Telegraphic Determinations of Longitude. 15 
work in the West Indies, differences of longitude having been 
measured between nearly all the important points connected by 
telegraph. The Latitude of all the stations, was also determined 
by the zenith telescope method, and the position of the stations 
was referred either to the observation spot previously used, when 
that could be identified, or to some prominent landmark. 
Between St. Thomas and Santa Cruz, the measurement was made 
twice, the observers exchanging stations at the completion of the 
first series of observations. This was to eliminate the effect of 
their personal errors, and to obtain a value of these, which might 
be applied to the other measurements. It has long been known 
that different people perceive the same phenomenon at different 
times, varying with different individuals, but reasonably constant 
with the same individual. In the particular case of observing the 
transit of a star, most people will record it on a chronograph from 
one to three tenths of a second after it happens. In the method 
of observing by eye and ear the error is generally much greater. 
The whole question of personal equation, however, is a mixed one 
-and I. will not attempt to discuss it, but will only give some of 
the results obtained in this particular work. In longitude meas- 
urements the error from this cause is half the difference of the 
personal equation of the two observers. If this difference 
remained constant, then it would be easy to find it once for all, 
and apply it to all measurements made by the same observers. 
In the West India work, it was assumed that it did remain con- 
stant, and half the difference between the two measurements 
made from St. Thomas to Santa Cruz, was applied to all the other 
links. The correction was quite small, being only 0°.025. In 
subsequent work by the same and other observers it was deemed 
wiser not to apply any corrections at all, rather than one that 
was probably not exact, and might be much in error. To show 
the fluctuations to which this elusive quantity is subject, I will cite 
the results of some observations made to determine it, by observers 
engaged in this same work at a subsequent period. In April and 
May, 1883, at Galveston, Texas, two observers D. and N. having 
just completed a telegraphic measurement between that place and 
Vera Cruz, Mexico, made some observations for the determination — 
of their relative personal equation, by observing transits of alter- 
nate stars under the same conditions as near as possible. Both 
used the same instruments, transit, chronometer and chrono- 
graph. On April 30, two sets of observations were made, show- 
