The astronomical
instruments hitherto used were, with the exception of the astrolobe, those
which had been employed by Ptolemy and the Arabians. The quadrant of
Ptolemy resembled the mural quadrant of later times; which, however, was
improved by the Arabians, who, at the end of the tenth century, employed a
quadrant twenty-one feet and eight inches radius, and a sextant fifty-seven
feet nine inches radius, and divided into seconds. The use of the sextant
seems to have been forgotten after this time; for Tycho Brahe is said to
have re-invented it, and to have employed it for measuring the distances of
the planets from the stars. The quadrant was about the same time improved
by a method of subdividing its limbs by the diagonal scale, and by the
Vernier. The telescope was invented in the year 1609, and telescopic sights
were added to the quadrant in the year 1668. Picard, who was one of the
first astronomers who applied telescopes to quadrants, determined the
earth's diameter in 1669, by measuring a degree of the meridian in France.
The observation made at Cayenne, that a pendulum which beat seconds there,
must be shorter than one which beat seconds at Paris, was explained by
Huygens, to arise from the diminution of gravity at the equator, and from
this fact he inferred the spheroidal form of the earth. The application of
the pendulum to clocks, one of the most beautiful and useful acquisitions
which astronomy, and consequently navigation and geography have made, was
owing to the ingenuity of Huygens.
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