208 THE COMMERCIAL GYROSCOPIC COMPASS. 
dled by electric lifting magnets, usually retain more or less magnetism and strongly 
influence the ship’s compasses. A cargo of iron ore, some of which may be mag- 
netic, often induces strong disturbances. 
It may not be generally known, but a perfectly innocent copper cable is one 
of the greatest enemies of the magnetic compass; for instance, in a submarine, 
where we have three thousand amperes direct current conducted by such a cable, 
the magnetic field set up will disturb a compass located at a remote point. One 
officer recently stated that it would disturb a compass in a “‘ten-acre lot,’ but it 
is evident that in the confines of a submarine it is impossible to get the compass at 
any such remote distance. The compass under the influence of this field often 
turns through 180°, and its magnetic conditions are frequently so changed that it 
never fully returns after the current has been cut off. 
Again, repairs to the ship’s hull, or alterations in the bulkheads invariably 
affect the ship’s magnetic moments, as is also the case when the ship is struck or 
jarred strongly, so that frequently a complete readjustment of the ship’s magnetic 
compasses is necessary. On the Great Lakes, as well as at other points of the 
globe, numerous local magnetic disturbances exist, which often throw the mariner 
off his course and probably serve to explain some of the serious accidents that 
occur, especially in fog, when observation is not feasible. 
Another difficulty with the magnetic compass is that without warning it may 
suddenly revert to any one of its well-known vagaries, at a time when the ship is 
out of sight of bearings and observation is impossible, as in fog. This may be 
brought about even by an internal accident, jarring of the ship, or change of 
temperature. On one occasion the firing up of an additional bank of boilers and 
the heating up of a stack, though located quite remotely from the compass, without 
warning caused a fourteen degree deviation. This was discovered only just in time 
to avert disaster. 
The electrical difficulty cited is true in degree of all direct-current wiring of all 
ships and yachts, but the electrical conditions and the electrical circuits do not in 
the slightest influence the gyro-compass. In the make-up of this compass the sup- 
ports are all mechanical and no mercury or similar liquid is required. 
The compass incorporates as an essential feature an indicator of its true 
meridional bearing. What a boon it would be to the magnetic compass if it could 
be supplied with an indicator, a glance at which would show at once whether the 
compass was giving a bearing that was true or false; in other words, whether it 
could be depended upon or not. 
All of our gyro-compasses are supplied with just such a device, namely, an 
independent indicator that shows whether or not the compass is on the meridian. 
This has been found to be of great value in operation under service conditions, 
and to fulfill another important purpose—namely, that of aiding in quickly placing 
the compass upon the meridian by hand, which obviates the waiting for its auto- 
matically finding the meridian through its natural settling oscillations after 
starting. 
ee ee 
