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Page 21
The next elements of accuracy must be perfect time and perfect
notation of time. As has been said, we get our time from the stars.
Thus the infinite and heavenly dominates the finite and earthly.
Clocks are set to the invariable sidereal time. Sidereal noon is
when we have turned ourselves under the point where the sun crosses
the equator in March, called the vernal equinox. Sidereal clocks
are figured to indicate twenty-four hours in a day: they tick exact
seconds. To map stars we wish to know the exact second when they
cross the meridian, or the north and south line in the celestial
dome above us. The telescope (Fig. 21, p. 61) swings exactly north
and south. In its focus a set of fine threads of spider-lines is
placed (Fig. 23). The telescope is set just high enough, so that
by the rolling over of the earth [Page 65] the star will come into
the field just above the horizontal thread. The observer notes the
exact second and tenth of a second when the star reaches each
vertical thread in the instrument, adds together the times and
divides by five to get the average, and the exact time is reached.
[Illustration: Fig. 23.--Transit of a Star noted.]
But man is not reliable enough to observe and record with sufficient
accuracy. Some, in their excitement, anticipate its positive passage,
and some cannot get their slow mental machinery in motion till
after it has made the transit. Moreover, men fall into a habit of
estimating some numbers of tenths of a second oftener than others.
It will be found that a given observer will say three tenths or
seven tenths oftener than four or eight. He is falling into ruts,
and not trustworthy. General O. M. Mitchel, who had been director
of the Cincinnati Observatory, once told one of his staff-officers
that he was late at an appointment. "Only a few minutes," said the
officer, apologetically. "Sir," said the general, "where I have
been accustomed to work, hundredths of a second are too important
to be neglected." And it is to the rare genius of this astronomer,
and to others, that we owe the mechanical accuracy that we now
attain. The clock is made to mark its seconds on paper wrapped
around a revolving cylinder. Under the observer's fingers is an
electric key. This he can touch at the instant of the transit of
the star [Page 66] over each wire, and thus put his observation on
the same line between the seconds dotted by the clock. Of course
these distances can be measured to minute fractional parts of a
second.
But it has been found that it takes an appreciable time for every
observer to get a thing into his head and out of his finger-ends,
and it takes some observers longer than others. A dozen men, seeing
an electric spark, are liable to bring down their recording marks
in a dozen different places on the revolving paper. Hence the time
that it takes for each man to get a thing into his head and out
of his fingers is ascertained. This time is called his personal
equation, and is subtracted from all of his observations in order to
get at the true time; so willing are men to be exact about material
matters. Can it be thought that moral and spiritual matters have
no precision? Thus distances east or west from any given star or
meridian are secured; those north and south from the equator or
the zenith are as easily fixed, and thus we make such accurate
maps of the heavens that any movements in the far-off stars--so
far that it may take centuries to render the swiftest movements
appreciable--may at length be recognized and accounted for.
[Illustration: Fig. 24.]
We now come to a little study of the modes of measuring distances.
Create a perfect square (Fig. 24); draw a diagonal line. The square
angles are 90�, the divided angles give two of 45� each. Now the
base A B is equal to the perpendicular A C. Now any point--C, where
a perpendicular, A C, and a diagonal, B C, meet--will be [Page 67]
as far from A as B is. It makes no difference if a river flows
between A and C, and we cannot go over it; we can measure its
distance as easily as if we could. Set a table four feet by eight
out-doors (Fig. 25); so arrange it that, looking along one end, the
line of sight just strikes a tree the other side of the river. Go to
the other end, and, looking toward the tree, you find the line of
sight to the tree falls an inch from the end of the table on the
farther side. The lines, therefore, approach each other one inch in
every four feet, and will come together at a tree three hundred and
eighty-four feet away.
[Illustration: Fig. 25.--Measuring Distances.]
[Illustration: Fig. 26.--Measuring Elevations.]
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