Again, the navigator takes the angle of the sun from the bridge or some other elevated part of his ship. The angle he gets from such a height is slightly different from the one he would get if he were at the water level. Therefore he must make a correction for the difference. This he finds by knowing his elevation above the water and looking up the correction.
USING A PELORUS
This apparatus consists of a movable plate marked with compass bearings, set in a stand. The observer sets the plate to correspond to the standard compass, and then sights across it in determining the compass bearings of points ashore from which he wishes to learn his exact position.
There are other corrections still, applying to the sextant angle, to the sun itself, and to time. All of these are necessary if one wishes to be accurate, and a navigator should always be as accurate as his science permits.
But often it is impossible to learn the angle between the horizon and any of the celestial bodies, for clouds and fog sometimes shut off the sky and the horizon. Sometimes one is clear while the other is obscured; sometimes both are hidden. But still it is necessary to know the position of the ship. As a matter of fact, the heavier the clouds or the thicker the fog the more desirable it is to know one’s position accurately. Until recently, however, seamen have had to depend only upon dead reckoning which often is anything but accurate. But now the radio direction finder and the method of learning one’s position by asking radio stations ashore to supply it by plotting the directions from which one’s radio message reaches two or more of them are coming into more and more common use.
Dead reckoning however, is still highly important, and is used by every careful navigator. It requires considerable experience for a navigator accurately to place his ship by dead reckoning alone. As a matter of fact, if the voyage is long and the sky has been obscured, the navigator expects to find himself somewhat wrong in his estimation of his position and is correspondingly careful. He has had to depend upon his log, which, as I explained in the last chapter, is a kind of nautical speedometer. As a check against this he often keeps a record of the revolutions of his propeller, for he knows, from experience, how far he will sail in an hour with his propeller running at any given speed. This is advisable because seaweed may foul the rotator of his log, or driftwood tear it away or bend it.
In addition to the distance he has sailed he must know accurately the direction he has sailed, and if he has changed his direction he must know when and how much. Furthermore, he must study his charts carefully in order to learn whether or not he is sailing in a part of the ocean in which there are currents, and if so he must figure out very carefully what effect the current has on his ship.
Suppose a ship was sailing by dead reckoning across the Gulf Stream directly east of Cape Hatteras. The Stream, let us say, is 100 miles wide, and he is ten hours in crossing it. The current flows at the rate of three miles an hour. Therefore, if he has headed straight across, the current has carried him thirty miles to the northeast, and unless he knows how wide the stream is, which direction and how fast it flows, and how long he has been in it, he cannot possibly know just where he is. It is as if you tried to cross a river in a rowboat and pointed its bow at right angles to the shore all the way. The current would certainly carry you downstream, so that you would not land on the opposite side directly across from where you started.
When it is necessary, then, for seamen to sail their ships entirely by “dead reckoning” they are always anxious to check up their positions by any outside aids that are available. It was for this reason that our captain, on the imaginary voyage we took from Philadelphia to Havana in the last chapter, sailed so close to Diamond Shoal Lightship instead of crossing the Gulf Stream and heading out to sea.