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Almanac, the elements given are for Greenwich mean noon, or 6 hours ahead of central time. In cases where strict accuracy is required, these elements should therefore be corrected for the longitude in time, multiplied by the difference for 1 hour; for azimuth determinations, however, such corrections are unnecessary, particularly so when observations are made early in the forenoon when the Greenwich mean time and local mean time are approximately identical. In the evening the mean of the elements of the day of observation and the succeeding day are approximately correct, but owing to the fact that the means of obtaining the compass bearing are liable to an error of a quarter of a degree, the error made in finding the true bearing by using the declination 10 minutes wrong, or the equation of time 10 seconds in error, sinks into insignificance. Therefore, the elements taken from either the Nautical Almanac or its auxiliary, for the purpose of azimuth determination, may be used without any correction. The price of this publication is 10 cents; it can be obtained from the Hydrographic Office or an agency.
1. On July 12, 1901, at 8.14 A. M., central time, in latitude 45° N and longitude 82° W, the bearing of the sun's center by the standard compass was N 78° E. Find the deviation, assuming the variation by chart to be 5° W.
Ans. 29° 26' E. 2. On May 21, 1901, in latitude 42° N, the bearing by compass of the sun's center at setting was WNW W. Find the deviation of the compass, assuming the variation of the locality to be 3° W.
Ans. 14.2° E. 3. On April 22, 1901, at 3.41 P. M., eastern time, in latitude 45° N and longitude 80° 30' W, the bearing by compass of the sun's center was S 88° W. Assuming the variation of the locality to be 6° 10' W, find the deviation of the point on which the ship is heading. Ans. 13° W.
4. On July 3, 1901, in latitude 47° N and longitude 86° W, the sun's bearing by compass at rising was N 72° E. Find the deviation, applying the variation of the locality as found on the variation chart.
Ans. 17.8° W. 5. On April 25, 1901, in latitude 45° N and longitude 83° W, bearing by compass of the sun's center, taken at 2.30 P. M., central time, was S 70° W. Apply the local variation as found on the variation chart and find the deviation of the compass.
Ans. 2.5o W.
USE OF THE PELORUS IN LAYING THE SHIP'S
34. At sea it is often desirable that the navigator have a convenient method of finding at any time during a clear day, the error or deviation of the compass for any desired point. This may be done by means of the pelorus in the the following manner:
On the date of observation select, beforehand, a suitable hour of local apparent time, and find also, in advance, the position of the ship for the hour in which the observation is to be made. With the latitude of the position selected and the declination, enter the Azimuth Tables and find the true azimuth, or true bearing, of the sun for the selected hour of apparent time; apply to this true bearing the variation of the locality; the result will be the magnetic bearing of the sun for the time selected. Shortly before the time selected, and when the ship has reached the position previously decided on, set that point of the pelorus corresponding with the required magnetic direction to the ship's head, and turn the sight vanes of the instrument to correspond with the magnetic bearing of the sun previously found. Then clamp the plate and sight vanes of the instrument. Turn the ship by means of the rudder until the sight vanes are directed toward the sun, and keep them in this position until the exact instant of the local apparent time selected. At that instant the ship's head will correspond with the correct magnetic direction required; any difference shown by the compass at that instant will be the deviation for that heading.
Illustration. Let it be required on July 11, 1901, to head the ship correct magnetic north at, for instance, 7.20 A. M., local apparent time. At the hour selected, the ship is estimated to be in latitude 45° N and longitude 82° 30' W, or about 37 miles W S W of Cape Hurd, the variation of that locality being about 3° 30' W.
Proceed as follows: First find from the Azimuth Tables the true azimuth corresponding to the selected apparent
time, the latitude of the place, and the declination for the given date. The declination according to Table I is N 22° 9'; the true azimuth is therefore N 87° 20' E. The variation applied to this will give the correct magnetic bearing of the sun at 7.20 A. M., local apparent time. Thus,
True azimuth = N 87° 20' E
Variation = 3° 30'W
N ' Now, supposing the clock to indicate central time, it becomes necessary to find the clock time that corresponds to the selected apparent time. This is found by applying to the selected apparent time the equation of time and the correction for longitude with contrary signs, as follows:
Selected L. App. T. = 7h 20m A. M.
6h 55m A. M. Having reached the locality decided on before 6.55 A. M., central time, set the north point of the pelorus to correspond with the ship's head, and the sight vanes to N 90° 50' E, or S 89° 10' E; clamp both plate and vanes. A few minutes before 6.55 A.M., turn the ship so that the vanes point directly toward the sun; keep them in this direction by means of the helm until the clock or watch set to central time shows 6h 55m A. M. At that instant the ship is heading correct magnetic north. Suppose the standard compass at that time shows NW; the deviation will then be į point or 5.5o E, because the correct magnetic bearing falls to the right of the compass bearing.
In this manner, it is evident that the ship's head may be laid in any correct magnetic direction and the deviation for any heading of the ship determined. In practice, it is usual to select not only one instant of time, as in the preceding illustration, but to choose several instants of apparent time at intervals of, say, 10 minutes, and to find the magnetic bearing of the sun at each instant.
36. Fields's Course Corrector. – Any form of pelorus may be used in the operation just described, and the selection of a particular instrument is of course optional to the user. Among the instruments recently introduced, and to which our attention has been called, is one called Fields's compass and course corrector, named after its patentee, Mr. J. M. Fields. This instrument, it is claimed, possesses many advantages over other instruments of the same class. Its dial consists of outer and inner circles, the former being graduated to degrees and the latter to points; the inner circle, which is reversible, has compass graduations on both sides. One side is called the shadow dial, and is used in combination with a shadow pin, which, when the instrument is set according to certain rules, will by its shadow indicate the magnetic course the ship is heading; the reverse side, which is called the arrow dial, is used in combination with a sighting bar that turns independently of both. Instructions for use accompany each instrument.
NOTES RELATING TO THE PROPULSION
PITCH AND SLIP OF THE SCREW PROPELLER
37. Explanations. – If a point be caused to rotate at a uniform distance from and about an axis, and if the point at the same time be caused to advance at a uniform rate in the direction of the axis, its path will be a helix. If the point, when moving away from the observer, moves in the direction of the hands of a watch, the helix will be right-handed; if in an opposite direction, left-handed. The distance the point advances in one complete revolution is known as the pitch. If a line passing through the axis be caused to rotate about the axis, and to pass along the path of the point mentioned above, its path will be the surface of a true