--------------+-----------------------+----------------

--------+
|  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.

Then, he asked, with what weapon is the ironclad going to vanquish these torpedo rams?  Guns cannot hit her when moving at speed; she is proof against machine guns, and, being smaller, handier, and faster than most ironclads, should have a better chance with her ram, the more especially as it is provided with a weapon which has been scores of times discharged with certainty at 300 yards.  The ironclad, he answered, must use torpedoes, and then he maintained that the speed and handiness of the Polyphemus would enable her to place herself in positions where she could use her own torpedo to advantage, and be less likely to be hit herself.  He then called attention to the necessity for well-protected conning towers in these ships, and prophesied that if a submarine ship, armed with torpedoes, be ever built, she will be the most formidable antagonist an ironclad ever had; and the nearer the special torpedo ship approaches this desideratum the better she will be.

* * * * *