Telegraphic Determinations of Longitude. 3 
lites by the shadow of the planet, the bursting of a meteor, and 
the appearance or disappearance of a shooting star. The difi- 
culty of identifying these last mentioned objects and the impos- 
sibility of foretelling their occurrence prevents the extended use of 
this method. ; 
Terrestrial signals may be used and among these can be 
included those sent by the electric telegraph. But when two 
stations are near together a signal may be made at either or at an 
intermediate station, which can be observed at both, the time 
may be noted at each of the stations and the difference found 
directly. These signals may be made by flashes of gunpowder, 
or the appearance and disappearance of a strong light, or a pre- 
concerted movement of any object easily seen. The heliotrope 
reflecting the image of the sun from one station to the other with 
an arrangement for suddenly eclipsing it, is a useful and efficient 
apparatus. 
Various truly astronomical methods have been employed with 
good results, of these may be mentioned moon-culminations, 
azimuths of the moon, lunar distances, ete. 
Coming now to the use of the electric telegraph for this pur- 
pose the following is a rough outline of the methods employed. 
Suppose two stations A and B connected by wire, and provided 
with clocks, chronographs and transit instruments. A list of 
suitable fixed stars is compiled and each observer furnished with 
a copy. The observer at A the eastern station, selects a star 
from his list and sets his transit instrument upon it. He is fur- 
nished with a key by which he can send telegraphic signals over 
the lime and also mark the time on his own chronograph. The 
instant he observes the star crossing the spider line which repre- 
sents the meridian, he taps his key, thus registering the time on 
his own chronograph and on that at station B and this operation 
he repeats with as many stars as necessary. B has his instrument 
set for the first star, and when it crosses his meridian, he taps his 
key marking the time on his own chronograph and also on A’s. 
Then, disregarding instrumental and personal errors and the rate 
of the clock, A has a record of the times at which the star passed 
both meridians. The difference of these times is the difference 
of longitude sought, except for an error due to the time occupied 
in the transmission of the signal over the wire between the sta- 
tions. B also has a record of the same difference of time with 
the same error affecting it in the opposite way. A mean of these 
