1118 



NAVIGATION-NAUTICAL ASTRONOMY. [DBVUTIOX OF TH OOMPAM. 



theoretical principle* on which bin rules of operation are 



folinil-d. it 'i ^'" " lto ft U 



the practical dutaiU of navigating a ship, nor to dwell 

 upon those facilitating expedients which could bo ren- 

 dered intelligible only by the aid of extensive nautical 

 tables. The present treatise is offered to the notice of 

 the mariner, more in the character of a companion or 

 supplement to the books of rules and tables in every-day 

 use at sea, and as serving to show him the scientific 

 theory on which his practice is based. But as fur as 

 could be done, without special tables, we have fully gone 

 into the calculations necessary for determining the 

 position of a ship on the ocean ; and have shown that 

 all the practical demands of navigation may be amply 

 satisfied by help of only the common logarithmic tubles, 

 and that knowledge of turning them to account, which 

 the elementary principles of plane and spherical trigono- 

 metry supplies. 



We slioll now add a few general remarks by way of 

 comment, subjoining some interesting and valuable in- 

 formation respecting the action of iron ships on the 

 compass. 



It cannot fail to strike an attentive reader, that the 

 subject of Navigation presents a forcible example of the 

 value of abstract science, even in circumstances where 

 the practical application of its principles would seem to 

 be almost precluded, on account of the unavoidable im- 

 perfections of our observations and experiments ; of the 

 instruments with which we work, and of the materials 

 upon which we operate. 



The mechanical tools or implements of the navigator, 

 are the log, the compass, the chronometer, and the sex- 

 tant ; these are to furnish him with the materials upon 

 which his science is to work, and from which he is to 

 u:t all his information in situations where no ex- 

 ternal aid can reach him, and where to err may involve 

 life and property in sudden destruction. Yet the me- 

 chanical means upon which he thus depends for guidance 

 and safety, are all confessedly imperfect ; he can measure 

 with accuracy neither the rate at which he sails, nor 

 the course upon which he steers ; and even if the log 

 and compass were perfect, hidden and unsuspected 

 agencies may vitiate, and falsify the indications of both. 

 The sextant, fortunately, is beyond the operation of 

 these disturbing causes ; it is, moreover, the least im- 

 perfect of all his nautical appliances, and accomplishes 

 the important end of rectifying and adjusting, to a very 

 close approach to accuracy, what the other instruments 

 may have done amiss. It is among the most valuable 

 gifts that science has ever presented to man to aid him 

 in his necessities. Nautical Astronomy could not exist 

 without it : and to say that it is not perfect, is only to 

 repeat what has been applied to every work of man's 

 hands. The best sextants, however, are subdivided to 

 no smaller arc than 10*, so that fewer seconds than 10 

 must be estimated, by help of the microscope, entirely 

 by the eye.* 



As just noticed, the sextant including, of course, in 

 this term the quadrant is of the utmost use in correct- 

 ing the results of the dead-reckoning. But the ship's 

 account continuously accumulates, and its errors must 

 run on till the weather and the sky furnish opportunities 

 for celestial observations. In the interim, the vessel is 

 trusted almost entirely to the guidance of the compass ; 

 it must unfortunately happens that, from the local 

 attraction, the ship may often be said rather to direct 

 the compass than the compass to direct the ship. It is 

 most important, therefore, that the intervals b> 

 observations at sea' be shortened as much as possible by 

 seizing every occasion that offers for making tliem. 

