458 Dr. HersChel’s Catalogue of Om Thoufand 
preferable to the perpendicular one commonly ufed in the 
Newtonian court ruction. 
In the prefent improved ftate of the apparatus this telefcope 
will, in general, give the relative place of an object by a lingle 
obfervation true to within if or 2 minutes of polar diftance, 
and 4 or 6 feconds of time in right afeenfion. But when there 
is an opportunity of repeating the obfervation, it will hardly 
differ a fingle minute in the former, and feldom fo much as 
3 or 4." in the latter. My apparatus, however, has not been 
equally perfect from the beginning ; for, being from time to 
time adapted to the different views I had in fweeping, it could 
only arrive to its prefent degree of perfection by many expe- 
riments and gradual improvements. 
To begin a fhoit biftory of this 20-feet telefcope. In the 
month of October of the already mentioned year I began to 
ufe it, being then mounted on its prefent hand, but with a 
lateral motion under the point of fupport of the great fpeculum, 
bv which its direction could be changed about 1 5 degrees. It had 
alfo a kind of moveable gallery in front, about nine feet long, 
which permitted me to follow a celeftial objeCt near 1 5 degrees 
more; by which means I obtained a range of 30 degrees with- 
out moving the ftand. The Newtonian form has the capital 
advantage of rendering obfervations equally commodious in all 
altitudes ; I had therefore placed the inftrument in the meri- 
dian, that I might view the {tars in their mod favourable 
fituation. 
When I had feen mo ft of the objeCts I wifhed to examine, 
I proceeded to the work of a general review of the heavens. 
The firft method that occurred was, to fuffer the telefcope to 
hang freely in the center ; then, walking backwards and for- 
wards on the moveable gallery, I drew the inftrument from 
that 
