U08 



NAVIGATION NAUTICAL ASTRONOMY. [Ktupui nr uuroRVDB, 



hu just been done, that the mean time at the ship at the 

 ineUnt of observation, U 4h. 9m. 54*. P.M., and tliat tlio 

 u 39 IV 45" out Such a large error as the 

 abora, an well in the estimated time as in the longitn<!<>. 

 could arise only from a long continuance of foul 

 weather, which, preventing observations of the heavenly 

 bodies, left the ship entirely dependent upon the dead- 

 rock' > .<t without taking iuto consideration the 

 : of unknown currents, leeway, <tc., even the devia- 

 of the compass alone, from the magnetic action of 

 the vessel, might in a few weeks load a ship astray to the 

 above extent* 



Under the circumstance* here imagined, it would be 

 better to take the Greenwich time at once from the 

 chronometer, if this can ut all be depended on, as already 

 recommended at page 1097 ; for the longitude and tim<- 

 by account are employed, only as subsidiary, to finding 

 an approximation to the time at Greenwich when the 

 observations ore mode. If the Greenwich time, as de- 

 termined from the lunar distance, differ considerably 

 from the chronometer time, the oj>eration for the time at 

 the ship should be repeated with the more correct 

 Greenwich time thus obtained, and thence the longitude 

 accurately deduced. 



2. On June 2, 1849, at lOh. 17m. P.M., mean time by 

 estimation, in latitude 50 61' N., and longitude by 

 account 41 W., the following observations were taken : 



Regulus W. of meridian. 



Observed altitude. Observed alt. moon's L. L. 



20 21' 40* 31 11' 0* 



Index 3* 50* Index 4' 10* 



20 17' 50* 31 



Obs. dUt. nearest limb. 



72 36' 30" 

 Index 9' 10" 



6' 50" 



Required the 



72 27' 20" 



The height of the eye was 20 feet, 

 longitude. 



Ship time, June 2 . . . lOh. 17m. 

 Longitude W. in time . . 2h. 44m. 



Approximate Greenwich time 13h. 1m. 



Referring now to the Nautical Almanac, we take out 

 the moon's semi-diameter and horizontal parallax for 

 midnight of June 2, the right ascension of the mean sun 

 at noon, and the right ascension and declination of the 

 star. 



From the Nautical Almanac. 



Moon's semi-diam. midnight 

 Horizontal parallax 

 Star's R.A. lOh. Om. 20s. 

 Decl. 



Fun's R.A. noon . 

 Cor. for 13b. 



Sun's R.A. at 13h. 



14' 49" 

 54' 23" 



12 42 1 4"N. 



4h. 43m. 2ls. 

 + 2m. 8s. 



4h. 45m. 29s. 



In Iron ihlpa, propelled by ateam, the local attraction i mbject to 

 rreat and fnquent chanfea, and the compaaa U ao powerfully acted upon 

 by thla fluctuation influence the effect of which U often unsuspected 

 that the moat fearful oonteqneneea aometlmea reault the true cnurac of 

 the Tcaeel beiac, from thii oaoae, widely different from that indicated. 

 At the conelutlon of the next Chapter, we .hall gin a brief abitract of 

 Dr. Hcomby'i reeearchea and lucirraliona in reference to Ihi. very im- 

 portant .ubjm. If icience fail to apply a remedy to counteract the ',-,..- 

 fine Influence of Iron iteamen on the compaaa, tbli naeful initru- 

 bad almoat w well be abandoned in >uoh ahip. The mere hoi -ling 



iu.il 



. 



of lk reaael will greatly affect the local deviation, and the contin 

 tr.m awl Tlbralioo to which a reeee! under .team ii lublccted, 

 tolimma Itt murnetk condition, aa to render the compaw a dangerou! 

 raid*. Th ihip Taylevr, which wa> wrecked in the Irtnh Channel in 

 Juuary. laM, waa led oat of Ur pioper coune by tbeae un.u.pected in- 

 The capum relied upon hi. helm compaaa, and 290 penona 



ui|ht err ere* u, UM utent .uppoaol in th* abute example. 



Fur the Apparent and True Altitude*. 

 Star's Altitude. 



Obs. alt . . . IN IV : Mi- 

 Dip ... 



App. alt. 



. 



True alt 



20 13' 26" (a) 



L' 



20 10' 49" (A) 

 DO" 



Co. alt. Z S - . . 69 49' 11" 



Moon's Altitude. 

 Obs. alt 31" 6' 60" 



Dip 4' 24" 



Semi-dia. ) 14' 60" 

 + 7 Aug. } 



Par. and Ref. 



+ 10' 32" 



31 17' 22" (a) App. alt 

 + 44' 54" 



32 Z 10* (A') True alt. 



For the Time at the ,S'A I/,. 



To compute the time we have the star's co-altitude 

 Z S =- C9 49' 11 ", its polar distance, or eo-deolination 

 P S = 77 17' 56", and the co-latitude PZ = 39 10', to 

 find the star's hour angle P, which, since the star is west 

 of the meridian, if added to the star's right ascension, 

 will be the right ascension of the meridian ; ;un 1 since the 

 time is P.M., this right ascension, diminished by that of 

 the mean sun, will be the mean time at the ship. 

 (See page 100G). 



Z S, 69 49' 11* 



sin. PS, 77 17' 6C* Arith. Comp. OlOV.V.i-J 

 sin. P Z, 39 W 0' Arith. Comp. -1995728 



2)186 17' 7" 



sin. s, 93 8' 33* 

 sin. (s ZS),23 19' 22* 



cos. J P, 36 46' 24* 

 2 



0-9093464 

 0-5976978 



2)19-8072756 

 9-9036378 



P= 73 32 1 48* 

 2 



3)14,6 6,4 9,6 



52 

 2 



S 

 3 



4h.54m.lU. 



R. A. of meridian 14 

 R.A. of mean sun 4 



54 



45 



31 

 29 



Mean time at ship 10 9 2 



For the Time at Greenwich. 



In computing the true lunar distance in this example, 

 we shall adopt the recommendation at the bottom of 

 page 1103, and so modify the observed, or rather the 

 apparent altitudes, that the seconds in each altitude may 

 be a multiple of 10 : whatever be added or subtract*.*! in 

 this way, must of course be in like manner applied 

 to the true altitudes. We shall also introduce a similar 

 mollification into the apparent distance, and apply the 

 necessary correction to the true distance : the object of 

 tlio.su changes is to reduce the work of computing for odd 

 Moonda, 



