of the Equatorial Instrument . 75 
ness of their dimensions, or defect of their constructions, were 
totally unfit for the accuracy of modern astronomy, where an 
error of a few seconds only, in an observation, is all that can 
be admitted, to entitle it to any credit.* With respect to the 
precision of astronomical instruments in general, I may no- 
tice by the way, that from the time of Hipparchus and 
Ptolemy, before and at the commencement of the Christian 
sera, to the age of Walther and Copernicus, in the begin- 
ning of the 16th century, few observations can be depended 
on to within less than 5, 8, or perhaps even 10 minutes; 
those of Tycho Brahe, indeed, that princely promoter of as- 
tronomy, to within one minute. The errors of Hevelius's 
large sextant of 6 feet radius, towards the middle of the last 
century, might amount to 15 or 20 seconds. Flamsteed's sex- 
tant to 10 or 12 seconds ; and lastly, those of Mr. Graham's 
mural quadrant of 8 feet radius, with which Dr. Bradley 
made so many observations from 1742, might amount to 
7 or 8 seconds. 
( 13.) Having said thus much generally upon the subject of 
this ingenious instrument, and not more, I trust, than will be 
deemed, by every lover of this science, what its importance de- 
serves, I proceed to the description of one I have caused to be 
made by a very able artist of this metropolis, Mr. Jesse Ramsden. 
AB, CD, EF, GH, (Tab. IX.) are 4 columns composed 
of hollow brass tubes g\ inches in diameter, and 5 feet 10 
inches long ; these, with two others, one of which appears 
* I must except from this remark the two large equatorial sectors made by Mr. 
Sisson, for Greenwich observatory; and also an instrument of this kind, made by Mr. 
Ramsden, for the late General Roy, and now in the possession of Mr, Aubert, 
whose circles are about 30 inches in diameter. 
L 2 
