stars. The correction of an inaccuracy of no greater magnitude than
that has reduced our estimate of the distance of our sun 3,000,000
miles.

[Illustration: Fig. 21.--Mural Circle.]

Consider the nicety of the work. Suppose the graduated scale to
be thirty feet in circumference. Divided into 360°, each would
be one inch long. Divide each degree into 60', each one is 1/60
of an inch long. It takes good eyesight to discern it. But each
minute must be [Page 62] divided into 60", and these must not only
be noted, but even tenths and hundredths of seconds must be
discerned. Of course they are not seen by the naked eye; some
mechanical contrivance must be called in to assist. A watch loses
two minutes a week, and hence is unreliable. It is taken to a
watch-maker that every single second may be quickened 1/20160 part
of itself. Now 1/20000 part of a second would be a small interval of
time to measure, but it must be under control. If the temperature of
a summer morning rises ten or twenty degrees we scarcely notice it;
but the magnetic tastimeter measures 1/5000 of a degree.

Come to earthly matters. In 1874, after nearly twenty-eight years'
work, the State of Massachusetts opened a tunnel nearly five miles
long through the Hoosac Mountains. In the early part of the work
the engineers sunk a shaft near the middle 1028 feet deep. Then the
question to be settled was where to go so as to meet the approaching
excavations from the east and west. A compass could not be relied
on under a mountain. The line must be mechanically fixed. A little
divergence at the starting-point would become so great, miles away,
that the excavations might pass each other without meeting; the