10V) 



NAVIGATION. 



east or wart, U of court* 180'. A pUoe of which the 

 longitnde u 180 U on the meridian opposite to that of I 

 Greenwich. 



The learner most bear in mind that, in Geography and 

 Navigation, the meridian of a place U limited by tlie 

 pole* of the earth : it U the semicircle pawing through 

 the place, and terminating in the north and south poles. 

 As already remarked, the terrestrial meridian is extended 

 to the concavity above us, and marks out the correspond- 

 ing celestial meridian. Certain stars which are ob*ervd 

 to come to this meridian are seen to pass the opposite 

 meridian without setting ; but the distinction, noticed 

 above, between a meridian and the opposite meridian U 

 not preserved here : the star is generally said to come to 

 the meridian twice once above and once below the pole. 

 In the case of a ship, however, sailing round the pole 

 from one meridian to its continuation, it is always said 

 to have arrived at the opposite meridian, and to have 

 advanced 180" in longitude. 



8. Horizon. The horizon of any place is a plane con- 

 ceived to touch the surface of the earth at that place, and 

 to be extended to the heavens that is, to the region of 

 the remotest of the stars. This plane in called the 

 tauibU horizon. A plane parallel to this, but passing 

 through the centre of the earth, is called the rational 

 Horizon. These two imaginary planes, though separated 

 from each other by an interval equal to the semi-diameter 

 of the earth, cannot but bo regarded as coincident at the 

 distance of the stars. An eye, whether at the centre of 

 the earth, or at the point on its surface immediately over 

 it, would see a star in precisely one and the same direc- 

 tion : tin- altitude of it, referred to the rational horizon, 

 would be exactly the same as the altitude referred to the 

 sensible horizon. The observer, at the centre, sees the 

 tar higher above hit horizon (that is, the parallel to the 

 sensible horizon) than the observer at the surface sees it 

 above his, by the apparent interval between the two 

 horizons, at the distance of the star ; which interval, 

 however, although we here call it apparent, is in reality 

 too minute to appear as any interval at all. 



An eye situated abvce the surface of the earth, as is 

 always practically the case, has an horizon different from 

 the sensible horizon : the Utter is an extended plane ; 

 the former is a conical tur/ace, everywhere dipping below 

 the sensible horizon, and of wider visible boundary. A 

 very moderate elevation above the surface will give a 

 sensible increase to the observed altitude of a heavenly 

 body ; namely, the whole of the angular distance between 

 the two horizons the whole of what may be called the 

 angle of the dip. 



The apparent circular boundary of the sea, as seen by 

 an eye thus elevated above its surface, is called the 

 visible, or tea-horizon ; or, more frequently by sailors, the 

 offi ny. 



9. Thr Compau. The straight line through any place, 

 in which the plane of the meridian of that place cuts the 

 sensible horizon, is called the horizontal meridian, or 

 simply the meridian line, or the north and south line ; 

 and the horizontal straight line perpendicular to this is 

 the east and west line of the horizon. The horizon is 

 represented in miniature by a circular card, connected 

 with a magnetised needle, which points out the direction 

 of the horizontal meridian, and consequently also that of 

 the east and west line : the pointt thus marked out on 

 the rim of the card, namely, the North, South, East, 

 and West points, are called the four Cardinal Points of 

 the compass ; the quadrantal arcs, intermediate between 

 these, are subdivided each into eight equal parts, called 

 also point*, and these again each into four equal parts, 

 called quarter-poinU. The above engraving is a repre- 

 sentation .f \\i> Mnrinerf Compatt, divided into its thirty- 

 two point*, with the intermediate quarter-points also 

 marked. 



This important instrument is so suspended on ship- 

 board as always to assume a horizontal position nn.l.r 

 every change of motion in the vessel, so that the 

 direction in which the whip is sailing, at any tinio, is 

 always known by observing what point is in that direc- 

 tion ; that is, in general, what point coincide* with the 



line, through the centre of the compass-card, from stem 

 to stein of the ship. 



Fig. 



It is of importance to mention, however, that the 

 magnetic needle does not point accurately north ami 

 south ; the points to which it it directed are called the 

 magnetic north and south : and the angular departure 

 from the true north and south, at any place, is called 

 the variation of the compass at that place. Its amount 

 may be discovered, and the necessary corrections for it 

 made, by Nautical Astronomy. 



10. Counts . A line on the globe, which cuts the succes- 

 sive meridians at the same angle, is called a rhumb-line ; 

 it marks the track of a ship, the constant anijlf nfvRM 

 to being called the ship's course. It is indicated by the 

 compass ; but if no corrections be made for variation, 

 the course thus indicated is called the common-course ; 

 after correction and allowance for what is called deviation, 

 it is the true-course. These modifications of the compass- 

 courses will be more fully noticed as we proceed. 



11. Leeway. Another correction of the course is also 

 frequently requisite. The ship's progress is not always in 

 the direction of her length ; the wind often impels her 

 sideways, or, as it is called, to leewurd of the line from 

 fore to aft The necessary allowance for this divergence 

 from the path indicated by the compass is the correction 

 for leeway ; its amount can be estimated only by practical 

 experience. 



12. Hate of Sailing. The rate at which a ship sails on 

 any course is measured by an instrument called the /,..;. 

 and aline attached to it called the log-line, about li'ii 

 fathoms in length. The log is a piece of wood in Kli;q e 

 of a sector of a circle, and with its arc or rim so loaded 

 with lead that, when thrown into the sea, it stands ver- 

 tically in the water, with only its centre just above the 

 surface.* The log-line is so attached as to keep the face 

 of the log towards the ship, in order that it may offer 

 the greater resistance to being dragged after it ; the 

 length of line unwound from a reel, by the advancing 

 motion of the vessel, half a minute, gives the distance 

 run in that time, and thus is inferred the rate of sailing. 

 The log-line is divided into equal parts by means of a bit 

 of string passed through the strands and knotted, the 

 number of knots showing the number of parts each 

 part, which is the 120th of a nautical mile, is hence 

 called a Knot; so that as many knots as are run out in 

 half a minute, or the 120th of an hour, so many nautical 

 miles per hour is the ship's rate of sailing. Sailors thus 

 say that the rate is so many knots an hour, mean in;; so 

 many nautical miles an hour. A nautical mile is the OOtli 



Thl< i the common log, still too much u>cd; a more accurate tn- 

 itruiurnl U Huwy'i lo. 



