The small side spans could obviously be left out in each case. The comparison was made with a view not only to arrive at a decision in this particular case, but also of answering the question of the economy of the arch more generally. The following table contains the weights of geometrically similar structures of three different spans, of which the second is the one here described. The so-called theoretical weight is that which the structure would have if no part required stiffening, leaving out also all connections and all wind bracing. The moving load is taken at one ton per foot lineal, and the strain on the iron at an average of four tons per square inch. The proportion of the girder is taken at 1 in 8. --------------+-----------------------+------------------------+ | Theoretical Weight. | Total Weight. | Span in Feet. +-----------------------+------------------------| | Arch. | Girder. | Arch. | Girder. | --------------+---------+-------------+------------+-----------| 100 | 0.0724 | 0.1663 | 0.1866 | 0.2443 | 220 | 0.1659 | 0.4109 | 0.4476 | 0.7462 | 300 | 0.2414 | 0.6445 | 0.6464 | 1.2588 | --------------+---------+-------------+------------+-----------+ |<------------Tons per foot lineal.------------->| It can be seen from these results that the economical advantage of the arch increases with the span. In small arches this advantage would not be large enough to counterbalance the greater cost of manufacture; but in the arch of 220 ft. span the advantage is already very marked. If the table were continued, it would show that the girder, even if the platform were artificially widened, would become impossible at a point where the arch can still be made without difficulty. The calculations leading to the above results would occupy too much space to make it desirable on this occasion to produce them. Our two views are from photographs.--_The Engineer._ * * * * * TORPEDO SHIPS. Commander Gallwey lately delivered an interesting lecture on the use of torpedoes in war before the royal U.S. Institution, London, discussed H.M.S. Polyphemus, and urged as arguments in her favor: 1. That she has very high speed, combined with fair maneuvering powers. 2. That she can discharge her torpedoes with certainty either ahead or on the beam when proceeding at full speed. 3. That her crew and weapons of defense are protected by the most perfect of all armor possible, namely, 10 ft. of water. 4. That she only presents a mark of 4 ft. above the water line.