The Determination of Longtitude. 399 
puting the final longitude difference, Montreal west of 
Cambridge, 9". 47°.51045019., the 0°19 indicating the 
probable error of the result, is equivalent to a distance east 
or west of about 20 feet. 
In longitude determinations carried on through land 
lines the signals obtained are sharp and the transmission 
time small, so that there is little or no difficulty in compar- 
ing the clocks at the distant stations. When, however, an 
ocean intervenes between the stations, and the signals have 
to be sent through 3,000 miles of cable the difficulties in 
the way ofthe clock comparisonsare greatly increased. In 
fact until quite recently it was not practicable to get any 
automatic record such as we obtain by our chronographs 
on land lines. In the longitude determinations between 
Europe and America, carried outin 1866, ’70 and’72 by the 
U.S.C. & G., Survey, the receipt of the signal through the 
cable was observed by the deflection of a beam of light from 
the galvanometer mirrors, at that time exclusively employed 
in ocean telegraphy. This was a matter of considerable un- 
certainty and the cause of much anxiety and trouble to the 
observers engaged in the work. 
In our recent longitude work between Montreal and 
Greenwich the siphon recorder was found, after some little 
difficulty in its adaptation had been surmounted, to answer all 
the requirements for an accurate comparison of the clocks 
at the cable ends. This is believed to be the first occasion 
on which time signals in longitude work have been received 
and automatically recorded through an ocean cable. While 
the wave time was of course large, (amounting on the aver- 
age to 0°.26) the variation from night to night was only a 
few hundredths of a second. 
There is here, however, a possible source of error. The 
wave time may not be the same for a signal passing east- 
ward as forone passing westward. ‘This we are not able at 
present to determine, but we have good ground for assum- 
ing that the difference cannot in any case amount to any 
more than a very few hundredths of a second, if it exists at 
all, The difficulty in the comparison of the clocks is not 
