for determining the difference of meridians ^ &c. 107 



ingly be allowed for. Now, as the intervals at which the 

 signals are made at the successive stations are small (only 

 five minutes), the gain or loss of the watches used may be 

 calculated for such small times to great nicety ; and, if the 

 watches were regulated to sidereal time, and of any ordinary 

 degree of goodness, the correction on this account would be 

 almost insensible ; or, if regulated, as is generally the case, 

 to mean time, the reduction from mean to sidereal time only 

 need be applied, neglecting the deviation of the rates from 

 strict mean time. The calculation then becomes of extreme 

 simplicity ; for since the watches have equal rates, we have 

 no occasion to apply aiiy correction to their observed dif- 

 ferences ; and it will suffice to apply to the uncorrected 

 value of A (= A — Z, or) 



^^ (A —B) + (B' — C) + (C - Z") 

 the mere reduction from mean to sidereal time for the in- 

 terval elapsed between the first and last signal ; or in other 

 words (regarding the whole operation as a species of tele- 

 graphing), for the time the message has occupied in its trans- 

 mission from one observatory to the other.* 



For example. On the 19th, a signal was made at Mont 

 Javoult, and noted at Paris to have happened at 18^ 39"" 5^' -5 

 true sidereal time at Paris, and at Lignieres at 10^ 49°* 4i'.o 

 by the Lignieres Chronometer. About 5°* after this, a signal 

 made at La Canche was observed at Lignieres to happen at 

 .10" 54° 53'-s, and at Fairlight at 10^ 46" sT-S by the Fair- 

 light chronometer. Finally, a third signal was made about 

 5" later still at Wrotham, and observed at 10^ 51°" 59^.4 by 



• Might not telegraphs be employed to ascertain the difference of longitudes of 

 the stations between which they are established ? 



