Scientific American Supplement, No. 484, April 11, 1885 | Page 2

the character of the site. On the other hand, a cantilever or
continuous girder bridge in three spans--although such structures have
been erected in similar localities--could not enter into comparison of
simple economy of material, because such a design would entirely
disregard the anomaly that the greater part of the structure, viz., the side
spans, being necessarily constructed to carry across a large space,
would be too near the ground to justify the omission of further supports.
The question was, therefore, narrowed to a comparison between the
present arch and a central independent girder of the same span,
including the piers on which it rests. 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.
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 PLUMBING TEST.
A recent trial of a smoke rocket for testing drains, described by Mr.
Cosmo Jones in the _Journal of the Society of Arts_, is deserving of
interest. The one fixed upon is 10 in. long, 2½ in. in diameter, and with
the composition "charged rather hard," so as to burn for ten minutes.
This gives the engineer time to light the fuse, insert the rocket in the
drain, insert a plug behind it, and walk through the house to see if the
smoke escapes into it at any point, finishing on the roof, where he finds
the smoke issuing in volumes from the ventilating pipes. The house
experimented upon had three ventilating pipes, and the smoke issued in
dense