age, one eighth of an inch thick above, increasing to six sixteenths of
an inch below, to give it steadiness when suspended, which apparently
was intended to be increased by hanging a weight on the little
projecting ring at the bottom of it, in using it on ship-board. Its
suspending ring is attached by a double hinge of the nature of a
universal joint. Its circle is divided into single degrees, graduated
from its perpendicular of suspension. The double-bladed index, the
pivot of which passes through the centre of the astrolabe, has slits
and eyelets in the projecting fights that are on it."

We give on the preceding page an engraving of this astrolabe from a
photograph, which presents a sufficiently accurate outline of the
instrument. The plate was originally made to illustrate Mr. Marshall's
article in the Magazine of American History, and we are indebted to the
courtesy of the proprietors of the Magazine, Messrs. A. S. Barnes and
Company of New York, for its use for our present purpose.

The astrolabe, as an instrument for taking the altitude of the stars or
the sun, had long been in use. Thomas Blundevile, who wrote in 1622,
says he had seen three kinds, and that the astrolabe of Stofflerus had
then been in use a hundred years. It had been improved by Gemma
Frisius. Mr. Blagrave had likewise improved upon the last-mentioned,
and his instrument was at that time in general use in England. The
astrolabe continued to be employed in Great Britain in taking altitudes
for more than a century subsequent to this, certainly till Hadley's
Quadrant was invented, which was first announced in 1731.

The astrolabes which had the broadest disks were more exact, as they
were projected on a larger scale, but as they were easily jostled by
the wind or the movement of the ship at sea, they could with difficulty