288 Mr. Henderson on the difference of meridians 
made with the utmost care and attention on the part of the 
observers. 
The details of the new computation are as follows. 
As mentioned by Mr. Herschel, rockets were exploded 
at Wrotham, which were observed at Greenwich and Fair- 
light down ; at La Canche, on the French Coast, which were 
observed at Fairlight and Lignieres ; and at Mont Javoult, 
which were observed at Lignieres and Paris. 
Before the difference of meridians can be eliminated from 
the observations, the rates of the chronometers employed at 
Lignieres and Fairlight must be ascertained. Let P be the 
sidereal time at Paris, L the corresponding time indicated 
by the chronometer at Lignieres ( these times being deter- 
mined by simultaneous observations of signals ) P' and L', 
the same times for a subsequent night, then (P 7 — - P) — 
(L' — L) is the retardation of the chronometer at Lignieres 
on sidereal time during the chronometer time (L 7 — L) ; 
24 x [(P P) (L L)] __ ^ t j ie rate t j ie chronometer, 
(L — L) 
or the equation to be added to 24 hours of chronometer time 
to obtain the corresponding interval of sidereal time ; and 
r ^ L ' T i s *h e equation to be added to any portion of chro- 
nometer time (L' — - L) to reduce it to sidereal time. 
In like manner with regard to the chronometer at Fair- 
light, let G and F be the corresponding sidereal time at 
Greenwich and chronometer time at Fairlight, G 7 and F 7 
the same times for a subsequent night, then 
24 * G ) (F — — r' is the chronometer’s rate, or the 
(* — G 
