often the one which gives the least weight of metal in the trusses.

The author finds by experience that, for trusses with polygonal top chords, the economic depths, as far as weight of metal is concerned, are generally much greater than certain important conditions will permit to be used. For, instance, especially in single-track bridges, after a certain truss depth is exceeded, the overturning effect of the wind-pressure is so great as to reduce the dead-load tension on the windward bottom chord to such an extent that the compression from the wind load carried by the lower lateral system causes reversion of stress, and such reversion eye-bars are not adapted to withstand. A very deep truss requires an expensive traveller, and to decrease the theoretically economic depth increases the weight but slightly ; hence it is really economical to reduce the depth of both truss and traveller.

Again, the total cost of a structure does not vary directly as the total weight of metal, for the reason that an increase in the sectional area of a piece adds nothing to the cost of its manufacture, and but little to the cost of erection; so it is only for raw material and freight that the expense is really increased. Hence it is generally best to use truss depths considerably less than those which would require the minimum amount of metal. For parallel chords, the theoretically economic truss depths vary from one fifth of the span for spans of 100 feet to about one sixth of the span for spans of 200 feet; but for modern railway through-bridges the least allowable truss depth is about 28 feet, unless suspended floor-beams be used, a detail which very properly has gone out of fashion.

In two five-hundred-foot spans of a combined railway and highway bridge the author employed a truss depth of seventy-two feet; but this was determined by the reversal of stress in bottom chords through wind-pressure. A greater depth, if permissible, would have caused a saving in total weight of metal.

In a design of the author's for a five-hundred-and-sixty-foot span  a  truss  depth of ninety  feet  was  adopted,  but  in  this  case  the  live  load  was  very  great,  varying  from  ten