CHRONOMETERS.] 



NAVIGATION NAUTICAL ASTRONOMY. 



1097 



The general mode of proceeding is this : Observe the 

 gun's altitude shortly before noon, and note the time. 

 Note the instant of the same altitude in the afternoon. 

 For great accuracy, several pairs of equal altitudes should 

 be obtained. Take the mean of the A.M. and P.M. times 

 by watch : this, when the ship does not change her lati- 

 tude, is the time by watch of apparent noon. To render 

 any correction for change of latitude unnecessary, it 

 would be desirable to alter the course of the ship, so as 

 to preclude this change, when the interval between the 

 observations is but short, as it should be to warrant 

 the dispensing with correction for change of declination. 



On account of the constancy of its declination, a star 

 is the best suited for the purpose of the present problem; 

 but a change in the state of the atmosphere, during the 

 interval of the observations, may interfere with the pre- 

 cision of the result, especially in the case of low altitudes ; 

 attention to the indications of the barometer and ther- 

 mometer is therefore necessary to the attainment of ac- 

 curacy. But the observations, to be free from all source 

 of error, should be made on shore ; for if the interval be 

 large, there is, at sea, some uncertainty as to the cor- 

 rection for change of latitude ; and if the interval be 

 small, there is some uncertainty ag to the altitudes, 

 especially in high latitudes, on account of the slowness 

 with which altitude changes near the meridian. 



O.N FINDING TUB LoNGITUDE BY A CHRONOMETER. 



From what is shown in the preceding articles, it appears 

 that the time at the ship can be accurately obtained by 

 is of altitudes of the sun or fixed stars, assisted 

 by the longitude, by dead-reckoning, and the time by 

 estimation ; and therefore all that is necessary for the 

 d ;terinination of the ship's longitude is, that we know 

 also the time at Greenwich at the same instant. If we 

 can only discover what o'clock it is at the same instant 

 at two different places on the globe, however distant, we 

 may at once infer the difference of longitude of those 

 places. We have only to convert the difference of time 

 into degrees, at the rate of 15 to one hour, to effect this 

 object. To find the time at Greenwich at any instant is 

 therefore to solve the problem of the longitude ; and as 

 noticed at page 1092, it is to supply this important in- 

 formation that BO much skill and perseverance havu been 

 bestowed on the chronometer. 



It is not necessary to the perfection of this instrument 

 that it should show the exact time at Greenwich, which, 

 in fact, a chronometer never does ; what is required is, 

 that its action be uniform and regular. The difference 

 between Greenwich mean time at any instant and the 

 time shown by the chronometer, is the error of the chro- 

 nometer at that instant ; and its average gain or loss in 

 24 hours is its mean daily rate. There is a depot estab- 

 lished at the Royal Observatory, Greenwich, for the 

 reception of such chronometers as the makers choose to 

 Ki'iicl, and where their errors and rates are determined by 

 tlie Astronomer Royal ; and few persons would now pur- 

 chase a chronometer for use at sea, without the proper 

 official certificate of these particulars. If the original 

 error on Greenwich mean time be allowed for, and the 

 proper correction be made for the accumulated daily rate, 

 the Greenwich time at any instant will of course be cor- 

 rectly obtained, provided that nothing has disturbed the 

 action of the chronometer after it left the observatory. 



It will have been observed in the foregoing articles, 

 that tho estimated longitude and time at the ship have 

 i employed tolely for the purpose of getting the 

 Greenwich time when the altitude was taken, with suffi- 

 cient accuracy to enable us to get the co-declination or 

 polar distance of the object observed ; and we have seen 

 that even a considerable error in the estimated quantities 

 will not sensibly affect the precision of the result. But 

 if the chronometer can at all be depended upon, the 

 Greenwich time of the observation will become known at 

 once, and, unless great derangement of the chronometer 

 have taken place, with m>>ru accuracy than it can be 

 found by the estimated longitude and time at the ship. 

