a certain
amount of light, which mitigates the blackness of terrestrial shadows
and tends to soften their outline. No such influences are at work on the
moon, and the sharpness of the shadows is taken advantage of in our
attempts to measure the heights of the lunar mountains.

It is often easy to compute the altitude of a church steeple, a lofty
chimney, or any similar object, from the length of its shadow. The
simplest and the most accurate process is to measure at noon the number
of feet from the base of the object to the end of the shadow. The
elevation of the sun at noon on the day in question can be obtained from
the almanac, and then the height of the object follows by a simple
calculation. Indeed, if the observations can be made either on the 6th
of April or the 6th of September, at or near the latitude of London,
then calculations would be unnecessary. The noonday length of the shadow
on either of the dates named is equal to the altitude of the object. In
summer the length of the noontide shadow is less than the altitude; in
winter the length of the shadow exceeds the altitude. At sunrise or
sunset the shadows are, of course, much longer than at noon, and it is
shadows of this kind that we observe on the moon. The necessary
measurements are made by that indispensable adjunct to the equatorial
telescope known as the _micrometer_.

This word denotes an instrument for measuring _small_ distances. In one
sense the term is not a happy one. The objects to which the astronomer
applies the micrometer are usually anything but small. They are
generally of the most transcendent dimensions, far exceeding the moon or
the sun, or even our whole system. Still, the name is not altogether
inappropriate, for, vast though the objects may be, they generally seem
minute, even in the telescope, on account of their great distance.

We require for such measurements an instrument capable of the greatest
nicety. Here, again, we invoke the aid of the spider, to whose
assistance in another department we have already referred. In the filar
micrometer two spider lines are parallel, and one intersects them at
right angles. One or both of the parallel lines can be moved by means of
screws, the threads of which have been shaped by consummate workmanship.
The distance through which the line has been moved is accurately
indicated by noting the number of revolutions and parts of a revolution
of the screw. Suppose the two lines be first brought i