eper. At a recorded depth of 123 feet the sinking of the vessel
became so much more rapid that the crew with frantic endeavours
sought at once to stop the leaks and pump out the water which had
entered. At that depth there was a pressure of 153-1/2 pounds upon
every square inch of the surface of the submarine. This the workers
at the one hand pump had to overcome. It was a savage and a
desperate struggle but the men finally won and the vessel regained
the surface. As a result of this experience every navy prescribed
submergence tests for its submarines before putting them into
commission. How to make these tests was perplexing at first. A
government did not want to send men down in a steel casket to see
just how far they could go before it collapsed. But if no observer
accompanied the ship it would be impossible to tell at what depth
leakage and other signs of weakness became apparent. An Italian
naval architect, Major Laurenti, whose submarines are now found in
every navy of the world, invented a dock in which these tests can be
made up to any desired pressure while the observers inside the
submarine are in communication with those without and the pressure
can be instantly removed if signs of danger appear. In the United
States Navy boats to be accepted must stand a pressure equivalent to
that encountered at 200 feet. In the German navy the depth
prescribed is 170 feet. Under normal conditions submarines seldom
travel at a depth of more than 100 feet although the "F-1" of the
United States Navy accomplished the remarkable feat of making a
six-hour cruise in San Francisco Bay at a depth of 283 feet. At this
depth the skin of the ship has to withstand a pressure of no less
than 123 pounds per square inch.

Specific information as to the nature of submarine construction in
the United States since the beginning of the war in 1914 is
jealously guarded by the Navy Department. In broad general terms the
number of ships under construction is revealed to the public, but
all information as to the size of individual vessels, their armour
or the qualities of novelty with which every one hopes and believes
American inventive genius has invested them, are kept secret. The
_Navy Year Book of 1916_ summarized our submarine strength at that
time as follows:

_Displacement_

Submarines fit for action 42 15,722 Tons
" under construction 33 21,09