companying the movement of any star upon which it may be
fixed. It accordingly forms part of the large sum of Fraunhofer's merits
to have secured this inestimable advantage to observers.

Sir John Herschel considered that Lassell's application of equatoreal
mounting to a nine-inch Newtonian in 1840 made an epoch in the history
of "that eminently British instrument, the reflecting telescope."[339]
Nearly a century earlier,[340] it is true, Short had fitted one of his
Gregorians to a complicated system of circles in such a manner that, by
moving a handle, it could be made to follow the revolution of the sky;
but the arrangement did not obtain, nor did it deserve, general
adoption. Lassell's plan was a totally different one; he employed the
crossed axes of the true equatoreal, and his success removed, to a great
extent, the fatal objection of inconvenience in use, until then
unanswerably urged against reflectors. The very largest of these can now
be mounted equatoreally; even the Rosse, within its limited range, has
been for some years provided with a movement by clockwork along
declination-parallels.

The art of accurately dividing circular arcs into the minute equal parts
which serve as the units of astronomical measurement, remained, during
the whole of the eighteenth century, almost exclusively in English
hands. It was brought to a high degree of perfection by Graham, Bird and
Ramsden, all of whom, however, gave the preference to the old-fashioned
mural quadrant and zenith-sector over the entire circle, which Roemer had
already found the advantage of employing. The five-foot vertical circle,
which Piazzi with some difficulty induced Ramsden to complete for him in
1789, was the first divided instrument constructed in what may be called
the modern style. It was provided with magnifiers for reading off the
divisions (one of the neglected improvements of Roemer), and was set up
above a smaller horizontal circle, forming an "altitude and azimuth"
combination (again Roemer's invention), by which both the elevation of a
celestial object above the horizon and its position as referred to the
horizon could be measured. In the same year, Borda invented the
"repeating circle" (the principle of which had been suggested by Tobias
Mayer in 1756[341]), a device for exterminating, so far as possible,
errors of graduation by _repeating_ an observation with different parts
of the limb. This was perhaps the earliest systematic effort to c