NAVIGATION NAUTICAL ASTRONOMY. 



[LONOITVDE. 



2. In latitude 48* W N. the true alt it u.!,- ,.f the sun's 

 centre was 23 2 : the declination at the time of the obser- 

 vation was 10 13- S. ; and the magnetic bearing S. 161 

 SSf E. Required the variation of the compass. 



An*. Variation, 22 s1 40 40* W. 



3 November 10, 1835, in latitude 60 22" N. , longitude 

 34 SO' W., at about 9 o'clock in the morning, the alti- 

 tude of the sun's lower limb was 8 10', and the bearing 

 by compass 8. 21 18- E. ; also the height of the eye was 

 20 feet Required the variation to the nearest minute. 



Sun's dec. at noon, Nov. 19, O. T., 19 23' 8. Semi- 

 diajii.-i, r, ir, . 



Variation of dec. in Ih. + 40', so that 2' must be sub- 

 tracted for three hours before noon. 



Ans. Variation, 24 12 West. 



NOTE. In the foregoing examples, if tlio local devia- 

 tion of the compass remain uncorrected, what is named 

 r.irmfion will be compounded of variation proper and 

 deviation. Kach result must then be regarded as the 

 difference between the uncorrected compass-bearing and 

 the i rui' bearing. 



It is proper to remark also, in concluding this division 

 of our subject, that a single altitude or a single azimuth 

 taken at sea is not considered so trustworthy as the mean 

 of several ; consequently, when all attainable accuracy is 

 desired, the observations are repeated at short intervals 



<if time (a minute or so), and the moan rviult of the set 

 is taken for (A observation employed in tin' calculation. 

 the mean of the times being the corresponding time i>f 

 that observation. 



Attention to the local deviation of the compass is a 

 matter of great practical importance; and I '..irl.uv's cor- 

 recting plate is a valuable check to its influence. Al..>nt, 

 a quarter of a century ago the Thetis, with a million of 

 dollars and other treasure on board, sailed umlor tho 

 most favourable prospects from Rio Janeiro ; the next 

 day, in consequence of unfavourable wind, they tacked 

 ship, in full confidence of being clear of land ; the fatal 

 mistake was first discovered by the jib-boom striking u 

 high perpendicular cliff ; all tho three masts were at onco 

 sent over the side, and vessel and cargo were lost. In a 

 paper in the Phil. Trans, for 1831, Mr. Harlow shows 

 that the local deviation of the compass was exactly such 

 as to be likely to occasion this great mistake in the ship's 

 reckoning. The distance the vessel had run was alxmt 

 80 miles ; and assuming the local attraction to have been 

 equal to that of the QlawsaUr a similar ship she 

 would have passed five miles nearer to the scene of her 

 destruction Cape Frio than she would have done li.nl 

 the compass been undisturbed by tho attraction of tho 

 vessel, or had this disturbance been counteracted by any 

 neutralising apparatus. 



CHAPTER IV. 

 ON FINDING THE LONGITUDE AT SEA. 



IXTIIODUCTIOK. To determine the longitude of a ship a( 

 sea, is justly regarded as the greatest achievement of 

 Nautical Astronomy ; it is often considered, too, as the 

 most important achievement ; but since both the lati- 

 tude and the longitude are equally necessary to enable 

 the mariner to ascertain the position of his ship on the 

 ocean, one of the*e determinations has, in fact, no claim 

 to superior importance over the other, as respects its 

 practical value to the navigator. 



A higher degree of consequence has been attached to 

 the problem of finding the longitude, solely because of 

 the greater difficulties with which the solution of it is 

 beset, and of the larger demaud made upon the re- 

 sources of science both mechanical and astronomical 

 for the means of overcoming them. 



We have seen, in the preceding Chapter, that there are 

 several ways of determining latitude : of these the 

 simplest is that wherein the data are the altitude and 

 declination of a celestial object when on the meridian. 

 So there are several methods of determining longitude ; 

 but in each of these we have two distinct problems to 

 solve instead of one : the problems are, 1 st, to find the 

 time at the ship ; and, 2nd, to find the time at Green- 

 wich. The difference of the times is the longitude of the 

 ship in time. The first of these problems is compara- 

 tively easy ; the second has exercised the ingenuity and 

 tasked the exertions of the greatest minds ; and two 

 very different forms of solution have resulted from these 

 effort* 



As the great object to be accomplished is to discover, 

 at any instant at the ship, what time it is at Greenwich, 

 or at the meridian from which longitude is reckoned, it 

 U natural that the problem of finding thu longitude 

 should have more especially engaged the attention of 

 cnroi. ikers ; and, accordingly, when, in 1714, 



the first parliamentary reward was offered for the solu- 

 tion of this problem, within certain limits, a most laud- 

 alile emulation WM called foith among the more scien- 

 tilic of this class of artists. The pecuniary stimulus 

 d by government was this namely, 10,000 for a 

 id which should determine the longitude to within 

 60 mile* of tho truth ; 16,000 if the method should 

 the longitude within 4<i miles ; and '2O,(XH) if within 30 

 miles. The most xucouSNful competitor for these rewards 

 was John Harrison, who, with indefatigable industry, 



applied himself to the construction of a chronometer that 

 should keep time with sufficient accuracy to accomplish 

 tho last of these objects. In 1736 his first chronometer 

 was subjected to trial in a voyage to Lisbon ; ami in 

 1739 a second, still more perfect, was produced ; but in 

 1758 he completed a third, which ho affirmed to bo so 

 accurate as to entitle him to the highest reward offered 

 by the Commissioners of Longitude. In compliance with 

 Mr. Harrison's request, the Admiralty directed tho 

 watch to be tried on a voyage to Jamaica, in the ship 

 Deptford, which sailed from I'ortsmouth, November IS, 

 1701, and arrived at Port Royal, Jamaica, January 19, 

 17li2. The time at Portsmouth for the watch hail been 

 set to Portsmouth time was found, on tho 2Cth of the 

 same month, to be oh. 2m. 47s., as shown by the chrono- 

 meter, at the instant of noon at Port Uoyal ; and tho 

 exact difference of longitude, in time, between the same 

 two places, as found by careful astronomical observatii ms, 

 was oh. 2m. 51s. ; differing from the determination of 

 tho chronometer by only 4s., which, in the parallel of 

 Jamaica, is less than one nautical mile. 



On January 28, 17C2, the chronometer was sent b;ick 

 in the Merlin, which arrived at Portsmouth on March 

 26. On the passage, the ship encountered a violent 

 storm, and the chronometer had to be removed to a 

 place where it was likely to suffer from exposure. Tim 

 time of mean noon, as shown by the chronometer, April 

 2, was llh. 58m. 6s'5. Hence, from Nov. 6, 17(il, to 

 April 2, 1702, during which period the chronometer hail 

 passed through a variety of climates, and been subjc. 

 to violent agitations of the sea, its error was no more 

 than 1m. 53s'5, or 28J minutes of longitude in time, 

 which, in tho parallel of Portsmouth, does not amount 

 to 18 nautical miles. 



On this occasion Mr. Harrison received 5000 ; and 

 another trial voyage was proposed namely, from Ports- 

 mouth to Barbadoes which was made in 17<>4, and 

 which proved so satisfactory that an additional 5000 

 was ordered to be paid to him, and the remainder of the 

 lushest reward promised as soon as he had sufficiently 

 explained the principles of his time-keeper to enable 

 another artist to construct one as good. Mr. Harrison 

 accordingly explained fully the whole of his mechanism 

 to a properly qualified committee; and Mr. Kendal, a 

 member of that committee, was directed to construct a 



