154 



NAVIGATION. 



a glass of fifteen instead of thirty seconds, counting 

 the knots double. 'Hie rate of sailing, per hour, 

 imilti|>lit> 1 by the hours sailed, thus gives the mariner 

 tlie measure of his run. 



In addition to these essential instruments for direct- 

 ing the course and ascertaining the distance, the 

 navigator must be provided with octants of double 

 reflection, to measure the altitude of the heavenly 

 )> ulics ; and a circle, or sextant, more nicely gra- 

 duated, to measure distances between the moon and 

 stars. He should also have with him a book con- 

 taining the logarithms of numbers, sines, tangents, 

 and secants, to facilitate trigonometrical calculations; 

 tables for correcting altitudes for dip, parallax, and 

 refraction ; also lists of latitudes and longitudes for 

 every part of the world ; and of time of high water 

 at every port, at the period of full and change of the 

 moon, from which, at all times, to be able to find the 

 tide ; and a variety of tables, to facilitate the various 

 problems of navigation. He should also have with 

 him the Nautical Almanac, containing the places 

 and declinations of the fixed stars and planets, and 

 especially the distances of the moon from the sun 

 and other stars, and all that relates to that body, 

 with a view to calculate the longitude by observation. 

 Finally, he must be provided with the general and 

 local charts applicable to his contemplated voyage. 

 Thus furnished, the mariner may set sail with con- 

 fidence ; many do so with no other aids than their 

 compass, log, quadrant, a single chart and book of 

 navigation, and arrive in safety. 



But it is less our business to show with how little 

 care a ship may be navigated, than to show how she 

 may be carried from port to port with the greatest 

 possible certainty. Having taken leave of ttie port, 

 and, when the last land is about to disappear from 

 Tiew, either from the growing distance or the inter- 

 vention of night, the mariner selects some conspicuous 

 headland, of which the latitude and longitude are 

 noted in his tables, and, placing a compass in some 

 elevated position, remote from any iron object to dis- 

 turb its polarity, proceeds to determine its bearing, 

 and estimate his distance from it, either by the pro- 

 gress made from it, or by the ready estimate of a 

 practised eye. Or, taking the simultaneous bearings 

 of two distinct points of coast, he has still surer data 

 for deducting his position. This is called taking the 

 departure, and is carefully noted on the log-slate, with 

 the time of making the observation. Thenceforth 

 the. log is thrown every hour, and the course and 

 distance are entered upon the slate, to be copied into 

 the log-book at the end of the day. The first thing 

 which the navigator attends to, after making the 

 offing which prudence dictates to clear the dangers 

 of the land, is to shape his course for the port of his 

 destination. And first he searches in the chart if 

 there be any point of land, island, or rock, interven- 

 ing in his way. If there be, the course is primarily 

 shaped with reference to the danger ; if not, the dif- 

 ferences of latitude and longitude between the two 

 places being taken, the course and distance are 

 obtained by the aid of trigonometry. The shortest 

 distance between any two places on the surface of 

 our sphere, is the arc of a great circle passing through 

 those two places. Thus, between cape Henry, in 

 latitude 37, and the island of St Mary, in the same 

 latitude, but 50 Ion. farther E., the distance is thirty 

 miles less in sailing on a great circle, than if you 

 were to sail due E. on a parallel of latitude, and con- 

 sequently on a lesser circle of the sphere. In a 

 higher latitude, the difference between sailing on a 

 great or small circle becomes more considerable, as 

 uie small circles grow smaller ; thus, in the latitude 

 of 6O, a distance equal to that between cape Henry 

 and St Mary would offer a disparity of nearly 200 



miles. But, as it is only in sailing on the equator, 

 or on a meridian, that the compass points us uniformh 

 along a great circle of the sphere, in most cases r. 

 would be necessary to change the course at short 

 intervals, in order to attain even an approximation 

 towards this desideratum. For instance, in sailing 

 from cape Henry to St Mary, on a great circle, it 

 would first be necessary to sail more than a point 

 northward of E., gradually approaching that direc- 

 tion towards the middle of the distance, when the 

 course should be due E.; thence declining southward, 

 until the land would be made upon a course as much 

 south of E. as, on starting, it was north of it. In high 

 latitudes, when the reduction of distance would offer 

 a sufficient inducement, it may be advantageous to 

 attempt following a great circle ; but in the seas 

 ordinarily traversed by mariners, the trifling increase 

 of distance which results from following a uniform 

 course, as obtained by Mercator's sailing, is far more 

 than compensated by its convenience and freedom 

 from all perplexity. For the rest, the wind not 

 unfrequently deprives the fastidious navigator of all 

 choice between a great circle and a loxodromic. At 

 the first noon succeeding the time of taking his depar- 

 ture, the mariner works up his reckoning. This is 

 an epoch fixed by nature, being determined by the 

 passage of the sun over the meridian, and is therefore 

 well chosen as the beginning of the day. The log- 

 slate being marked, he copies the courses and dis- 

 tances, if from head winds or other cause they have 

 been various ; the departure from the land is also 

 converted into a course ; as is also the current, if there 

 be any known one. He next proceeds to find the 

 difference of latitude and departure from the meridian 

 corresponding to each course, either by geometrical 

 calculation, or, more expeditiously, by reference fx> 

 tables, then he adds the several differences of latitude 

 and departure, and, if they be of different names, as 

 some north and some south, some east and others 

 west, deducts the less from the greater. With the 

 remaining difference of latitude and departure, he not 

 only finds the course and distance made good, but 

 also the latitude and longitude in ; the difference of 

 latitude being applied to the latitude left, by adding 

 or subtracting, in sailing from or towards the equa- 

 tor, at once gives the latitude of the ship. But be- 

 fore the departure can be thus applied to find the 

 longitude, it is necessary to reduce it for the con- 

 verging of the meridians towards the poles ; for, 

 though all degrees of longitude are divided, like 

 those of latitude, into sixty minutes or miles, yet they 

 decrease in length, from being equal to a degree of 

 latitude at the equator, until they become nothing at the 

 poles. There are many ways, more or less accurate, 

 of deducing the difference of longitude from the de- 

 parture, the latitude being known ; they are founded 

 upon this principle : the circumference of the earth 

 at the equator is to its circumference at any given 

 parallel of latitude, as the departure is to the differ- 

 ence of longitude. The most easy and correct way 

 of obtaining the difference of longitude, on an oblique 

 course, is by the aid of a table of meridional parts ; 

 for, having taken out the meridional difference of 

 latitude, the mariner has this simple proportion : the 

 proper difference of latitude is to the meridional dif- 

 ference of latitude, as the departure to the difference 

 of longitude. The difference of longitude thus 

 obtained, is applied to the longitude left, adding or 

 subtracting, in sailing to or from the first meridian, 

 and the result will be the ship's longitude ; which, 

 with the latitude previously ascertained, determines 

 her position on the chart. The method of navigating 

 thus described is called dead reckoning. It is far from 

 infallible, and leaves much to desire. It will, indeed, 

 do pretty well in short runs ; but as errors daily creep 



