TAKING A DEPAUTUEB.] 



NAVIGATION. 



10C7 



''The counter-currents, or those whicli return beneath 

 the surface of the water, are very remarkable. In some 

 parts of the Archipelago they are at times so strong as to 

 prevent the steering of the ship ; and, in one instance, on 

 sinking the lead, when the sea was calm and clear, with 

 shreds of bunting of various colours attached to every 

 yard of the line, they pointed in different directions all 

 round the compass." 



For the foregoing quotations, on the under-currents ol 

 the ocean, we are indebted to a recent publication of great 

 value and ability The Physical Geography of the Sea, by 

 Lieut Maury, of the United States' Navy, the author oi 

 The Wind, and Current Charts, in which the results of 

 extensive observation and experience, in different seasons, 

 and during a course of years, in all parts of the globe, are 

 recorded. Speaking of these charts, the distinguished 

 writer says "The young mariner, instead of groping his 

 way along till the lights of experience should come to him 

 by the slow teachings of the dearest of all schools, would 

 here find, at once, that he had already the experience of 

 a thousand navigators to guide him on his voyage. He 

 | might, therefore, set out upon his first voyage, with as 

 much confidence in his knowledge as to the winds and 

 currents he might expect to meet with, as though he 

 himself had already been that way a thousand times 

 befora"* 



OBLIQUE SAILING TAKING EEpAKTUREs.-r-From what 

 has hitherto been explained, the learner has perceived 

 that oblique-angled triangles may be dispensed with in 

 all the ordinary computations which enter into the dead 

 reckoning. But, at the commencement of her voyage, 

 before the ship is out of sight of some known point of 

 laud, or other object oil shore, the distance and bearing of 

 that object is found, and thus a first course and distance 

 obtained. This is called taking a departure. It is a 

 common piactice to observe the bearing by compass, and 

 to estimate the distance by guess ; and experienced navi- 

 gators can, in this way, come pretty near the truth : but 

 as our object is to-describe the most accurate methods of 

 performing the various calculations of Navigation, we 

 shall devote a short article to the more correct way of 

 taking a departure, which is by observing two bearings 

 of the object, and measuring, by the log, the distance 

 sailed by the ship in the interim between the observa- 

 tions. The solution of an oblique-angled triangle will 

 thus become necessary, as in the following example. 



Example*. 



L In sailing down the Channel the EdJystone Light- 

 fig 21 house bore N.W., and after running 



W. bS. 8 miles, it bore N.N.E. Re- 

 quired the ship's course and distance 

 from the Eddystone at the last place of 

 observation. 



In the annexed diagram (Fig. 21), A 

 represents the first position of the ship, 

 A. and B the lighthouse, the bearing of 

 which from A, that is, the angle of 

 N AB, is N.W., or 4 points. C is the 

 second position of the ship ; and the 

 bearing of B from this position, that is, 

 the angle NC B, is N.N.E., or 2 points. 

 Also the course of the ship in the in- 

 terim, that is, the angle SAC, is W. b 

 S., or 7 points. Hence the angle BAG 

 is 16 points 11 points + 5 points ; also, 

 since the angle S A C= N A, we have B C A= 7 points 

 -2points = 5 points. Consequently, the angle B is 16 

 points 10 points = 6 points ; so that in the triangle 

 A B C are given AC = 8, A=56* 15', and B=67 3V, to 

 find B C as follows : 



As sin. B, 67 3tf Arith. Comp. . -0344 



: sin. A, 66 iy . . . . 9-9198 



; ; A C =1 8 . . . . -9031 



iCI5= 72 . . . . '8573 

 Consequently, the distance of the Eddystone from the 



Tht Phytiml aeojraphy of the Sei. By M. F. Maury, LL.D. Lieut. 

 U.S. Nry. 18M. Introduction, p. ri. 



last place of observation is 7 "2 miles ; and, as the course 

 to the Eddystone is N. X.E., the course from the Eddy- 

 stone to the place of the ship must be S.S.W., the 

 opposite point. 



