Again,
provision will have to be made for sending the water through the
propeller in such a way that it shall have as little as possible of
the motion of the ship imparted to it. But as soon as we begin to
reduce these principles to practice, it will be seen that we get
something very like a paddle wheel hung in the middle of the boat and
working through an aperture in her hull, or else a screw propeller put
into a tube traversing her from stem to stern.
We may sum up by saying that the hydraulic propeller is less efficient
than the screw, because it does more work on the water and less on the
boat; and that the boat in turn does more work on the water than does
one propelled by a screw, because she has to take in thousands of tons
per hour and impart to them a velocity equal to her own. Part of this
work is got back again in a way sufficiently obvious, but not all. If
it were all wasted, the efficiency of the hydraulic propeller would be
so low that nothing would be heard about it, and we certainly should
not have written this article.--_The Engineer._
* * * * *
THE NEW ARMY GUN.
The cut we give is from a photograph taken shortly after the recent
firings. The carriage upon which it is mounted is the one designed by
the Department and manufactured by the West Point Foundry, about six
months since.
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