

AM> 



LONGITUDE AND LATITUDE. 



i 



Unit 



i he umfaasJ that few observers have the patience or skill 

 Mat |.sible accuracy out of either. The obeerva- 

 i confined to stare, as neither of these instrument* will 

 iDMfj Ha dnMtmonta well under the sun. 



The naming ciroU was need by the French astronomers to deter- 

 Min. the UtiUMk. in their great survey. Since that time the instru- 

 hos been much better made, and the catalogue* of stars which 

 from Kooia^berfcOreoiwich, and Cambridge, hare supplied 

 ate and convenient mean* of using it. If the level* are 

 good and sensible, we think that the observations of one fine 

 everything being favourable, should bring out the latitude 

 8* or V. 



4. The but class of inst niments to be noticed U that of reflecting 

 including the reflecting circle of Troughton, the repeating 



1le of Borda, and the sextant of Hadley. These will be 

 Oder UM article SKXTAXT, as the title best suited to their 

 liiwllil quality of reflection. At present we must suppose a general 

 knowledge of their nature. And first we will suppose the observer 

 to have a eland and a mercurial or other horizon. In this case, standard 

 i should be observed seven! minute* before and after the meridian 

 to the north and south, between the altitude* of 15 and 60, 

 and as much a* possible in pairs, that is, for each star to the north, a 

 tar to the south should be observed about the same altitude, or two, 

 one higher and the other lower, so that the mean altitude should 

 nearly correspond. Whatever errors may exist in the division, glasses, 

 Ac., will be the same in each star of the pair, and as the error will 

 affect the latitude differently, the mean latitude will be free from the 

 error very nearly. In this way several pairs may be observed, taking 

 the stars of the ' Nautical Almanac,' and the mean of the whole will 

 come out very near the truth. With a good sextant or circle, and a 

 mercurial horizon, we believe that a careful observer would get the 

 latitude within 5', in one fine night It is, however, supposed that 

 everything U favourable, and especially that the instrument is supported 

 on a stand. This is absolutely necessary for the accurate observation 

 of stars, which dance very perplexingly when the instrument is held 

 in the hand, and a high power applied. If a stand cannot be afforded, 

 the sun i* far the best object to observe with a reflecting instrument. 

 It is always supposed that the observations are made for several 

 minute* before and after the meridian passage, and the time noted for 

 computing the reduction to the meridian. The meridian altitude of 

 the sun, such as it would be if observed on the meridian and freed 

 from instrumental and other errors, is then computed, and as the 

 longitude of the place is known, at least approximately, the declination 

 of the sun at its passage over that meridian may be computed from 

 the ' Nautical Almanac.' The meridian altitude -f- the south declination 

 of the sun, or the north declination, in, in the northern hemisphere, 

 the co-latitude of the place. With Troughton's circle, the limbs ore 

 alternately observed, to get rid of the sun's diameter, and the number 

 of observations, forwards and backwards, should be equal, so as to get 

 rid of the index error. With the repeating reflecting circle, the obser- 

 vations should also be of the upper and lower limbs alternately, and 

 should be carried quite round the circle, so as to get rid of eccentricity. 

 In the sextant the index error should be carefully determined before 

 and after each day's observations, and the alternate limbs observed 

 exactly a* with the circles. There is, however, no way of getting rid 

 of exosntricity in the sextant by observing one object, and any fault in 

 determining the index error will vitiate Uie latitude to half its amount. 

 While the circles will probably give a latitude to nearly 5', with a very 

 careful series of observation* of the sun, the sextant used with equal 

 care might be out 10" or 15'. It is evident therefore, that where 

 accuracy i* an object, the observer ought, if possible, cither to use a 

 circle or to mount the sextant iijx.n a stand, and observe stars as we 

 have above described. It U an additional reason to carry a stand, 

 when practicable, that in luw latitudes the sun cannot be observed at 

 all for the latitude, nor any object which is elevated 65" or 70. In 

 this caw star* must be used; and without a stand, the observation. 

 neing high magnifying powers, is difficult and unsatisfactory. In 

 peering of the horizon we always mean a m-i m d borbwo, . 

