39 4 Canadian Record of Science 
(the precise fraction is at present not quite assured but we 
have hitherto called it 64) west of Greenwich. 
Now imagine one of our meridians—such as that of 
Greenwich—to be fixed, then the next adjo ining meridian 
to the west will, as the earth revolves, come up to it in one 
hour, the second in two and so on. Suppose a star or the 
sun to be on the meridian of Greenwich (i. e., in the plane 
of the meridian) then the next meridian will contain the 
star one hour afterwards. That is, in the case of the sun it 
is apparently 12 o’clock at Greenwich, when, at the westerly 
station, it is 11 o’clock. 
We sev, therefore, that if we can determine the local time 
at any given instant at any two stations and compare these 
times we have the difference,in longitude of the stations, 
If we know the time at Greenwich now to be one hour and 
thirty minutes past midnight, when here it is eight hours 
and thirty minutes past noon, our longitude is the differ- 
ence in these times, or five hours. The determination of 
longitude then consists in obtaining the local time at the 
two stations, and as in all English-speaking countries longi- 
tudes are referred to Greenwich, we must, directly or indi- 
rectly, know the time at Greenwich. Comparatively few, 
however, of the longitude determinations made are by 
direct connection with Greenwich, the usual practice be- 
ing to determine with the greatest possible accuracy the 
longitude of a conveniently situated observatory, or station 
in the given country and from this observatory to establish 
the various points required by differences of longitude 
with it. 
There are various methods of determining longitude, but 
none so accurate as what is callel the telegraphic method, 
This method admits of several modifications, but the plan 
which I shall first describe is now almost exclusively 
adopted. An observer is required for each station and 
usually two stations only are concerned, A programme of 
work having been agreed upon, the stations are occupied 
simultaneously by the observers. The instrumental outfit 
at each station should be a transit instrument, a clock and 
