4 National Geographic Magazine. 
two differences, will be the true difference with the error of trans- 
mission eliminated. This method has the advantage of not 
depending upon the computed position of the star. The instru- 
mental errors may be allowed for, as well as the rate of the 
clocks, and the personal error may be eliminated by the exchange 
of stations. 
There are disadvantages inseparable from this method, how- 
ever, especially when the meridian distance is great. A star 
observed at the first station, may be obscured by clouds at the 
time of its meridian passage at the second. And the weather 
generally, at the two stations may be cloudy, so that while stars 
can be observed at intervals, yet it may be impossible to note the 
meridian passage of the same star at both places on the same 
night. Then the telegraph lines are usually the property of some 
commercial company and while their use for a short time might 
be freely granted, yet a protracted occupation of them as neces- 
sary when the meridians are distant from each other, would prove 
a serious hindrance to their regular business. 
The method at this time most generally employed, is to observe 
at each station a number of stars entirely independently of the 
other. From these stars are deduced the clock errors and rates 
upon the respective local times. Then at some prearranged 
period, communication is opened between the stations, and a com- 
parison of the clocks made which shows their exact difference at 
a given instant. By applying the error to the time as shown by 
the clock at this instant, the exact local time at each station is the 
result, and applying the difference between the clocks as shown 
by the comparison, the required difference of longitude is readily 
obtained. 
These methods originated, as did the electric telegraph, in the 
United States, and soon after Morse’s invention came into prac- 
tical use, they were extensively employed by the Coast Survey, in 
accurately determining points in every part of the country that 
could be reached, no pains being spared to make the determina- 
tions as accurate as possible. Upon the completion of the first 
successful Atlantic cable in 1866, an expedition was organized 
and placed in charge of Dr. B. A. Gould, for the purpose of 
measuring the meridian distance between Greenwich and the 
Naval Observatory at Washington. This was successfully car- 
ried out im spite of numerous difficulties, and the result proved 
that the determinations already made upon which the most 