 another i* specified. The glasses of the roof should l~- truly pi me and 

 parallel, but by reversing the horizon for half the observation!! any 

 error of this kind is destroyed. The mercurial horizon w unfortunately 

 heavy and inconvenient, and troublesome from iU tremors wl> 

 there to any motion. Several substitute* have been uaed. Oil or 

 tirade ha* been adopted with good success where the shaling from 

 earriagse, *c.. has prevented the use of mercury. Sometime* a piece 

 of glass I* art horizontal by a level applied to iU surface, or by a fluid 

 below it. to as to get a reflecting surface, but these generally absorb 

 too much light to be need conveniently for (tan, and are not very 

 tra-tworthy. The bast substitute mm* to be a piece of speculum 

 metal, ground plane, and laid horizontal by a level. It is certainly the 

 brightest, and therefore the best for stars, but it must In- remembered 

 that horizons which are not wlf-ragoUted, by bring fluid, are scarcely 

 tobetnarfedunderahotaim. Troughton's reflecting circle U rather 

 heavy atd reading three vernier* lor every observation Is t 

 poaallT at night, but uxurate, and fewer ol-ervati.m. are 



I Juris rrlUctin*; circle may be made inu.-h smaller and 





 umlerof obaerraUuns, and more reduction. The simple ** 



more manageable, but require* greater precautions and checks in its 

 use. But with any of these a skilful observer will get the latitude 

 very nearly. Sextants are made of all sizes from 10 inches radius 

 (which is probably not so good a* 8 inches) down to the snuff-box 

 sextant of 1 4 inches radius. For travellers who cannot afford to carry 

 much weight, the 3-inch sextant is very convenient. In a com- 

 munication to the Royal Astronomical Society, Mr. Lowell states, that 

 with a 3-inch sextant made by Dollond, which |iacks up, 

 horizon, and all, in a box 43 inches square and 27 deep, he found 

 that he could get the latitude within 10', and the time to 1"0 by 

 observations of stars. The horizon was of speculum metal, i 

 by himself, and m-t true by a level. The observations sent with the 

 account completely justify Mr. Lowell's opinion, but one observer 

 differs more from another in sextant observations than in any other 

 class of astronomical instruments ; with the snuff-box sextant, altitudes 

 may be got within 1'. The state of the barometer and thermometer 

 must be noted at the time of all observations for the latitude in 

 to compute the true refraction. At the same time we may n-maik, 

 that if the observations be balanced, that is, if the altitudes 

 north have nearly corresponding altitudes to the south, the rel'i 

 will affect the observations like an instrumental error, and the 

 variations depending on the barometer and thermometer will be quite 

 insensible. 



5. Observations of Polaris may be taken at any time for the latitude, 

 and there ore tables for approximate reduction given in the ' Nautical 

 Almanac ' for each year. 



6. At sea the sextant is the only instrument which con be used, and 

 the latitude is generally got by observing the altitude of the sun's 

 lower limb wheu on the meridian, above the sea horizon. This U 

 rather a rude process, but the resulting latitudes ore generally true to 

 1', or at worst to /. The moon, since 1834, the date of the improved 

 and extended ' Nautical Almanac,' may be very conveniently usod for 

 finding the latitude at sea, and the brighter planets and stars arc often 

 observable on the meridian. The latitude may also be deduced from 

 two altitudes of the sun, and the time elap.-ed lietwcen the 

 vations, or indeed from any two altitudes of two known < 

 bodies, one of which is near the meridian, and the other distant 



it, as persona not acquainted with spherical trigonometry may - 

 themselves on the celestial globe. There is a considerable difficulty in 

 seeing the sea horizon by night, which is somewhat reduced by getting 

 as near the level of it as you can. 



Determination of the Longitude. The determination of the longitude 

 of nny place on the earth's surface, astronomically considered, resolves 

 itself into two parts, the finding the time at the place of observation, and 

 finding the time, at Me taint moment, on the first meridian (we shall 

 always apeak of Greenwich), or at any place the longitude of which 

 from Greenwich is well known. It will be more convenient to classify 

 the methods of finding the longitude by the phenomena than by thu 

 instruments. 



Determination of Tim' at the Pliifr. 1. This is beat and most easily 

 done by a transit instrument, and the time, when found, is kept by a 

 clock or chronometer. [TBANSIT.] The transit however is neither a 

 very portable instrument, nor U a proper situation for it, we mean unc 

 sufficiently steady, readily found. 



\L. The time can also be found from the altitude of the sun, planets j 



or stars ..nt. ..f lh.. meridian. Thus let r IKS the pole, 7. the zenith, and 

 /eiiith dixt.-mc.i-, or K li the altitude of any heavenly body, tin) 

 right ascension oiid derlinatii.il of which are well known, and conse- 

 quently the ].ol ir di-t .nee p . Kr..m thcw- d.ila and I- /. the co-l.r 

 of the plioe, Ut* angle zrg can be computed, colled the hour 

 and tl,>. if the b,>dy be the sun, and to the west of the meridian, in the 



i noon ; or if the sun bo to the east, the lion, 

 i< the n/i/xirriii tune U-f,,rc noon. Thi* apparent tini. 

 to m ran time with the data of the ' Nauti. d Almanac.' \Vliun the 

 object observed is a planet or star, the hour angle being added < 

 right ascension when the body is to the west, or subtracted from th 

 right ascension when the Uxly is cast, will give the sidereal time, which 

 can be reduced to mean solar time with only an approximate knowledge 

 of the longitude. The problem therefore of finding the time 00 

 generally in observing the altitude or zenith distance of any known 

 -nid determining the hour angle from it. The repeating or alti- 

 1 azimuth circl.-s arc v.rv lit i'..r ihi. ]>nr|>.j.w, but the most 

 usual nnd |H.tt.il,lc instrument is a reflecting circle or sextant with 

 iU horizon and a chronometer. The observations of altitude should 

 H iiiii.-h as ]M)niblc on the prime vertical, that i, when 



1 or west. Again, to gel rid of instrun, 

 and also to save computation, the mm should bo observed, when 



