12 National Geographic Magazme. 
times in quick succession, thus sending a series of impulses 
through the line, which appeared at the other end as a rapid 
movement of the light to and fro. This was a warning signal, 
and the observer at the second station with his eye on the light, 
tapped his chronograph key in the same way making a series of 
marks, which indicated the beginning of the comparison. The 
first observer exactly at the sixtieth second by his chronometer 
pressed his key quickly and firmly and repeated this operation at 
every fifth second for one minute. The second observer tapped 
his key promptly as soon as he saw the light move, thus register- 
ing the time on his chronograph. The minute at which the first 
signal was sent, was then telegraphed, and repeated back, to 
_ insure against error, and the operation was repeated until sixty- 
five signals had been sent from one station and received at the 
other. Then the second observer sent the same number of sig- 
nals to the first in precisely the same manner, thus giving sixty- 
five comparisons of the chronometers in each direction. The 
results derived from this method are affected by errors from two 
causes. One is the personal error of the observers in sending 
and receiving signals and the other the time consumed by the 
electric impulse in traveling over the line and through the instru- 
ments. If the same strength of battery is used at each station, 
and the resistance of the instruments is the same, the errors aris- 
ing from this latter source will be eliminated by the double 
exchange. The observer sending the signals kept his eye on the 
chronometer and counted the second beats by both eye and ear, 
moving the hand which he had on the key slightly in unison with 
the beats, and could thus be sure of pressing the key at the proper 
time within a very small fraction of a second. At the other end 
of the line, considerable time is lost after the actual movement of 
the light before the observer can press his chronograph key, and _ 
the principal error affecting the result is the difference of this 
time in the two observers, which was found to be very small. 
As I have said, the cable was first used in the measurement 
between Kingston and Aspinwall, Lieut. Commander Green 
occupying the former station, and Mr. Rock the latter. After 
the successful completion of this link, measurements were made 
from Santiago de Cuba to Kingston, and to Havana. It was the 
intention to measure from this last. point to Key West, but about 
this time yellow fever broke out there and the expedition was 
ordered by the Secretary of the Navy to return. The Fortune 
