402 Canadian Record of Science. 
leaves port, Greenwich time may be obtained and a com- 
parison made with the local time as determined by astrono- 
nomical observation at any point on the voyage. The in- 
strument used in the observations is of course the Sextant. 
Before the successful laying of the cable in 1866, our longi- 
tudes in America depended upon results obtained by this 
method, the chronometers being carried between Harvard 
College Observatory and Greenwich. The chronometer 
errors were of course determined at both observatories by 
the transit method. 
Amongst other methods, the chief are lunar distances 
and moon culminating stars, in both of which the moon 
stands for the hands of the clock, the vault of the heavens 
for the dial and the fixed stars amongst which the moon 
moves, for the marks on the dial. The nautical almanacs 
furnish the data by which, from an observation of the 
position of the moon with reference to certain fixed stars, 
Greenwich time may be computed at any instant. Owing, 
however, to the slow movement of the moon, the irregu- 
larity of its motion, and the unavoidable errors of observa- 
tion, these methods have not hitherto furnished results of 
any great accuracy. ‘The lunar distance method is suited 
to navigation and chiefly employed in long voyages where 
the chronometer rates are not sufficiently reliable to estab- 
lish Greenwich time. 
In fact, the honor of having improved the methods of 
determining longitude at sea is about equally divided be- 
tween the astronomers and the chronometer makers. This 
was recognized by the British when a long standing offer 
of a reward of £10,000 to any one who would find a suc- 
cessful method of determining longitude at sea, was divided 
between an astronomer who greatly improved the tables of 
the moon’s motion and a watchmaker who improved the 
marine chronometer. 
It was owing chiefly to the difficulty experienced in the 
determination of longitude at sea and the importance of the 
problem to navigation and commerce that the Royal Ob- 
servatory at Greenwich was founded. The duty of the 
