88 |
--------------+---------+-------------+------------+-----------+
|<------------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.

* * * * *

A PLU