 From what is taught in the preceding pages, the reader 

 will easily perceive how it happens, that even very gross 

 errors in the dead-reckoning become comparatively in- 

 operative in the results deduced from astronomical ob- 

 servation, although the calculations founded upon these 

 observations virtually involve the data furnished by the 



; > rrror of thii ntimitifm can Hem reach Jlw nvontln, no th it Ike 

 OUHevuuntflny error In longitude cannot b* mor* than about two mile* ; an 

 *nv Uut U quiir corn] feet af. t), <<]( ill very eilra- 



41ui7 ctreuutuee*. (Sec PM' 



ship's account These data, however, do not dii 

 enter into the work ; it is the timt which corr.Mponds to 

 . that is employe. 1 ; and, fortunately, tli 

 .! elements, taken from tin u re- 



ference to this time semi-diameter declination right 

 ascension horizontal parallax, <tc., vary NO little, 

 in a large interval, that an error in the ship's place to the 

 extent of a quarter of the glol>e, would not, in general, 

 entail an error of a quarter of adegree in the adjustment 

 of that place by the lunar observations, ]>ro\idcd, at 

 least, that the time be not deduced from the moon. 



As already remarked at page 553, in the MATH 

 TICAL SECTION, the reasonings of pure geometry tolerate 

 no errors in the premises ; but practice can never satisfy 

 these rigorous conditions ; and in proportion as they are 

 departed from, will be the geometrical shortcoming of our 

 conclusions. At first sight, therefore, it would seem 

 chimerical to hope for any close approach to accuracy 

 from data so widely erroneous ; but when it is o 

 that these data connect themselves with other dependent 

 data, which are incapable of error beyond a very limited 

 range, we at once perceive that these latter may be very 

 near the truth, though the former may greatly depart 

 from it ; and that if the inquiry involve the dependent 

 data only, and not in a direct manner the original, the 

 inaccuracies of thac need give us but comparatively 

 little concern. .' 



It is thus that a very close approximation may be 

 made to the true position of a ship, though the dead- 

 reckoning may displace her many decrees, and though, at 

 the same time, we employ this reckoning in the o; 

 tion, as if it involved no error at all. But in the in- 

 tervals between these adjusting observations, the safety of 

 the ship is often wholly dependent on the trustworthi- 

 ness of the compasses ; and, of late, much mischief has 

 arisen from placing too implicit a confidence in them in 

 certain circumstances. 



Since the prevalence of iron vessels, the disturbances of 

 i the compass have been seriously forced upon the attention 

 I of scientific men ; and the subject of local deviation, still 

 involved inconsiderable obscurity, is becoming more and 

 more a matter of anxious scrutiny and investigation. 

 The importance of the inquiry was strongly urged by Dr. 

 Scoresby, at the meeting of the British Association, at 

 Liverpool, in 1854. "There were certain principles," 

 observes the Rev. Dr., " connected with the navigation 

 of iron ships, which were universally admitted. Those 

 principles were, that iron, being more especially disposed 

 to the magnetic condition, was a material, of coi. 

 calculated above all others to disturb the action of the 

 compass on board the ship. Again, it was admitted that 

 there were difficulties in the navigation of iron ships, 

 arising, not merely out of the original or primarily mag- 

 netic condition and disturbing influence of the iron, but 

 also in respect of certain changes which had been held as 

 mysterious changes which took place not unfrcquently 

 in regard to ships whose magnetic condition had been 

 supposed to be very well ascertained." Dr. Scoresby 

 then adverts to the circumstances connected with the 

 melancholy wreck of the Tayleur, the fate of which 

 must be in the recollection of all our readers. The 

 ship Tayleur, a new vessel, bound to Australia, sailed 

 from Liverpool on Thursday, January 19th, 1844. 

 She was 1,971) tons burthen, new measurement, and 

 she had on board about 458 passengers the crew and 

 passenge.rs, altogether, making a total of 528 persons. 

 She left the Mersey about noon on the above-nam -d 

 day. The pilot left her between seven and eight 

 o'clock in the evening, in a position between Point 

 Lynas and the Skerries. On Friday she encountered 

 very heavy weather ; and about eight o'clock on the 

 following morning (Saturday), it was for the first time 

 ascertained that there was any material difference be- 

 tween the compasses. 



There were three compasses on board. Dr. Scoresby 

 makes special reference to two of these. One of the two 

 was near the helmsman, and was the one by which he 

 steered ; and the other was near the mizen-mast. Both 

 of these compasses had been what is called adjusted, by 



