whilst taking into account the altitude of the sun at the
moment of observation. This method also allows the calculating of the
depth of craters and cavities on the moon. Galileo used it, and since
Messrs. Boeer and Moedler have employed it with the greatest success.

Another method, called the tangent radii, may also be used for measuring
lunar reliefs. It is applied at the moment when the mountains form
luminous points on the line of separation between light and darkness
which shine on the dark part of the disc. These luminous points are
produced by the solar rays above those which determine the limit of the
phase. Therefore the measure of the dark interval which the luminous
point and the luminous part of the phase leave between them gives
exactly the height of the point. But it will be seen that this method
can only be applied to the mountains near the line of separation of
darkness and light.

A third method consists in measuring the profile of the lunar mountains
outlined on the background by means of a micrometer; but it is only
applicable to the heights near the border of the orb.

In any case it will be remarked that this measurement of shadows,
intervals, or profiles can only be made when the solar rays strike the
moon obliquely in relation to the observer. When they strike her
directly--in a word, when she is full--all shadow is imperiously
banished from her disc, and observation is no longer possible.

Galileo, after recognising the existence of the lunar mountains, was the
first to employ the method of calculating their heights by the shadows
they throw. He attributed to them, as it has already been shown, an
average of 9,000 yards. Hevelius singularly reduced these figures, which
Riccioli, on the contrary, doubled. All these measures were exaggerated.
Herschel, with his more perfect instruments, approached nearer the
hypsometric truth. But it must be finally sought in the accounts of
modern observers.

Messrs. Boeer and Moedler, the most perfect selenographers in the whole
world, have measured 1,095 lunar mountains. It results from their
calculations that 6 of these mountains rise above 5,800 metres, and 22
above 4,800. The highest summit of the moon measures 7,603 metres; it
is, therefore, inferior to those of the earth, of which some are 1,000
yards higher. But one remark must be made. If the respective volumes of
the two orbs are compared the lunar mountains are relatively higher than
the terrestr