MARITIME CONVEYANCE. 



log ; and the distance between the knots on the 

 line is made to bear the same proportion to a 

 mile as the duration of the sand-glass bears to 

 an hour. The mode of measuring the ship's speed 

 is as follows : a sailor holds the reel on which 

 the line is wound ; and the officer, holding in his 

 hand a quantity of loose line, throws the chip 

 into the water, well to leeward. Before the stray 

 line is exhausted, the chip is floating astern of the 

 ship, and the officer watches the line until the 

 first knot passes through his hands, and at that 

 instant gives the order to turn the glass. As soon 

 as the glass runs out, the person holding it cries 

 ' Stop ! ' The line is grasped, and the number of 

 knots passed oft" mark the speed of the ship. Self- 

 indicating logs are also used besides the kind 

 above described. These consist of a brass box, 

 shaped in such a manner as to offer very little 

 resistance to the water, with a vane-wheel like a 

 screw-propeller attached to them. This wheel is 

 made, by means of clockwork, to mark on an 

 index the distance run as deduced from its revolu- 

 tions. This log, instead of remaining stationary 

 like the chip, is, of course, towed at a suitable 

 distance behind the vessel. 



The position of a vessel when it is deduced 

 from the compass and log alone, is called her 

 place by dead-reckoning. The compass points 

 out the direction in which the vessel moves, but 

 its indications must be corrected for variation, for 

 deviation (caused by any peculiarity in the con- 

 struction of the ship), and for leeway. The current, 

 if there is any, must also be noticed. The 

 Nautical Almanac, a copy of which every captain 

 has in his possession, gives the variation of the 

 compass for every part of the world ; the devia- 

 tion has been found out when the compasses were 

 adjusted previous to the ship leaving port ; and 

 the officer, guided by his experience, makes what 

 allowance he thinks necessary for leeway ; depend- 

 ing on the force of the wind, the direction of the 

 course, and the amount of canvas spread. The 

 log-line is hove every hour in the navy, and every 

 two hours in the merchant service. A journal 

 called the log-book is kept, in which are entered 

 (for each time of heaving the log) the speed of 

 the vessel, her apparent course, the state of the 

 weather, leeway, and everything else that can 

 affect her position. From the entries in this 

 journal, the ship's place at noon each day is care- 

 fully calculated. 



The position of a ship is, however, calculated 

 from observation of the sun, planets, and other 

 heavenly bodies, as well as by dead-reckoning. 

 For this purpose, another instrument is required 

 besides those already mentioned namely, a sex- 

 tant or quadrant, instruments for determining 

 the height of an object above the horizon. The 

 principle of both is the same : each has a gradu- 

 ated arc and a system of lenses and mirrors, by 

 which, when the object observed is made to 

 appear in contact with the horizon, its angular 

 distance above it can be read off by an index 

 on the arc. The quadrant measures angles up 

 to 90 ; it has no telescope attached, and can only 

 be used for altitudes or latitude observations. 

 The sextant is a more elaborate instrument, 

 measures up to 120, and besides altitudes, is 

 adapted for measuring the distance of the moon 

 from the sun, or a star, &c. which is sometimes 

 useful in determining the longitude. Just as 



observations by the compass must be corrected 

 for variation, &c. the indications of these instru- 

 ments must be corrected for parallax (as all angles 

 are supposed to be measured from the centre of 

 the earth, and not from its surface), for the refrac- 

 tion of the atmosphere, for the dip of the horizon, 

 and other things, for all of which tables are pro- 

 vided in the Nautical Almanac. 



The situation of any place (or the position of 

 any ship) on the surface of the globe is deter- 

 mined by its distance from two imaginary lines, 

 the equator and the first meridian. Its position 

 in reference to these is called its latitude and 

 longitude respectively. The equator is a fixed 

 circle on the earth's surface equidistant from each 

 pole. A meridian is a line on the earth's surface, 

 extending from pole to pole in a plane perpen- 

 dicular to the plane of the equator. Different 

 nations reckon different meridians as being the 

 first ; the English reckoning from the meridian 

 which passes through Greenwich, the French from 

 that which passes through Paris, &c. Latitude 

 is the distance from the equator measured on a 

 meridian in degrees, minutes, and seconds. It 

 must, therefore, always be from i to 90 ; and it 

 is called north or south latitude, according as it is 

 measured towards one or other of the poles. It 

 is evident that all degrees of latitude are equal,* 

 and that each of them is equal to one three- 

 hundred-and-sixtieth of the earth's circumference. 

 A degree is equal to 60 nautical miles, or knots 

 or about 69^ ordinary miles. Longitude is also 

 measured in degrees, minutes, and seconds, and 

 is the arc of the equator intervening between the 

 first meridian and the meridian of the place named. 

 At the equator, therefore, a degree of longitude is 

 equal in length to one of latitude, but its length 

 continually diminishes as the latitude increases, 

 until it is zero at the poles. Longitude is 

 measured east and west to 180 in each direction, 

 and it is obvious that 180 E. longitude and 180 

 W. longitude coincide, and are in the plane of the 

 first meridian. 



The places of the sun, moon, and many stars 

 for every day of the year are all carefully calcu- 

 lated and tabulated beforehand, and the mariner 

 has the tables with him on his voyage, so that on 

 finding by observation the altitude and position of 

 any of the bodies named, he may be able, by 

 reference to the tables, to infer from that the posi- 

 tion of the ship. As the earth revolves round its 

 axis, successive meridians come under the sun ; 

 and as it always revolves through the whole 360" 

 in exactly 24 hours, it passes through precisely 15 

 of longitude in an hour. In determining the 

 longitude, it is, therefore, all-important to know 

 the exact time ; for this purpose, chronometers are 

 used ; they are made with the utmost precision, 

 and so as to be unaffected by any changes of cli- 

 mate or temperature. The chronometer is set, on 

 starting, to Greenwich time, so that when it shews 

 12 o'clock noon, the sun is exactly on the meridian 

 of Greenwich ; and during the voyage the captain 

 has only to ascertain by the sextant the exact 

 hour at which the sun passes the meridian in 

 which the ship is, and convert the difference be- 

 tween that hour and noon at once into degrees 

 of longitude, four minutes of time corresponding 

 to one degree. 



* Disregarding the spheroidicity of the earth. 



461 



