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The result is two noon positions, the first without considering the current, which in the other is taken into account.
The current could be considered on each course, but such a degree of precision is seldom necessary.
The ship's position differs slightly from that found by the preceding method, but is quite near enough to meet the demands of practice.
Lat. Pro. L. 21° 17'. N. Long. Pro. L. - 89° 39'-5 W.
25 02.0 N.
90 57.2 W. Dif. 225'.0= 3 45.0 N. Dif.
77'.7= I 17.7 W. Mid. lat. 23° dif. long. 77'.7 = dep. 71'.8
Lat. ship - 25° 02'. N. Long. ship - 90° 57' 2 W. Lat. H. B. L. S.
29 06.1 N.
Long. H. B. L. S. 94 12.4 W. Dif. 244'.1= 4 04.1 N. Dif. 195'.2= 3 15.2 W. Mid. lat. 27° dif. long. 195'.2= dep. 173.7
Dif. lat. 244'.1 N. ?
293:7 W.}=Co. N. 35° W. dist. 298.0
By this method the distance is in error about 1'.4, which is quite satisfactory in practice. However, the distance found by Mercator sailing by inspection is in error by about the same quantity, which at once indicates that logarithms should be used in place of the inspection method.
FOR THE STUDENT
Bowditch Navigator with Tables
FOR SALE BY ALL DEALERS
R. M. PUGSLEY
17 South Street
New York 67, 35. The Log and Line.
This instrument is now known as the "old-fashioned log" on account of the logs in use now, having taken its place generally; but strange as it may seem, the old method of ascertaining the speed is still in use on board some ships. While it is not so convenient as the instruments more generally used for the purpose, it gives the speed correctly at the time it is used.
With its many disadvantages, it has one important advantage over the logs now used. That is, owing to its simplicity, one can be very easily and quickly made in case the modern log could not be had; a circumstance that most any officer is liable to encounter, and for that reason it is necessary that every officer should know how to make and use the "old-fashioned log.”
The contrivance is composed of the log chip, line, reel and glass. The second hand of a watch may be used in place of the glass.
The Log Chip.
This part of the instrument consists of a quadrantal piece of thin board about seven inches radius, the curved side of which is loaded with lead to make it float upright. One hole is in each corner of the chip, and through the upper one the log line is rove and knotted. . Both ends of a piece of line about ten feet long the same size as the log line are rove through the two lower holes and form a bridle. Exactly in the middle of the bridle a toggle is seized and this toggle fits into a socket seized on to the log line at the same distance from the chip as the toggle is.
The object of this toggle and socket is to reduce the labor of hauling the chip in, as the toggle will pull out as soon as the line is checked, which is done as soon as the glass runs out. The chip will then lie flat on the water and very little strain will be on the line.
The Time Glass.
The time glass may be for any number of seconds. Those used are fourteen, twenty-eight and thirty seconds. It is not uncommon to find a time glass in error a second or two, which may be determined by comparison with the second hand of a chronometer or watch. When the sand in the glass is dry, it will run faster than when damp. Usually it will be dry or damp according to the weather, a matter that must be considered.
The Log Line. The line is marked off at certain intervals, the length of which depending on the number of seconds in the glass used.
Measuring from the chip, allow about fifteen fathoms for stray line, and put in a red rag as a mark. From this mark the knots are measured. Their lengths are as follows:
For a fourteen second glass, the length of a knot is twenty-three feet and seven and three quarter inches.
For a twenty-eight second glass, the length of a knot is forty-seven feet and three and one half inches.
For a thirty second glass, the length of a knot is fifty feet and eight inches nearly.
The line is measured and marked when wet and tested by frequent measurement. Put one knot in the first mark, two in the second and
For half knots use a piece of cod line without
If the line is marked for a twenty-eight second glass and a fourteen second glass is used, it is only necessary to double the number of knots run out to get the speed.
The length of knot is found on page forty-eight, by using logarithms and the following shows the same result without their use.
The proportion is—As the number of seconds ( 3600) in an hour is to the number of feet (6080) in a nautical mile, so is the number of seconds (14, 28 or 30) in the glass to the number of feet ini a knot.