knows, there are only a very few bridges in which the desired intersection in a single point of the axes of all members assembling at each apex is accomplished; and in most structures where eccentricity exists for want of such intersection, its prejudicial effects are not duly recognized and provided for.

Principle XIX.

Truss members and portions of truss members should always be arranged in pairs symmetrically about the plane of the truss, except in the case of single members, the axes of which lie in said plane of truss.

One occasionally sees a violation of this principle even in important bridges; but experience with structures in which it was ignored has been such as to show most clearly that this cannot be done with impunity, for the torsion resulting from eccentrically connected adjustable members is patent even to the uninitiated.

Principle XX.

In proportioning main members of bridges, symmetry of section about two principal planes at right angles to each other is a desideratum to be attained whenever practicable.

Of course in top-chord and inclined end-post sections, which should be designed with a cover-plate, symmetry about both principal planes is not attainable. The objectionable features caused by want of it, however, are provided against by the next axiom.

Principle XXI.

In both tension and compression members the centre line of applied stress must invariably coincide with the axial right line passing through the centres of gravity of all cross-sections of the member taken at right angles thereto.

 Until  very  lately  this  important  principle  has  been  simply ignored, the  effect  being that  the  allowed  intensities  of  work-