NAVIGATION. 



[SAILINO IS CtTERK!fT8. 



mid. latitude, or Heritor's nailing be employed, u 25 



\ ., and the latitude and longftad* left Wing given, 



the direct course and distance, and the corresponding 



departure, may be readily found by Mercator's tail- 



<hip, from latitude 63 30* N., and longitude 

 14 38' W., ha* nail<d tin- following conned, namely 



1st E.S.E.,431 : mil.-s : :rl. S. E. 



f.s mile* ; 4ti 00 mile*, Required tho 



latitude aii>: in. 



An*. Latitude, 49" 67' N. Longitude, 13 67' W. 



.-.]., fn.in latitude 62 30' N., and longitude 

 21 45' W., has aailcd the following courses, namely 



1st, N.E , 36 miles ; 2nd. X. '> W., 14 miles ; 3rd, N.E. 

 b E.* B., 68 miles ; 4th, N. /. K ., 42 miles ; 5th, E.N.E., 

 29 miles. Required tho latitude and longitude in. 



Ann. Latitude, 64 35' N. Longitude, 18 4' W. 



4. A ship, from latitude 66 14' N., and longitude 

 3 12' E., him nailed as follows : 



1st, X.N.K. i E, 4r, miles ; 2nd, N.E. 4 E., 28 miles ; 

 3rd, N. J W., 52 miles ; 4th, N.E 6 E. J E., 57 miles ; 

 r.th. 1 : s. 1 : , _' I miles. Required tho latitude and longi- 

 tude in. 



Ans. Latitude, C8 24' N. Longitude, 7 54' E. 



6. A ship, from latitude 67 30' N., longitude 8 46' 

 \V., has sailed the following courses, namely 



1st, N.E. ,64 miles; 2nd, N.N.E., 50 miles ; 3rd. N.W. 

 b N., 88 miles; 4th, W.N.W., 72 miles ; 5th, W., 48 

 miles ; 6th. S.S.W., 38 miles ; 7th, S. b E., 45 miles ; 

 J<tli. !'.. .-. !' . 10 miles. Required her present latitude 

 ami longitude by Mercator's sailing. 



Ans. Latitude, 68 43' N. Longitude, 11 43' \V. 



I.IM; is CORRECTS. In what has preceded, the 

 course of the ship is supposed to have been indicated by 

 the compass, and that she has actually moved through 

 the water in the direction of her length. It is plain. 

 however, that if a current act upon the ship, she will bo 

 diverted from the course shown by the compass, unless 

 the set of the current be itself in the direction of the 

 ship's length, when the rate of sailing only will be inter- 

 fered with. 



The sr t of a current is the point of the compass tovxirdt 

 which tho stream runs ; its rate, or velocity, is called the 

 drift of the current 



The common way of ascertaining the set and drift of 

 a current unexpectedly met with at sea, is to take a 

 boat, if tho weather permit, a small distance from the 

 ship ; and, in order to keep it from driving with the 

 stream, to let down, to the depth of about 100 fathoms, 

 a heavy weight attached to a rope fastened to the stem 

 of the boat ; steadiness being in this way secured, tho log 

 is hove into the current ; the direction in which it is 

 carried is observed by means of a boat-compass, and the 

 number of knots run out in half a minute gives the hourly 

 drift In this way the set and velocity, or drift, of the 

 i-nn.-nt is estimated. 



The set of the current being ascertained, we can apply 

 it to correct the compass-course of tho ship ; and the 

 drift, or velocity, of the current being known, wo can 

 apply it to correct the ship's rate of sailing as indicated 

 l>y the log. The rate indicated by the log, it will be ob- 

 1, is only the rate at which the ship moves faster 

 tli. in tho log moves in the ship's direction, and, in still 

 water, is the velocity of tho ship itself, since the log 

 remains stationary ; but, as in a current, the log also 

 moves in the direction and with the rate of the current, 

 the absolute velocity of tho ship cannot be directly de- 

 termined by tho log in the customary manner : yut the 

 distance sailed, as indicated in this way by tho log, and 

 tho coarse steered as indicated by the compass, though 

 l'th modified by the action of the current, and tlin.-- 

 fore both inaccurate, are necessary helps to the determi- 

 nation i.f tho true course and distance. 



