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A "WRINKLE" TO SAVE TIME IN NAVIGATION
By COMMANDER F. D. PRYOR, U. S. Navy
When one must work in a sweat box, such as a chart house becomes when the ship is darkened, any method that shortens the work at that time is most welcome. Though there is nothing startling in the following process, it is something that no navigator I have met has used, and one of which none of the younger
officers I have come in contact with have ever heard. Therefore, having never seen anything in print along these lines and having found the process reliable and a great help, I pass it along for what it may be worth to others.
The process is simply to combine, before sunset, all the elements of the work of the haversine formula possible, leaving very little to do after star sights are taken.
To begin with, one determines the time of sunset and from that time assumes the “middle time” of the series of times of the various sights. For example, if the sunset is at 6.30, and the navigator knows from experience that it will be dark at 7.15, and that he will get his first star about 15 minutes after sunset, he can safely assume 7 p. m. as the "middle time" of his sights.
One then determines the D. R. position from which to work and obtains C-W.
The“ middle time ” of sights is converted into G. M. T., and the R. A. M. S. taken out of the almanac and corrected for this Greenwich interval, giving the correct R. A. M. S. for the "middle time."
This R. A. M. S., the C—W, C. C. and the longitude (expressed in time) are then all combined, being careful to observe the algebraic signs. (C. C. may be plus or minus, longitude will always be minus when west, and plus when east.) The result is a "constant" to be used in all the sights. The watch time of sight plus the “constant ” gives the L. S. T. at once.
Besides the “constant” it is possible to do much else to shorten the time in the sealed up chart house.
One decides what stars he will observe and the declinations and right ascensions of these stars are taken out and entered in the forms. The cosines of the latitude and the various declinations are taken out entered in the forms and added up and the sum entered in the sight form, L-D is determined and the nat hav of this taken out and put down in the form.
Having done this preliminary work, when the stars are observed the following only remains to be done: (1) Correct the observed altitude; (2) add the “constant” to watch time of sight; (3) combine the L. S. T. thus obtained with the R. A. and obtain t. The log hav of t is added to the previously combined logs of L and D (the addition of two numbers is simpler and therefore faster than three), and the nat hav taken out and added to the nat hav of L-D
Below is an example showing the saving it is possible to obtain in a sight:
SHORT METHOD (All work that can be done in p. m. is entered in ordinary type; evening
work in black face type.) P. M. Star LINE, 19 May, 1918.
I0 42 00 N.
1" 21" 48"
Obs. h 34 44 30 C-W
True h 34 38 00 C. C. ( )
6 40 00 G. M. T.
Corr.=(C-W+C.C.) 1 15 26 Z 74° true by Weir
diagram of course Long.()
R. A. M. S.
3 45 23 C-W2 L. M. T.
R. A. M, S.
3 46 41 ^ W. (-) I 21 48 Corr. (-)
0 06 22+ Corr. (-) 0 06 22 * Corr. 1 340 19
3 40 19 - This is the conL. S. T, 10 25 19
stant used in all *R. A.( ) 14 11 58
19.96644 d 19 36 18 N. log cos
19.97406 log hav
9.31904 19.96644 nat hav
7 55 26
34 38 00
II 10 away.
* These may also be combined in p. i. if desired.
Obs. h 34 44 30
7 59 86
Weir Z 74° true
P. M. STAR LINE, 19 May, 1918.
Long. 20 27 oo W.
I 21 48 W
6 45 00 C-W
I 04 32
10 42 00 N. log cos 9.99238
log hav 9.31898
nat hav .20844 L-d
8 54 18 nat hav .00603
55 10 35 nat .21447 h (comp.) 34 49 25 h (obs.)
34 38 oo
II 25 away.
THE PHYSICAL CHARACTERISTICS OF THE
By G. W. LITTLEHALES
As a result of the marine hydrographic coast surveys of the various maritime countries, undertaken in the interests of navigation, the coast line of the world is now among its best known geographical features, and hence, since geodesists have determined the size of the terrestrial spheroid, the area of the oceanic surface of the globe is known with closeness to be 140 million square statute miles, which is an expanse exceeding the total area of the lands of the globe by 83 million square statute miles. In other words, 71 per cent of the surface of the globe is covered by the waters of the ocean. Although the depth of the ocean has been measured in many thousand places throughout the world, since the middle of the nineteenth century when deep-sea soundings first began to be successfully made, yet, as will be seen from the accompanying world-chart showing where the ocean has been sounded, there are oceanic areas as large as the United States where no soundings have been taken, and many others where the present soundings are but widely spaced. The contours of the oceanic basins cannot, therefore, be completely drawn at present, and the volume of the ocean may only be stated by estimation to be 324 million cubic statute miles, or 14 times the bulk of all the lands in the world above sea level. The accepted measurements of the areas within the different zones of depth are shown in the diagram on page 46.
The mean depth of the ocean is estimated to be 2080 fathoms; and the greatest depth, which is found east of the Island of Mindanao, is 5348 fathoms.