[COPYRIGHTED] U. S. NAVAL INSTITUTE, ANNAPOLIS, MD. ANOTHER "WRINKLE" IN NAVIGATION By COMMANDER H. K. HEWITT, U. S. Navy There are many conditions under which it is desirable to have as much work as possible completed before the sight is actually taken. Most navigators are entirely familiar with this situation and it is unnecessary to go into the many advantages often derived therefrom. Under the conditions upon which one can decide beforehand what stars he will use, with a little experience, he can also predict, within a few minutes, the time of his observation. Having selected a watch time, and a position point, he can, using Marcq Saint-Hilaire or Aquino, work out the whole sight beforehand and have his azimuth and computed h ready. Going further, the altitude correction can be picked out and applied to this computed h with sign reversed, giving a computed sextant h. Setting the latter on the sextant, the star is readily picked up, and the necessity of setting on zero and then “bringing down" the star or "bringing up the horizon " is eliminated. This means much, especially to a destroyer navigator in a seaway. The above work being completed, nothing remains but to observe the star at the selected time, apply the altitude obtained to the computed sextant altitude for the altitude difference, and plot. One can even go so far as to have the position point and azimuth 'line ready plotted. The foregoing can, of course, be used for a sight of the sun as well as of any other heavenly body. Moreover, should one, for some reason such as clouding over, be unable to make the observation exactly at the selected time, his work is not lost, provided he can get his altitude within a few minutes of this time. Should the watch time be different from that selected, it is simply necessary to shift the longitude of the position point by a like amount in order that the t used in the computation will remain unchanged. Following this through, it will be seen that this is simply a matter of converting the time difference into arc, and applying it to the assumed longitude, west if the sight was taken late, and east if taken ahead of time. The limit of time difference that can be safely used without introducing an appreciable error depends principally upon the rate of change of azimuth. If the hour angle is large and the azimuth changing slowly a difference of two or three minutes is not too much. However, should the hour angle be small, and azimuth be changing rapidly, the time difference should be kept as small as possible. If the navigator is one who prefers the average of a series of observations, instead of one observation, he can take his observation so that the average watch time will be near the selected time, and apply his time difference as before. There is nothing new or original about the above, as I believe it to be used by a number of navigators. It is not as flexible in the matter of time as the method proposed by Commander Pryor, but where it can be used, has many advantages. An example is given: Suppose that it is decided to observe star Sirius at 7" oo" oo" p. m., watch time, March 1, 1919. Position point: Lat. 18° 56′ N., Long. 74° 50′ W. All the above work is completed before taking the sight. Now suppose, for some reason, that watch time of observation is 700 35' instead of 7" oo oo". m Diff. 35 later = 8'.8 W. X' is then used in plotting the line. [COPYRIGHTED] U. S. NAVAL INSTITUTE, ANNAPOLIS, MD. HAULING 'EM OFF BEFORE DINNER By CAPTAIN R. DRACE WHITE, U. S. Navy The skies are blue in the island of Jamaica. Blue with a blue that comes only from chalk soil underfoot and a cloudless dome overhead. The air is balmy, the flowers are bright, and a comfortable laziness envelops you from the time you finish your bath before breakfast till you take your swim before dinner. Not an overpowering laziness, however. The well-kept tennis courts and the genuine, enthusiasm that pervades the atmosphere at the Tennis Club sound a call that is irresistible to the fan. The golf is fair. The roads are a motorist's delight. And last, but not least, the so-called invasion of personal liberty exemplified in the latest nervous stride of a more powerful and prosperous community on this side of the Atlantic in its forced march toward higher civilization has left Jamaica untouched-and exchange is only $3.26 to the pound. It was in the midst of this setting that the Prometheus found herself on February 7, 1920, in the harbor of Kingston. Her mission (to quote verbatim from her orders) was, among other things," to promote contentment." Practical interpretation meant principally to give liberty to the crew. It was a fulfillment of the "enlist and see the world" promise so prominently featured in recruiting propaganda. It was also probably intimately related to the "improve the morale" campaign now prominently vogue, which more frequently, however, finds expression in intensely moral motion pictures produced by the greatest motion picture producers in captivity. Be that as it may, Kingston on that particular day bade fair to prove herself an ideal spot for the promotion of contentment. A motor ride and picnic had been arranged for the men. There was a tea and a dance for the officers. And for me there was to be a dinner. Not an ordinary |