As the triangle B C A, in this example, happens to be 

 isosceles, the angle at A and being equal, a perpen- 

 dicular upon A C from B will bisect AC, so that we 

 shall have 



C B=4 sea 56 15'=4 -f- cos. 56 15'; 

 that is, C B= 4 -=--550= 7 '2, cos. 50 15'= -5,5,6)4 (7 -2 

 as in the margin. And this 389 



is an easier way of finding . 



the distance than that ex- 11 



hibited above ; but when, 11 



as is usual, the triangle is scalene, the operation by 

 logarithms, as in the specimeu above, is to be preferred. 



2. Sailing down the Channel the Eddystone bore 

 N.W. 6 N., and after running W.S.W. 18 miles, it bore 

 N. 6 E. : required the course and distance from the Ed- 

 dystone to both stations. 



Ans. Course, S. E. 6S. Distance, 21-2 miles from the 

 first station. Course, S. 6 \V. Distance, 25 miles from 

 second. 



3. Coasting along shore, a headland bore N.E. 6N. ; 

 then having run 15 miles E. 6 N., the headland bore 

 W. N. \V. Required the distance from the headland at 

 each time of observation. 



Aus. First distance, 8i miles ; second, 10'8 miles. 



4. Two ships sail at the same time from one port ; one 

 sails E. S. E. , and the other S. S. E. Required their bear- 

 ing and distance when each has run 37^ miles. 

 Bearing from first ship, N. 45 E. Distance, 28 '7 miles. 



GKEAT CIKCLE SAILING SHORTENING OF PASSAGES. 

 The great object of the navigator is to reach the port he 

 intends to, inako by the shortest route. The path de- 

 scribed by a ship, sailing on what is called a direct course 

 from one place to another, is not the shortest path, unless 

 the ship sail either on the equator or on a meridian. Tho 

 oblique spiral track, or rhumb-line, of a ship, sailing in 

 any other direction, exceeds the arc of the great circle, 

 joining the two places, by an amount that becomes more 

 iind more considerable as the distance increases. Tho 

 shortest distance between any two points on the globe is 

 the arc of the great circle between them ; for the curva- 

 ture of this arc is less than that of any other line drawn 

 on the surface, from point to point, and, therefore, ap- 

 proaches more nearly to a straight line. 



If, therefore, a ship could sail accurately on.tllo araoff 

 a great circle, voyages might, in general, be considerably 

 shortened. But a great circle cuts all the meridians it 

 crosses in different angles ; so that to, keep on. such a. 

 circle the course must be continually varying^ The 

 practical impossibility of changing the course every in- 

 stant, renders Great Circle Sailing strictly speaking a 

 matter of mere theoretical speculation. Much advan- 

 tage, however, has been found to arise from .dividing the 

 great circle path, from one place to another, into short 

 portions, or stages, and to reach these . in .succession by 

 the ordinary methods of sailing. A, ship .making these 

 several stage:], though never actually moving on the great, 

 circle, always keeps so closely in its .neighbourhood as to. 

 be a considerable gainer, in point, -of time, in a long, 

 voyage. 



Mr. Towson, of Devonport, has. been at the pains of 

 constructing tables, by which the, proper successive 

 courses, necessary to cause the entire track of the ship 

 to approximate as closely as conveniently. practicable to> 

 the great circle path, may be found ; they 'are .published; 

 by the Admiralty, under the title of Tables to ^Facilitate 

 the Practice of Great Circle Sailing. We can do no, 

 more than allude to them here, and recommend them to. 

 the notice of the practical seaman. 



But, without the aid of tables, a twelve or eighteen- 

 inch globe would be of service in suggesting how to shape 

 the different courses : a line stretched from one point to, 

 another, on such a globe, would show the great circle 

 track between them. If a brass circular rim, two or,, 

 ihree inches in diameter, made to lie close to the surface, 

 of th,e globe, were divided, like a compass-card, aud if, 



