Jim -- possibly you are referring to the two pelorus bases. A pelorus is a sighting instrument used to take relative bearings of objects around the ship. It has a compass card fixed so that the 000/360 mark (North) is always pointing toward the bow. The card does not pivot and has no magnets. Instead, it is used in conjuction with a sight vane to measure the angle of objects seen from the bridge.
A pelorus is useful in navigation for certain bearing combinations which allowed the navigator to determine distance off short. Or, other bearing combinations could be used to place the ship a desired distance off a headland.
The easisest of these bearing combinations is the "bow and beam." If a bearing is taken of an object (say a lighthouse ashore) at 45 degrees relative to the ship, and a second taken when the object is abeam (90 degrees relative); then the distance of equals the distance run between bearings.
If anyone is interested in this sort of geometry, look up "Rule of 60" in any navigation text.
A pelorus can also be used in relative motion problems. For instance, by using a series of bearings and a polar plotting sheet it is possible to predict the "closest point of approach" (CPA) of another ship. This sort of plot will tell in advance if collision is likely.
If fitted with the proper sight mechanisms, a pelorus can also be used to take azimuths of celestial objects. These can be used to check the compass. It is unlikely the pelorus instruments on Titanic were used in this manner as a specific sight for azimuths was fitted to the standard compass.
Titanic had two pelorus instruments because the bridge did not have 360-degree sight lines. There was no single place where the entire horizon could be seen. So, two instruments were fitted giving coverge from dead ahead to well aft on both sides. Neither pelorus was useful for objects dead astern.
--David G. Brown
