THE HAMILTON ASSOCIATION. 73 
transit of the true sun is called apparent noon, and the transit 
of the second mean sun is called mean noon. A knowledge 
then must be had of the daily differences between the true and 
mean suns; this difference is called the equation of time, and is 
tabulated each day of the year in the Nautical Almanac. 
Thus, by the revolution of the Earth on its axis we can 
obtain measures of time of different values according to the 
reference points in the celestial sphere. By referring the 
meridian to transits of the stars we obtain sidereal time, to that 
of the true sun we have apparent solar time, by the mean sun 
mean solar time, and if we take the Moon, apparent lunar time, 
and so forth. — 
Time is almost universally obtained at observations by star 
transits, occasionally supplemented by solar transits. These 
observations are made with the meridian transit instrument, 
which is generally fixed so as-to be used as nearly as can be in 
the meridian only. The ordinary surveyor’s transit is in real- 
ity a small portable transit, and is capable, when well made and 
used with care, to give results especially in time work with al- 
most the precision of a large stationary transit in an observa- 
tory. There are a variety of forms in transit instruments, but 
the most common in use is that in which the telescope and hori- 
zontal axis bisect each other, and the two ends of the axis, 
called pivots, are supported on pillars with V-shaped tops, in 
which the pivots revolve. 
In the principal focus of the objective is placed a ring, 
upon which is fixed five or more parallel threads made from 
spider lines or very fine drawn platinum wires. Sometimes 
these are replaced by a glass diaphram with ruled lines. The 
threads when in proper adjustment should be parallel to the ver- 
tical plane of the telescone and perpendicular to the optical axis. 
This ring is held in place by four screws, by means of which the 
wires are properly adjusted. The wires are rendered visible 
at night by light thrown into the interior of the telescope 
through the hollow rotation axis from a lamp, and is reflected 
down the telescope by a small silvered mirror at the intersec- 
tion of the rotation axis and the axis of the telescope. The 
