14. National Geographic Magazine. 
cause the star’s transit to be recorded too early or too late, and to 
get the true result they must all be found and applied with their 
proper signs. ‘The inequality of pivots and the flexure correction 
are found by delicate measurement and observations, when the 
instrument is first used, and are recorded as constants to be applied 
in all subsequent work. The level tubes are graduated and the 
value of their divisions obtained in angular measure. The collima- 
tion error is found by observing stars near the zenith in one 
position of the instrument and then reversing and observing 
others, or by taking the transit of a slow moving star over a 
portion of the spider lines then reversing and observing the same 
intervals in the opposite order. The error of azimuth, or devia- 
tion from the north and south line, is found by comparing the 
observations of stars whose zenith distances differ considerably. 
These corrections all being found and applied to the observation 
of each star, the result is the correct time of transit as shown by the 
chronometer, and the difference between that time and the true 
time, is the error of the chronometer. A mean of the observations 
of several stars on the same night, gives a very accurate value for. 
this clock error, and by comparing the results of several nights’ 
work, the rate is found. By applying the rate to the clock error 
it is reduced to any required epoch, -as for instance, the mean 
time of the exchange of time signals, and the difference of longi- 
tude is easily found. As may be imagined the computation and 
application of all these errors, exercising the greatest care to 
msure accuracy is a long and tedious process. The operations 
described give a very close result, but in order to arrive at the 
greatest accuracy obtainable the computations are made again by 
the method of least squares. 
In the Autumn of 1875, the expedition again took the field, 
this time in the side wheel steamer Gettysburg, which was much 
etter adapted to the work than the Fortune. The first link 
measured was between Key West and Havana. Key West had 
aiready been telegraphically determined by the Coast Survey, and 
now afforded a base for the system of measurements completed 
_and for those to follow. The next measurement was between 
Kingston and St. Thomas. Then from the latter place to Anti- 
gua and to Port Spain, Trinidad. From Port Spain, measure- 
ments were made to Barbadoes and Martinique. The position at 
St. Thomas was then re-occupied, and measurements made thence 
to San Juan, Porto Rico, and to Santa Cruz. This ended the 
