CHARTS, ETC.] 



NAVIGATION NAUTICAL ASTRONOMY. 



1115 



sequently called Hadley's Quadrant ; there is little reason 

 to suspect that the invention was not his own. But a 

 similar instrument had long previously been invented by 



THE SHIP'S JOURNAL. A Sea Journal is a record of 

 the daily transactions and occurrences in connection with 

 the navigation of the ship, including whatever observa- 



Newton, and communicated to Halley, who kept the | tions and remarks that may be necessary to give a brief 

 contrivance to himself. " The description of the instru- but connected professional history of the voyage. 



ment was found, after the death of Halley, among his 

 papers, in Newton's own handwriting, by his executor, 

 who communicated his papers to the Royal Society, 

 twenty-five years after Newton's death, and eleven after 

 the publication of Hadley's invention, which might be, 

 and probably was, independent of any knowledge of 

 Newton's, though Hutton insinuates the contrary." 

 "But," adds Sir John Herschel, "the priority of in- 

 vention belongs undoubtedly to Newton, whose claims to 

 the gratitude of the navigator are thus doubled, by his 

 having furnished at once the only theory by which his 

 vessel can be securely guided, and the only instrument 

 which has ever been found to avail in applying that 

 theory to its nautical uses." 



CHARTS. A chart is a map upon a plane surface of a 

 portion of the sea, including whatever within its limits 

 it may be useful to the mariner to have exhibited, such 

 as rocks, shoals, itc. ; and in some, the directions of 

 currents, and the variations of the compass. Charts are 

 of two kinds the Plane Chart and Mercator's Chart. 

 As in the former the meridians are all parallel lines, and 

 the degrees of latitude all of equal length, the proper 

 relations of latitude and longitude are grossly violated, 

 and such charts are of no use except for mere coasting 

 purposes. At sea, the only charts of any use are those 

 constructed on Mercator's principles, described at page 

 ]0fi2, the degrees of longitude remaining invariable ; while 

 those of latitude are enlarged more and more from the j 

 equator to the pole, agreeably to the law investigated 

 and explained at the page referred to. 



The accompanying map is a representation of the globe 

 projected upon Mercator's plan, but on a scale far too 

 minute, of course, to serve any other purpose than mere 

 illustration, for which alone it is here introduced. On 

 the actual sea chart, representations of the compass are 

 placed at intervals, for the purpose of setting off courses. 

 The meridians which limit the chart employed at sea 

 are graduated, as also the parallel of latitude which 

 bounds the extent of the chart north or south. A point 

 on the chart being marked, we find its latitude by simply 

 observing where the edge of a straight ruler applied to 

 the point, and held parallel to any parallel of latitude, 

 cuts the graduated meridian ; and the longitude is found 

 by placing the edge, while on the point parallel to any of 

 the meridians, and observing where the edge cuts the 

 graduated parallel. 



To find the course between two places on the chart, 

 apply the edge of a parallel ruler to the two places, and, 

 holding it there, move the other part of the ruler till an 

 edge passes through the centre of one of the compasses : 

 the course will thus be indicated. 



To lay down on the chart the position of the ship by 

 dead-reckoning that is, from her course and distance 

 from a given point of departure, as the preceding noon 

 place the ruler on the point of departure, and parallel to 

 the given course : from the graduated meridian at the 

 side of the chart, and in the latitude of the ship, take 

 the distance, in degrees, <kc , by the compasses : this dis- 

 tance, applied from the point of departure along the edge 

 of the ruler in the direction of the course, will mark out 

 the position of the ship by dead-reckoning. To lay down 

 the position as given by the latitude and longitude is 

 sufficiently obvious : the intersection of two pencil lines, 

 through the given points of latitude and longitude, and 

 parallel to the boundaries of the chart, will be the posi- 

 tion sought. 



