The criterion, equation (27), for the greatest bending moments in a bridge is applicable to any truss whatever, whether the chords are parallel or inclined, but it is not so with equation (33). If the chords of the trusses are parallel, the quantity _i_ in equation (33) becomes infinitely great, and the equation takes the following form: _l_ _W₁ + W₂ + ... + Wₙ_ = ---- (_W₃ + W₄_ + etc.) (34) _p_ Ordinarily the span _l_ divided by the panel length _p_ is equal to the number of panels in the span. Hence equation (34) shows, in the case of parallel or horizontal chords, that when the moving load is placed for the greatest web stress in any panel, the total load on the bridge is equal to the load in that panel multiplied by the total number of panels. =99. Influence Lines.=—A graphical method, known as that of “influence lines,” is used for determining the greatest shears and bending moments caused by a train of concentrated weights passing along a beam or bridge-truss. Obviously it must express in essence that which has already been shown by the formulæ which determine positions of moving loads for the greatest shears and bending moments. In reality it is the application of graphical methods which have become so popular to the determination of the greatest stresses in beams and bridges. =100. Influence Lines for Moments both for Beams and Trusses.=—It is convenient to construct these influence lines for an arbitrary load which may be considered a unit load; the effect of any other load will then be in proportion to its magnitude. The results determined from influence lines drawn for a load which may be considered a unit can, therefore, be made available for other loads by multiplying the former by the ratio between any desired load and that for which the influence lines are found. [Illustration: FIG. 25.—Bending Moment in a Simple Beam.] _AB_ in Fig. 25 represents a beam simply supported at each end, so that any load _g_ resting upon it will be divided between the points of support, according to the law of the lever. Let it be desired to determine the bending moment at the section _X_ produced by the load _g_ in all of its positions as it passes across the span from _A_ to _B_. Two expressions for the bending moment must be written, one for the load _g_ at any point in _AX_, and the other for the load at any point in _BX_. The expression for the first bending moment is _z_ _M = g_ ----(_l-x_), (_a_) _l_ and that for the latter _l-z_ _M′ = g_ ---- _x_. (_b_) _l_