182 NEW METHOD or pLacine A 
mark being taken down at repairing the building to which 
it was fecured, the tranfit inftrument was. accidentally 
thrown out of its true pofition, and the obfervations with 
it were continued for a confiderable time before the error 
was detected. My meridian mark being thus rendered 
ufelefs, I contrived feveral other methods of fupplying its 
place, all of which were, on fufhcient deliberation, reject- 
ed for the following. 
I faftened the obje& glafs of a thirty fix feet telefcope, 
firmly, to the wall which fupports the tranfit inftrument,. 
oppofite to and as near as convenient to the obje& glafs of 
the tranfit, when brought to a horizontal fituation. In 
the focus of the thirty fix feet objet glafs I {crewed faft a 
piece of brafs to a block of marble, fupported by a brick 
pillar built on a good foundation, for this purpofe, in my 
garden. On this piece of brafs are feveral black concen- 
tric circles; the reft of the plate is filvered. The diverg~ 
ing rays of light which proceed from every point in thefe 
circles, after paffing through the thirty-fix feet glafs be- 
come parallel, and entering the tranfit inftrument, an image 
of the plate and its circles is formed in the fame place 
where the images of ftars or the moft diftant objects are 
formed. ‘The circles are therefore diftinétly feen through 
the tranfit,. and being placed in the fame meridian with 
the centre of the thirty fix feet glafs, the innermoft circle, 
about the fize of a brevier 0, ferves for a meridian mark, 
to the centre whereof the crofs hair of the tranfit may be 
nicely adjutted. 
This mark is in feveral refpedts preferable to.one placed 
in the common way. It is entirely free from parallax, 
which the other cannot be, unlefs placed at a very great 
diftance, when glafles of great magnifying powers are 
ufed. It is not fenfibly affeted by the undulation of the 
air, which very often renders it impoflible to fet the tran- 
fit accurately to a diftant mark. And it can be illuminat- 
ed at night without difficulty, fhould the fufpicion of any 
accident 