It will be perceived by the reader who has gone over 

 what has been stated in the Navigation respecting Mer- 

 cator's sailing, that although positions are correctly ex- 

 hibited on the chart, as regards latitude, longitude, ami 

 course, yet distances are exaggerated. Distances which 

 are the same on the globe, become more and more elon- 

 gated on the chart as we approach the pole. 



The entries in the Journal are made hourly after the 

 departure is taken : the ship shapes her course towards 

 a definite point ; and to do this, either a chart is con- 

 sulted, or the angle determined by computation ; allow- 

 ance tiien being made for the variation of the compass, 

 and the local deviation, the compass or steering course 

 for the first stage is obtained. The ship, however, is 

 usually considered to depart from the point of land or 

 other conspicuous object last seen ; and the bearing oppo- 

 site to that, of this point of departure, is regarded as the 

 first course, and the distance of it as the first distance. 

 The ship is not, however, considered as having fairly com- 

 menced her voyage till her final departure has been taken. 

 Time is generally recorded as in the affairs of civil life, 

 and not according to astronomical reckoning : noon and 

 midnight equally divide the twenty-four hours, as on 

 land. From the hourly registry of the course and dis- 

 tance, the leeway and the variation of the compass being 

 properly allowed for, the ship's position is determined 

 every day at noon. If no astronomical observations 

 have been made, the position thus determined is the 

 place of the ship by dead-reckoning ; but if the latitude 

 or longitude, one or both, have been computed from obser- 

 vations, a distinct entry to that effect is made, although 

 the day's account by dead-reckoning is still preserved. 



This clearing up the ship's account every day at noon, 

 so as to enable her to take a fresh departure daily at that 

 hour from a known position, just as she took her depar- 

 ture at first, is called a day's work. As noon is the time 

 invariably fixed upon for ascertaining tlie resultant of 

 all the preceding twenty-four hours' sailings, whatever 

 latitude or longitude may have been determined by 

 astronomical observations in the interval, is brought up 

 to that hour by help of the dead-reckoning. Hence the 

 entries, " Latitude by Observation," and " Longitude by 

 Observation," frequently inserted in the noon results, are 

 in general made up in some small part of the latitude 

 and longitude by account. 



In keeping the ship's reckoning, the position departed 

 from at each noon, is considered to be that which nauti- 

 cal astronomy assigns'; so that when observations have 

 been made in the interim, the dead-reckoning com- 

 mences afresh, and is not a continuation of the yester- 

 day's account. The record is then carried uninterrupt- 

 edly on till a noon arrives, immediately before which, 

 the true position of the ship has again been settled by ob- 

 servations, and soon. The meridian observation forliti- 

 tude is of course recorded for the noon on which it is made. 

 The working of a day's work may always be effected by 

 the Traverse Table, after the manner shown at page 

 1055 ; and as the twenty-four hours' sailings should be 

 regarded only as furnishing data for finding the position 

 of the ship at the end of that time approximately, it is 

 not considered as in general necessary to attend to 

 minutes in the courses. " It is mere waste of time," 

 observes Lieut. Raper, "to work the course nearer than 

 to the whole degree ; for if eveii the compass could be 

 depended upon, as it cannot be, to 1, the ship cannot 

 be steered to twice that quantity." We shall now give 

 a very short specimen of a Ship's Journal : there is no 

 settled and uniform plan of arranging all the entries ; 

 but there are certain prominent features in which all sea 

 journals .\re alike. The specimen here offered is, with 

 some slight modification, extracted from that given by 

 Mr. Riddle in the work before referred to. 



The reader is already aware that the principal entries 

 in the Journal are, in the first instance, written in 

 chalk on the black board called the log-board, from 

 which they are afterwards transferred to the log-book. 

 The correction of the several courses for leeway and 

 variation being made, and the position of the ship at 

 noon, as deduced from the sailings, and as determined 

 by observation when observations are taken, completes 

 the day's work, and renders the log-book a journal. 



