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 any correction to their observed dif- 
ferences ; and it will suffice to apply to the uncorrected 
value of A (= A — Z, or) 
A=(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 i8 h 39“ 52 s . 5 
true sidereal time at Paris, and at Lignieres at io h 4 9 m 4T.0 
by the Lignieres Chronometer. About 5 m after this, a signal 
made at La Canche was observed at Lignieres to happen at 
io h 54 m 53 s . 2, and at Fairlight at io h 46“ 37 s . 5 by the Fair- 
light chronometer. Finally, a third signal was made about 
5 m later still at Wrotham, and observed at io h 51“ 59 s . 4 by 
• Might not telegraphs be employed to ascertain the difference of longitudes of 
the stations between which they are established i 
