Telegraphic Determinations of Longitude. 13 
arrived at Washington in April, 1875, and the time until 
November was spent in working up the winter’s observations. 
Speaking in a general way this work is as follows :—From obser- 
vations extending over many years, the exact positions mm the 
heavens of a large number of fixed stars have been found, so that 
their times of passing any meridian can be computed with great 
accuracy. The transit instrument is furnished with an eye-piece 
containing a number of parallel lines usually made of spider silk. 
These are placed in the focus of the instrument, and it is set in — 
position, so that the middle line of the group is in the plane of 
the meridian. The observer provides himself with a list of 
desirable stars, and setting his instrument on those he may choose, 
records the time at which they pass each of the spider lines, by 
tapping his chronograph key. If there were no instrumental 
errors to be discovered and allowed for, if the star’s place were 
known absolutely, and the observer had no personal equation, 
then it would be only necessary in order to find the error of the 
clock, to observe one star upon the middle line of the reticle. 
The difference of the clock time of transit and the real time as 
already known, would be the clock error and no further trouble 
would be required. But as none of these conditions are fulfilled, 
it is necessary to multiply observations in order to eliminate acci- 
dental errors, and to obtain instrumental corrections which may 
be applied so as to get the most probable result. Accidental 
errors of eyesight and perception are nearly eliminated by taking 
the star’s transit over several lines instead of one and using the 
mean. Some of the instrumental errors are from the following 
causes. If the pivots which support the telescope are unequal in 
size the axis of the tube will be thrown to one side or the other 
of the meridian, and the star will be observed either before or 
after it crosses. The weight of all transit instruments causes a 
flexure of the horizontal axis and this effect is at its maximum in 
those of the prismatic pattern. The spider lines must be adjusted 
so that the middle one is exactly in the axis of the tube, or as this 
can seldom be done the resulting error, called the collimation, must 
be found. The horizontal axis of the instrument must be as 
nearly level as possible, and the error in this respect must be 
found by frequent applications of a delicate spirit level. Finally 
the instrument must be directed as nearly as possible to the north 
and south points of the horizon, and a correction must be made for 
any error in this respect. The result of each of these errors is to 