K.r the ship has, in fact, been making a sort of double 

 course : the wind has carried her on a certain course 

 (nh'iwn by the compass) a certain distance ; and the cur- 

 rent has carried her a certain other course and <li 

 in the same timi- ; t .n 1 ,' the same as if tho 



two courses and distances had been sailed in succession ; 



so that the case becomes a very simple one of traverse 

 tailing, as in tho following example. 



pin, 



1. A ship runs N.E. b N. 18 miles in three houn, in a 

 current settiu-,' \V. b S. two miles an hour. Required 

 the course and distance made good. 



This is evidently the same as saying A ship sails tho 

 following courses, namely 



1st, N.E. b N., 18 miles ; 2nd, W. b S., 6 miles. Re- 

 quired the course and distance. 



TRAVERSE TABLE. 



And, with the difference of latitude and departure thus 

 found, we have, by plane sailing, or by tho traverse til>le, 

 the distance made good, 14 miles, and the course 1 j 

 points, or N. 6 E. 4 E. 



2. A ship sails N.W. 60 miles, in a current that sets 

 S.S.W. 25 miles in the same time. Required tho course 

 and distance made good. 



Ans. Course, N. 69 38' W. Distance, 65 i miles. 



3. A ship has sailed the following courses and distances 

 in twenty-four hours : 



1st, S.W., 40 miles; 2nd, W.R.W., 27 miles; 3rd, S. 6 

 E., 47 miles ; but she has been the whole time in a cur- 

 rent setting S.E. b S. at the rate of 1$ mile an hour. 

 Required the ship's direct course and her distance made 

 good. Ans. Course, S. 6 W. Distance, 117 miles. 



4 . A ship has sailed by reckoning N. \ W. 20 miles ; but 

 it is found by observation, that, owing to a current, she 

 hits actually sailed N.N.E. 28 miles. Required the set 

 of the current and the amount of drift upon the ship. 



Aus. Sot N., 64 48' E. Drift, 14 miles. 



The method described above, of estimating the set and 

 drift of a surface-current, by sinking a weight, and ob- 

 serving the direction and velocity of tho log, is often 

 likely to lead to conclusions not strictly correct. Lieut. 

 Walsh and Lieut. Lee, of the United States' Navy, while 

 carrying on a system of observations in connection with 

 the wind and current charts, had their attention directed 

 to the subject of submarine currents, upon which they 

 made some interesting experiments. A block of wood 

 was loaded to sinking, and, by means of a fishing-line, 

 was let down to the depth of one hundred and five 

 hundred fathoms. A small float, just sufficient to keep 

 the block from sinking farther, was then tied to tho Hue, 

 and the whole let go from the boat. 



To use their own expressions " It was wonderful, in- 

 deed, to see this barrtga, move off, against wind, and sea, 

 and surface-current, at tho rate of over one knot an hour, 

 as was generally the case, and on one occasion as much as 

 1J knot. The men in tho boat could not repress excla- 

 mations of surprise, for it really appeared as if somo 

 monster of the deep had hold of the weight below, and 

 was walking off with it." 



The effects of such under-currents must sometimes in- 

 terfere with the results, deduced in the ordinary way, for 

 the set and drift of a surface-current, since to hi. Men 

 influences, of the existence of which tho observer has no 

 suspicion, part of what ho attributes to the surface-cur- 

 rent may be really due; the boat, though apparently 

 steady, may have an imperceptible drift. Such drifts 

 are not caused so much by the action of the under-cur- 

 rent upon tho sunken weight, as by the bellying of the 

 line in the direction of the set The surface current, 

 too, if of any considerable depth, may operate in a similar 

 way. 



Admin] Kir Francis Iteaufort, when in tho ModiS-r- 

 in, made several experiments on this subject. Ho 

 says 



