o
excellently have his needs been served, that the history of the
ingenious contrivances by which discoveries have been prepared would
supply a subject (here barely glanced at) not far inferior in extent and
instruction to the history of those discoveries themselves.

There are two chief modes of using the telescope, to which all others
may be considered subordinate.[335] Either it may be invariably directed
towards the south, with no motion save in the plane of the meridian, so
as to intercept the heavenly bodies at the moment of transit across that
plain; or it may be arranged so as to follow the daily revolution of the
sky, thus keeping the object viewed permanently in sight instead of
simply noting the instant of its flitting across the telescopic field.
The first plan is that of the "transit instrument," the second that of
the "equatoreal." Both were, by a remarkable coincidence, introduced
about 1690[336] by Olaus Roemer, the brilliant Danish astronomer who
first measured the velocity of light.

The uses of each are entirely different. With the transit, the really
fundamental task of astronomy--the determination of the movements of the
heavenly bodies--is mainly accomplished; while the investigation of
their nature and peculiarities is best conducted with the equatoreal.
One is the instrument of mathematical, the other of descriptive
astronomy. One furnishes the materials with which theories are
constructed and the tests by which they are corrected; the other
registers new facts, takes note of new appearances, sounds the depths
and peers into every nook of the heavens.

The great improvement of giving to a telescope equatoreally mounted an
automatic movement by connecting it with clockwork, was proposed in 1674
by Robert Hooke. Bradley in 1721 actually observed Mars with a telescope
"moved by a machine that made it keep pace with the stars;"[337] and Von
Zach relates[338] that he had once followed Sirius for twelve hours with
a "heliostat" of Ramsden's construction. But these eighteenth-century
attempts were of no practical effect. Movement by clockwork was
virtually a complete novelty when it was adopted by Fraunhofer in 1824
to the Dorpat refractor. By simply giving to an axis unvaryingly
directed towards the celestial pole an equable rotation with a period of
twenty-four hours, a telescope attached to it, and pointed in _any_
direction, will trace out on the sky a parallel of declination, thus
necessarily ac