The instrument which they employ to measure the angles from which to
deduce the height of the clouds is a peculiar form of altazimuth that
was originally designed by Prof. Mohn, of Christiania, for measuring
the parallax of the aurora borealis. It resembles an astronomical
altazimuth, but instead of a telescope it carries an open tube without
any lenses. The portion corresponding to the object glass is formed by
thin cross wires: and that corresponding to the eye piece by a plate
of brass, pierced in the center by a small circular hole an eighth of
an inch in diameter. The tube of the telescope is replaced by a
lattice of brass work, so as to diminish, as far as possible, the
resistance of the wind. The vertical and horizontal circles are
divided decimally, and this much facilitates the reduction of the
readings.
The general appearance of the instrument is well shown in the figure,
which is engraved from a photograph I took of Mr. Ekholm while
actually engaged in talking through a telephone to M.
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