Special Publications, Ausgabe 6

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U.S. Government Printing Office, 1900
 

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Seite 21 - Local magnetic disturbance of the compass on board ship. — The term "local magnetic disturbance" has reference only to the effects on the compass of magnetic masses external to the ship.
Seite 12 - The glare of a powerful light is often seen far beyond the limit of visibility of the actual rays of the light, but this must not be confounded with the true range.
Seite 13 - ... even in clear weather. Therefore too much confidence should not be felt as to hearing a fog signal.
Seite 13 - ... hearing a fog signal. The apparatus, moreover, for sounding the signal often requires some time before it is in readiness to act. A fog often creeps imperceptibly toward the land, and...
Seite 21 - Observation shows that disturbance of the compass in a ship afloat is experienced only in a few places on the globe. Magnetic laws do not permit of the supposition that it is the visible land which causes such disturbance, because the effect of a magnetic force diminishes in...
Seite 19 - The ordinary form of it, the so-called "bow and beam bearing," the distance from the object at the latter position being the distance run between the times of taking the two bearings, gives...
Seite 19 - If communication can be had with the shore, positions may be fixed with great accuracy by occupying with theodolite or sextant two known points of the chart. The third angle of the triangle, that between the two points at the position sought, should be measured as a check. The compass. — It is not intended that the use of the compass to fix the ship should be given up; in ordinary piloting the compass, with its companion, the pelorus, may be usefully employed for this purpose, although less accurate...
Seite 19 - In all such cases angles should be taken to several objects, the more the better; but five objects is a good number, as the four angles thus obtained not only prevent any errors, but they at once furnish a means of checking the accuracy of the chart itself.
Seite 11 - Caution in using small-scale charts. — In approaching the land or dangerous banks, regard must always be had to the scale of the chart used. A small error in laying down a position means only yards on a large-scale chart, whereas on one of small scale the same amount of displacement means a large fraction of a mile.
Seite 21 - ... of magnetic force of an amount absolutely unknown to affect a compass half a mile distant. Such deflections of the compass are due to magnetic minerals in the bed of the sea under the ship, and when the water is shallow, and the force strong, the compass may be temporarily deflected when passing over such a spot, but the area of disturbance will be small, unless there are many centres near together.

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