now increasedto E m, E n.

The strain upon the beams A B, C I), willremain the same as before, if a weight equal toE G be attached to an upright piece hanging