 The problem just disposed of may therefore be correctly 

 solved by aid of even an iii'lillrrcnt chronometer; but 

 if the time at Greenwich, after the corrections for error 



VOL. I. 



and rate, be not correctly furnished by the instrument, 

 there will of course be a proportionate error in the lon- 

 gitude inferred from it. 



We are anxious that the student should clearly under- 

 stand how it happens that, from erroneous data, we ob- 

 tain accurate results, even in a imtter of so much delicacy 

 as the determination of the time at sea. The errors re- 

 ferred to can affect only two elements of the calculation 

 the declination and the equation of time ; and from the 

 slowness with which these vary, a few minutes, more 

 or less, can have no sensible influence. 



ON FINDING THE RiTB OP A CHRONOMETER. It is 



evident that if the rate of a chronometer remained in- 

 variable, the time at Greenwich could always be accu- 

 rately deduced from it ; but a sudden jerk or concussion, 

 change of climate, local attraction, and even the motion 

 of transporting it from the observatory to the ship, may 

 cause the rate to vary. The scientific navigator, there- 

 fore, avails himself of every favourable opportunity of 

 ascertaining the rate of his chronometer. This is best 

 done when the ship is in harbour, and in the neighbour- 

 hood of a fixed observatory, at which the rate may be 

 ascertained like as it was at Greenwich. For this pur- 

 pose the chronometer need not be carried on shore. The 

 time may be taken from it by a good seconds-watch, and 

 the watch then carried to the observatory and compared 

 with the mean-time clock the error of the chronometer, 

 on the mean time at that place, will thus be seen ; and 

 this comparison being repeated day after day, the daily 

 deviation from the exact amount of the error will be dis- 

 covered, and hence the mean daily rate. The ship then 

 sails afresh with the time at the new meridian, and 

 therefore the longitude of that meridian being known 

 with the Greenwich time. 



Or, without resorting to a fixed observatory, the rate 

 may be ascertained, when the ship is in harbour, by find- 

 ing the mean time at the place, as explained in last prob- 

 lem. A comparison of this with the mean time shown 

 by the chronometer will, in like manner, show the error 

 on mean time at the place ; and the observations bein.; 

 repeated day after day, the rate will become known. 

 The altitudes may in general be obtained with greater 

 precision by taking them on shore, by means of an arti- 

 ficial horizon, the chronometer time being carried theru 

 by the watch. 



Every attention should be paid on ship-board to cir- 

 cumstances likely to interfere with the rate of the chro- 

 nometer : it should be kept as much as possible out of tho 

 influence of changes of temperature, and its horizontal 

 position should remain undisturbed : to secure this per- 

 manence of position, it is frequently suspended on gim- 

 bals, like the compass. It should be wound up by the 

 action of the key alone, the chronometer itself being kept 

 steady ; if the instrument be turned as well as the key, 

 the motion of the balance-wheel will be affected, and the 

 rate disturbed. We have hitherto spoken of the chrono- 

 meter, as if ships in general carried only one ; but in 

 long voyages, or in voyages tmdertaken for Hcientitie 

 purposes, ships usually take several chronometers as 

 checks to one another, and to provide against accident. 

 Captain Fitzroy, in his surveys of the coast of South 

 America, took so many as twenty-two chronometers. 

 The apartment in which the chronometers are kept is 

 called the chronometer-room, and the temperature of it 

 is sometimes regulated by lamps, with the aid of the 

 thermometer. This should be the permanent depository 

 of the time-keepers, and they should be meddled with 

 only for the purpose of winding them up, which should 

 be at the same regular interval ; and, of course, they 

 should not be allowed to run down. 



The error and rate of a chronometer being found, as 

 explained above, the longitude is determined, as in the 

 following example, from Captain Kater's article in the 

 Encyclopedia Metropolitana. 



Examples. 



1. On the 2nd of June, 1823, .the true altitude of the 

 sun's centre was 30 2', when the chronometer showed 

 5h. 1 in. Os. ; the latitude was 40 5 N. ; and the sun's 



