In such cases of enormous compression, it is of great importance that the compressing force bear equally on the whole abutting surface. The German carpenters are accus tomed to put a plate of lead over the joint. This prevents, in some measure, the penetration of the end fibres. M. Per ronet, the celebrated French architect, formed his abutments into arches of circles, the centre of which was the remote end of the strut. By this contrivance the unavoidable change of . form of the triangle made no partial bearing upon either angle of the abutment. This always has a tendency to splinter off the heel of the beam where it presses strongest. It is a very judicious practice.
When circumstances allow it, we should rather employ ties than struts, for securing a beam against lateral strains. When an upright pillar, such as a flag-staff a mast, or the uprights of a very tall scaffolding, are to be shored up, the depen dence is more certain on those braces that are stretched by the strain than on those which are compressed. The scaf
folding of the iron bridge near Sunderland had some ties very judiciously disposed. and others with less judgment.
Figure '25.-1. When a beam, A n, is firmly fixed at the end, A, and a straining force acts perpendicularly to its length at any point, a, the strain occasioned at any section, c, between B and A, is proportional to c B, and may therefore be represented by the product wXca; that is, by the product of the number of tons, pounds, &c. which measure the strain ing force, and the number of feet, inches, &c. contained in c B. As the Toads on a beam are easily conceived, we shall substitute this for any other straining force.
2. 11 the strain or load he uniformly distributed along any part of the beam Iying beyond c (that is, farther from A), the strain at c is the same as if the load were all collected at the middle point of that part ; for that point is the centre of gravity of the Ioad.
Figure 26.-3. The strain on any section, n, of a beam A B,