542 



THE LATITUDES AND LONGITUDES. 



and forms a conspicuous object from the river. The towers of the observatorv 

 ar<; at all times visible from ships sailing down the river. It was, therefore, 

 decided that a signal should be given at the instant of one o'clock in the after- 

 noon of each day ; by observing which, navigators within view of the observa- 

 tory could correct their chronometers. The signal adopted for this purpose 

 was the sudden fall of a large black ball, placed upon a pole raised from the 

 top of one of the towers of the observatory. 



Before elevating the ball, at five minutes before one o'clock, a signal is made 

 of the intention to do so by raising it half-mast high. Observers are then in- 

 structed to prepare their 'chronometers ; and as the descent of the ball occupies 

 several seconds, they should confine their attention to observing the moment 

 when the ball leaves the top, as it is that alone which indicates the hour. 



The use of this signal is not merely confined to the indication of the mean 

 time at Greenwich for navigators going down the river. By observing the 

 drop of the ball, repeated day after day, mariners who are in the river will be 

 enabled to ascertain the daily rate of their chronometers. Thus, if a clock 

 were found to show the time of 3 min. 5 sec. after 1 o'clock at the moment of 

 dropping the ball one day, it will appear that the clock is 3 min. 5 sec. faster 

 than the mean Greenwich solar time. On the following day, if you again ob- 

 serve the descent of the ball, and find that the clock shows 3 min. 7 sec. after 

 1 o'clock, you find that it gains 2 seconds per day. Thus you are enabled, not 

 only to ascertain the actual error of the chronometer, but also predict the man- 

 ner in which that error will be augmented or diminished for the future. 



In noticing the different methods which have been proposed for determining 

 the longitude, I ought not to omit one which has been recently resorted to with 

 considerable advantage, and which is called the method of determining the 

 longitude by moon-culminating stars. In the practice of this method a star is 

 chosen which culminates or passes the meridian nearly at the same time with 

 the moon, and which differs so little in declination with the moon, that it may 

 be seen at the same time in the field of view of the telescope. The transit of 

 the star and that of the moon's limb, is observed at both stations, and the differ- 

 ence of the time at the two stations noted. This difference being dependant 

 on the moon's change of position on the firmament, in passing from the meridian 

 of one station to the meridian of the other, will enable the observers to deter- 

 mine the time which the centre of the moon takes to pass from the one meridian 

 to the other, which will give the difference of the longitudes. 



The spirit of this method is derived from the great accuracy of the knowl- 

 edge we have acquired of the moon's motions, and the precision with which 

 we can observe its transits over the meridians. In the practice of this method, 

 it is indispensable that the moon and star should differ so little in declination 

 that the position of the telescope will not require to be changed to observe 

 their respective transits. Although the method has been called that of moon- 

 culminating stars, the only reason why the moon and star should be required 

 to pass the meridian nearly together is, that the same errors may, as far as 

 possible, affect both transits, and if so no effect would be produced on the ulti- 

 mate result. 



