POINTS OF THE COMPASS.] 



NAVIGATION. 



1051 



part of a degree of the equator or of the meridian ; that is, 

 about 6,080 feet. When the log is hove, about ten or 

 twelve fathoms of line are suffered to run out before the 

 counting commences ; this is called the stray-line, which 

 is an allowance for letting the log go clear of the ship, 

 and settle in the water. As soon as the end of the stray- 

 line, which is marked by a bit of red cloth, passes from 

 the reel, the half-minute commences. The time is 

 measured by a sand-glass, which runs out in thirty 

 seconds, and which is turned when the end of the stray- 

 line passes. The line is stopped as soon as the sand has 

 run out. 



13. A Day's Work. By a day's work is meant the ordi- 

 nary daily operations at sea to determine the position of 

 a ship, and the advances made from noon till noon ; it 

 consists in keeping a record of the compass-courses sailed 

 in the interim, of the different rates of sailing, of the 

 variation of the compass, the velocity and direction of 

 currents, <fcc., <tc., and, finally, of the latitude and lon- 

 gitude. These particulars, as they become known, are 

 recorded on the log-board ; and the latitude and longitude, 

 finally deduced, is called the dead-reckoning, or the lati- 

 tude and longitude by account. The particulars of the 

 log-board are transferred, from day to day, to the log- 

 book : and, with the addition of whatever else may give 

 the necessary completeness to the record -the astrono- 

 mical observations made to correct the dead-reckoning, 

 the state of the wind and weather, the bearings of points 

 of land, <fcc., <fec. form the journal of the voyage. 



POINTS OFTHE COMPASS. The following table gives the 

 several angles which the different points of the compass 

 make with the meridian, as also the angles for the 

 quarter-points : 



The angles in the above table nowhere differ from the 

 truth by more than a quarter of a minute ; a degree of 

 accuracy amply sufficient for all the purposes for which 

 it is used. It is necessary that the learner should 

 commit this table to memory at least, so far as to be 

 able to state, without reference to it, how many points 

 any of these courses, or rhumbs, are distant from the 

 meridian, or north and south line ; as each point 

 i 11 15', the angle corresponding to any number of 

 points is easily deduced. Repeating the points in order, 

 completely round the compass, as figured at page 1050, 



or according to this table, is called by sailors boxing tlie 

 compass. 



We shall now proceed to consider the various sailings, 

 first, however, making a few brief remarks in reference 

 to the data, or observed conditions upon which the com- 

 putations in the following articles are, in general, to be 

 founded. 



From what has been explained in the preceding pages, 

 the learner will perceive that the principal measurements 

 made at sea to determine the place of a ship, inde- 

 pendently of astronomical observations, are the measure- 

 ments of the course sailed on, and the rate of sailing 1 . 

 From the account given of the means and instruments 

 employed for these purposes, he must see that the results 

 furnished by them can scarcely ever be regarded as 

 rigorously correct. The compass-card, valuable and 

 indispensable as it is, has no divisions upon it to dis- 

 tinguish angles which differ from one another by less 

 than a degree ;* such differences, therefore, in steering a 

 ship, have to be roughly estimated by guess the course 

 is thus liable to error to some extent. Again, the rate 

 of sailing, as measured by the log, is equally exposed to 

 error from the very nature of the operation, and thus 

 the distance run, on any course, cannot be determined 

 with strict accuracy. Even if these sources of error 

 could be obviated, yet winds, currents, swells of the sea, 

 and the various other accidents to which a ship is exposed, 

 and which all act as disturbing causes, would often 

 seriously affect the correctness of the position of the 

 vessel, as deduced from the dead-reckoning. 



The aim of the careful and experienced mariner is to 

 be on the look-out for these external influences, and to 

 allow, as best he can, for their effects ; matters in refer- 

 ence to which there is room for the exercise of much 

 practical tact and judgment. 



These allowances being made, the courses on the log- 

 board are modified accordingly, and, being corrected for 

 the variation and local deviation of the compass, the 

 actual distance made, as also the difference of latitude and 

 longitude since the preceding noon, is computed, and the 

 latitude and longitude, by account, are thus ascertained. 

 These last important particulars the latitude and 

 longitude-^-being derived from data so exposed to error, 

 can be regarded, at best, as only approximately true j 

 and therefore the properly qualified navigator loses no 

 opportunity to correct his dead- reckoning by employing 

 the more sure and certain methods which nautical 

 astronomy supplies. In the following articles, however, 

 none of these can be introduced ; the object of this part 

 of our subject is to treat exclusively on what concerns 

 the dead-reckoning ; and we shall, throughout, suppose 

 that proper allowance for the variation of the compass, 

 for the leeway, <fec. , have been made, and that the courses 

 concerned are the true courses. The methods for ascer- 

 taining the variation of the compass must be deferred till 

 we come to treat of Nautical Astronomy. 



PLANE SAILING. Investigation of the Theoretical 

 Principles. Let the following diagram represent the 

 globe of the earth, P being one of its poles, and E Q the 

 equator. Let A B be a rhumb-line or track of a ship on 

 a single course. Imagine this oblique path to be divided, 

 by eq uidistant meridians, into portions Ai>, be, cd, ifcc., 

 so small that each portion may differ insensibly from a 

 straight line ; and, as in the figure, let the parallels of 

 latitude b'b, c'c, d'd, <tc., be drawn. A series of triangles 

 Abb', bcc', cdd", &c., will thus be formed on the surface of 

 the sphere, so small that each may be practically regarded 

 as a pline triangle. It is obvious that this may be con-, 

 ceived without any sensible violation of strict accuracy. 

 The triangles thus described, and thus assumed to be 

 plane triangles, are all similar ; for the angles at b', c' t 

 d', <to., are all right angles, and the ship's track cuts 

 every meridian which it crosses at the same angle. Con- 

 sequently, by Euclid, Prop. 4, Book VI., we have tho 

 proportions., 



A6 :Ab' :: be : US :: cd ;cd'&c.; 



The outer edge of the card is divided into 360 degrees ; but M it U 

 difficult to steer a ship to the nicety which these divisions imply, the 

 marks on the extreme rim of the compass-card are seldom much attended 

 to by mariners.. 



