LESSONS IN NAVIGATION'. 



LESSONS IN NAVIGATION. I. 



I>FIWITION OFTHB ART DEAD RECKONING LOO : PRACTICAL 

 WAY OF KEEPING TUB LOO DEFINITION OF TJCKMM. 



NAVIGATION may be defined as the art by which the mariner in 

 able at nil tiun's to ascertain the position of bin Teasel upon 

 the earth's surface, the course she has pursued nnd distance 

 traversed in any given time, and the course which she must 

 follow to reach a given point. The wider definition sometimes 

 given, and which, indeed, the name suggests the art of con- 

 du.-tmtf 11 ."Inp from place to place is misleading, - 

 would seem to include a thousand arts of practical seamanship 

 lying quito bfyond its province. 



Miing the navigator to bo provided with charts showing 

 the distance and course between any two given points, it is 

 obvious that his first and greatest necessity is the power of 

 fixing his position at any time upon the " trackless wasto ' ' as 

 represented by his chart, for, knowing his position, he has evi- 

 dently the moons of tracing his past and planning his future 

 course. 



There are two ways of fixing a vessel's position, one by 

 observations of the heavenly bodies (nautical astronomy), the 

 other by what is called dead reckoning that is, by deduction 

 from a close record of the vessel's 

 movements since her place on the 

 chart was hist determined. The 

 elements of this record are the 

 observed speed and the course or 

 courses sailed the former ob- 

 tained by "heaving the log" at 

 intervals, and the latter by watch- 

 ing the compass. A ship at sea 

 is so much the sport of winds and 

 waves, tides and currents, that 

 this record (which is called the 

 log* or journal) is liable to serious 

 error, and the ship's position, as 

 deduced from it every day at 

 noon, needa to be checked as often 

 as possible by the more certain 

 but troublesome and not always 

 feasible method of celestial obser- 

 vation. As cloudy skies and vio- 

 lent storms often forbid observa- 

 tion for days together, the import- 

 ance of a well-kept log cannot be 

 overrated, and the accurate keep- 

 ing of the log-board may be called 

 the first or practical division of 

 the science. The daily deductions 

 from the log may, on the other 



Fig. 1. THE MARINER'S COMPASS. 



hand, be called the second or theoretical part of the science. 

 The remaining branch the observation of the heavenly bodies 

 to check deductions from the log involves practical skill and 

 theoretical considerations of an equally high order. 



I. To commence with the practical work of keeping the log. 



The nautical day runs from noon of one day to noon of the 

 next. During that time the ship's movements at every hour, as 

 nearly as con bo estimated, are noted on the log-board i.e., the 

 course or direction followed at each hour, and the speed through 

 the water. The completed record is called the day's work, and 

 from it, at noon, is computed the dead reckoning, or longitude 

 and latitude by account. The particulars of the log-board are 

 then transferred to a properly ruled page in the log-book or 

 journal, together with the computed position, the position as 

 found by observation, the distance made during the day, and 

 the general course steered (i.e., the nett result of the various 

 courses and distances made, if the ship, as is often the case, 

 has not held one straight course throughout) ; particulars as to 

 weather, wind, currents, variation of the compass, amount of 

 sail set, duties of the crew, etc., and the distance and bearing 

 of the land next expected to be seen. 



The speed of the ship is noted every hour. A thin piece of 

 wood, shaped like a quadrant of a circle, and so loaded as to 

 stand upright in the water and offer resistance to being drawn 



* Tliis name is also given to an instrument for finding the speed of 

 the ship. 



151 X.E. 



along, U cant overboard. ThU U called the log, and to it if 



attached a thin cord called the log- Uie Utter are 



i series of knoti (or pieces of coloured cloth) about fifty-one feet 



;t, and the number of these which run oat during half 



minute pivea the number of nautical ntilet (hence called knoU) 



at which the ship is then moving per hour, fifty -one feet being 



about the 120th part of a nautical mile. The nautical mile U 



. the sixtieth part of a degree of latitude (= 1 minute), or about 



10 feet ; the statute mile is 5,280 feet, or about one-seventh 



less. The common log here described is far from being a period 



apparatus, though still generally used ; various improved form* 



are manufacture i. 



The course steered by compass is also noted every hoar, or M 

 often as changed. It is now necessary to explain the compact 



,\ hi< h a diagram is annexed in Fig. 1. 



Standing at any spot on the earth's surface (except the poles, 

 which are practically out of the question), and facing the north, 

 an observer has on his right the tast, on his left the west, and 

 behind him the south. In order that the direction of any line 

 on the earth's surface may be quickly described, in language 

 universally intelligible, the whole horizon is divided into thirty- 

 two points, eight of which lie in each of the spaces between 

 north and east, east and louth, etc. Half-way between north 

 and east comos north-east, half- 

 way between east and south comes 

 south-east, etc. The principle on 

 which the points are named in 

 apparent from the diagram. The 

 eight most important are N., 

 N.E., E., 8.E., S , S.W., W., and 

 N.W. A combination of two of 

 these indicates half-way between 

 the two; thus, 8.S.W. (south- 

 south-west) is two points on west 

 side of south and two south of 

 south-west. Again, one of the 

 eight principal points by another 

 means one point away from the 

 first-named towards the other; 

 thus, N.b.E., one point to east- 

 ward of north ; S.W.b.W. (south- 

 west by west) means one point 

 to west of south-west. This ac- 

 counts for the names of the whole 

 thirty-two, but a still greater 

 degree of accuracy may be thus 

 attained : N.E.b.E. i E., north- 

 east by east one quarter east, 

 equal to 5| points eastward of 

 north. This might with equal 

 truth bo called E.N.E.JN., or } 



of a point northwards of E.N.E. The horizon, like other 

 circles, is also divided into 360", whence each point = 11* 15' ; 

 a half point = 5 37' 30" ; a quarter point 2 48' 45". 



Sailors always describe a ship's course by the " point of th" 

 compass" towards which she is steering; but in computing 

 position, etc., the course is described as so many degrees and 

 minutes from the line running north and south through the 

 spot from which she sails : thus, S. 11 15' W., equivalent to a 

 course S.b.W., or one point westward of south. 



Beneath the compass card, in the north and south line, is 

 fastened a magnetised needle, and the card being free to revolvr , 

 and the apparatus hung so as to be little affected by the motion 

 of the ship, the bearing of every part of the horizon is accu- 

 rately shown, a certain known correction being made for what 

 is called variation of the compass. The magnetic pole to which 

 the needle points does not quite correspond with the actual 

 north pole of the earth, and, indeed, varies slightly from year 

 to year. The variation in England is about 2 j points to west- 

 ward of north. Hence true north is shown by the compass 

 as N.N.E.JE., and the ti-ue bearing of any object is obtained 

 by adding 2| points to the left of the compass or magnetic 

 bearing. If the ship's head point N.W. by compass, we know 

 that her true course is W.N.W.JW. The compeu* course if, 

 however, noted on the log-board, and correction made after- 

 wards in " working off the log." The compass course is found 

 by simply noting to what point the ship's head is directed 

 as shown by the compass card on board. Similarly, if we 



