SHIP'S JOURNAL.] 



NAVIGATION NAUTICAL ASTRONOMY. 



1117 



From what has already been said as to the unavoidable 

 imperfections in even the most careful measurements of 

 a ship's course and distance, the difficulty of making the 

 proper allowances for the leeway, <fec. , and of estimating 

 the effects of sqxialls, currents, <tc. , it will readily be in- 

 ferred that a ship's journal would soon become so erro- 

 neous, as a registry of actual facts, as to be quite value- 

 less, unless repeatedly rectified by astronomical obser- 

 vations. In the absence of these, such a journal would 

 describe little other than the imaginary route of an 

 imaginary ship, which, after a voyage of any length, 

 might terminate at almost any point of the globe as 

 likely as at the real point reached by the real ship. No 

 opportunity, therefore, should be lost to check this in- 

 creasing tendency to error in the dead-reckouing, by care- 

 fully determining the position of the ship from the safe 

 principles of nautical astronomy ; and the ship's account 

 should be regarded as of value, only in so far as it can 

 be made auxiliary to the application of those principles. 



The letters H, K, F, at the heads of the first three 

 columns in the preceding specimen of a journal, stand 

 for hours, knots, and fathoms, or tenths of a nautical 

 mile. The entries between noon and midnight are 

 marked J*iM. (post meridiem), and those between mid- 

 night and noon are marked A.M. (ante meridiem). 



The result of each day's work is inserted at the 



bottom of the page ; the courses being corrected for 



leeway and variation, the result exhibits the true course 



and distance from the point of departure ; together with 



'liff. lat. and diff. long, made in the twenty-four 



i, or rather the latitude and longitude reached. 



In reference to the two preceding days' work, it will 

 be observed, that as the variation of the compass is 

 westerly, it must be allowed to the left of the compass 

 courses ; and, therefore, when the ship makes leeway on 

 the larboard tack, the difference between the leeway 

 and variation is the correction to be applied to the 

 course to the left, if the variation be greater ; but to the 

 right if the leeway be the greater. 



When the ship makes leeway on the starboard tack, 

 the allowance for it, as well as that for variation, being 

 to the left, their sum will be the correction to be applied 

 to the compass course ; and when no leeway is made, the 

 only correction is for the variation. Now E. N. E., the 

 opposite point to the bearing of the land from which the 

 departure is taken, is the first course ; and this, and the 

 drift, being corrected for variation, and the other courses 

 for both variation and leeway (when there is any), and 

 the distances on each of these courses added up, we have 

 the following Traverse Table for the first day's work : 



TRAVERSE TABLE. 



Hence, by Mercator's sailing, page 1061, the difference 

 of longitude is 1 2' E.: 



Long, left . . . Za" 13' W. 

 Ditf. long. ... 1 2 E. 



M. diff. 



24 11 W. 



Therefore the ship's place, by account, is 

 Lat. 38 W N. Long. 24 11' W. 



From 11 A. M. till noon, the ship's true course was 

 N. J W., 5 miles nearly : hence the difference of latitude 

 also is 5 miles nearly ; and this added to the latitude, as 

 determined by double altitudes, at 11 A.M , gives 38 2^ 

 N. for the latitude by observation at noon. 



With this latitude and the longitude by account, the 

 bearing and distance of the Lizard are found to be 

 N. 49 iE., 1074 miles. 



The courses for the second day's work, being first cor- 

 rected for leeway only, as the variation is given degrees, 

 we have the following traverse table. 



TRAVERSE TABLE. 



Compass course 

 Variation 



True course 

 Distance 



N. 89 E. 

 20 W. 



N. 69 E. 

 63 miles. 



With this course and distance, 63 miles, found from 

 the Traverse Table, the dilf. hit. and departure are found 

 to be 22-8 N. and 5&'8 E. 



Lat. left . . . . 38 2V N, 

 Diff. lat 23 N. 



Lat. by account 



Long, left . . 

 Diff. long. . . 



Long, by account 



3843N. 



24 11 W. 

 115E. 



22 56 W. 



The departure made, from 9 A.M. till noon, is nearly 

 14 miles, with which, and the mid-latitude, about 38^, 

 the difference of longitude is found to be 17 miles ; 

 which, taken from 23 2', the longitude by the chrono- 

 meter at 9 A.M., gives 22 45' for the longitude by the 

 chronometer at noon. 



CONCLUSION . In here terminating this section on the 

 Principles of Navigation and Nautical Astronomy, we 

 would remind the reader of the objects proposed in un- 

 dertaking it, as sufficiently declared in the INTRODUCTION. 

 This object was twofold : first, to furnish to the mathe- 

 matical student a convincing proof of the great practical 

 value of the abstract sciences which he cultivates ; aud, 

 second, to supply the professional navigator with the 



