356 
POPULAR SCIENCE REVIEW. 
the month of November, is the 44 equation of time ” of our 
almanacs. Its amount for each day at noon to the nearest 
second of time is contained in common almanacs usually under 
the heading either of 44 clock before sun ” or 44 clock after sun : ” 
for greater accuracy reference must be made to the Nautical 
Almanac . 
Having shown the relation existing between apparent or 
sun-dial time and mean solar or clock time, we see how it is 
that, taking time from a sun-dial and allowing for the equation 
of time, ordinary clock time is obtained. But a sun-dial is 
useless for any accurate determination ; and of other instru- 
ments and methods, giving something more of accuracy, space 
will not allow us to speak. We must hasten to describe 
that special instrument, the 44 transit instrument,” which is 
always employed in fixed astronomical observatories. This 
instrument consists of a telescope fixed at its centre to a cross 
axis supported at the extremities on bearings firmly fixed in an 
east and west position, so that on turning the telescope on its axis 
it points successively to all parts of the meridian (that imagi- 
nary great circle in the heavens which corresponds to the 
brazen meridian of a celestial globe, and at which the heavenly 
bodies attain, between rising and setting, their greatest alti- 
tude). In order that it may do this precisely, the line of sight 
of the telescope must be at right angles exactly to the cross 
axis, and the axis itself must be truly level and also precisely 
east and west ; but no instrument, if placed in exact position, 
will long remain so. It is therefore usual to register its small 
deviations, and apply corrections as necessary to the observa- 
tions. The instrument made use of at Greenwich is the one 
meridian instrument of the Observatory, the noble transit-circle 
(designed by the present Astronomer Ko} r al). Such an instru- 
ment is used for many purposes besides the determination of 
time, but it is with this use of it only that we have to do here. 
On looking into its telescope we see a number of delicate 
vertical threads across which objects must pass in their transit 
through the field. The centre thread represents the meridian, 
the others being uniformly distributed, an equal number on 
each side. The time at which, by the sidereal clock (always 
that employed in an observatory), the object to be observed is 
upon each thread being noted, the mean of the observed times 
gives a more accurate value of the meridian transit. Usually 
an observer counts the beats (seconds) of his clock, and esti- 
mates the time at which the object is on each thread ; but at 
Greenwich this method is no longer pursued, for by means of 
the chronograph (brought into use in the year 1854) all transits 
are registered by galvanism. Of this instrument we cannot 
here attempt description further than to say, that by its means 
