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88
THE SUBMARINE TORPEDO BOAT
a tender or cruiser if in their company and opportunity
affords, but it must not be necessary to put another vessel
to the trouble and disadvantage of refilling tanks, etc.,
when the moment is not strictly opportune.
The radius of action submerged should be sufficient to
enable a successful under-water attack and a safe retreat.
In this respect it is my belief that to cover a distance of
thirty or thirty-five miles in two hours is of a greater mili-
tary advantage than to be able to cover fifty miles in seven
hours.
Air storage capacity should be supplied to enable enough
compressed air to be carried to supply the crew for seventy-
two hours with fresh air for breathing purposes when the
vessel is at rest under water.
Navigation
The problem of navigation has been greatly simplified
in recent years by the advent of the gyroscopic compass.
Before this valuable addition to the equipment of a sub-
marine, navigation was more or less a combination of dead
reckoning and luck, by reason of the fact that the ordinary
magnetic compass could not be relied upon to any extent
whatever. The hull being of magnetic material and there
always being present large electrical currents, fluctuating
and under various conditions, brought about variations
and deviations of the magnetic needle which one would not
realize until he has experienced them personally. The
character of these deviations made it impossible to com-
pensate and correct for with any degree of accuracy.
Attempts were made to overcome these difficulties by
mounting the compass in a composition helmet outside
the hull, and fitting with a reflector to cast the image down