43 2 JWr. South on the discordances between the 
collimation was frequently ascertained.* The results having 
satisfied my astronomical friends as well as myself, that the 
instrument fulfilled all the required conditions, further expe- 
riments were deemed unnecessary ; and on the 5th of August, 
the instrument being relieved from its two months* “torture/* 
was prepared to grapple with the delicate observations for 
which it was designed. 
The character which the instrument acquired shortly after 
its erection, four years* subsequent experience has unequi- 
vocally confirmed ; and exclusive of the property which it is 
the object of the subsequent pages to investigate, I know not 
whether most to respect it, for the unusual accuracy with 
which it obeys its adjustments, or for the extreme pertinacity 
with which it retains them. 
The object glass of the Greenwich Transit instrument is 
five inches in clear aperture ; its focal length is 10 feet ; its 
horizontal axis, including the pivots, is 3 feet 10 inches; in 
the focus of the object glass are seven fixed wires, and two 
moveable for micrometrical purposes ; the semicircles at the 
eye end of the telescope, being insufficient to enable the 
• The proximity of lofty buildings to the north and south of my Observatory, 
rendering it impossible to erect any object to perform the offices of a meridian 
mark, an apparatus was planted upon the top of my house, enabling me to examine 
the collimation, by the flag-staff on Severndroog Castle. The trouble, however, of 
frequently repeating the operation became so considerable, and from unfavourable 
state of atmosphere, occasionally so unsatisfactory, that sidereal observations were 
recurred to, generally of Polaris, and of a small star about 54 minutes from the 
pole : these, particularly the latter, offer severe tests for the accuracy of the adjust- 
ment; and where the instrument can be reversed, without risk of deranging its 
horizontally, (as is the case with mine,) no error of collimation, sensible to observa- 
tion, need remain uncorrected. 
