GREENWICH OBSERVATORY. 
273 
the second and decimal of a second at which the object passed 
each wire. Since 1851 they have been registered by an electric 
chronograph. A sheet of paper on a cylinder rotating uni- 
formly receives a puncture from a point on the armature of 
an electro-magnet at every beat of the Transit-clock : thus a 
time scale is formed by the clock. The observer taps a key 
mounted on the telescope eye-piece as the object under obser- 
vation crosses each wire ; his tap completes a galvanic circuit 
which works another electro-magnetic pricker ; his punctures 
fall among the clock punctures, and their times can be read off 
thereby. The prickers travel down the cylinder as it rotates, 
and thus give a spiral line of registers. Uniform rotation of 
the cylinder being of the highest importance, it is secured by a 
driving-clock controlled by a rotating in place of a vibrating 
pendulum. 
Since the Transit-clock is a measuring instrument of the 
most accurate kind, its excellence and the steadiness of its 
rate should be of the highest character attainable. The one in 
the Transit Circle room at Greenwich having grown old and 
exhibited some slight infirmities, a new Sidereal Motor has 
lately been mounted, in a basement where the temperature is 
nearly uniform, at a distance from the Transit instrument. It 
is, however, in connection with the chronograph, and nearly all 
Transit observations are made (galvanically) by it. It also 
gives controlling currents to several subsidiary clocks. Its rate 
is wonderfully regular ; so uniform, indeed, that it faithfully 
exhibits the small changes which are produced by barometric 
fluctuations. 
The Transit Circle being the chief instrument of the Obser- 
vatory, it is in almost constant use whenever the sky is clear. 
A daily course of work with it includes Transits and North 
Polar Distances of the sun, moon (when visible on the meridian), 
all the large planets that pass before 3 o’clock in the morning, 
all the small planets passing between 10 p.m. and 1 a.m. during 
the first half of the lunation,* transits of a selection of the prin- 
cipal or fundamental stars for error and rate of the clock, an 
upper and lower culmination of a polar star for azimuthal error, 
an observation of the reflected meridional wire for level error, 
and of each collimating mark for collimation error of the 
Transit telescope ; and observations of four stars, two north, two 
south of the zenith, each by reflexion and directly, for the hori- 
zontal-point error of the circle. These secured, as many extra, 
or small stars as can be are observed for the ultimate formation 
* By arrangement with the Paris Observatory the small planet obser- 
vations are divided ; Greenwich observing from new to full moon, and Paris 
from full to new moon. 
VOL. XI. — NO. XLIV. T 
