MERIDIAN MARK. 183 
accident to the tranfit-make it neceflary. But it has like- 
wife one difadvantage. Should the pillar in fettling, carry 
the mark a little to the eaft or weft, the error will be 
greater in proportion to its nearnefs. 
I am, dear fir, your humble fervant, 
DAVID RITTENHOUSE. 
P. S. The great improvement of object glaffes by Dol- 
land has enabled us to apply eye glafles of fo fhort a fo- 
cus, that it is difficult to find any fubftance proper for the 
crofs hairs of fixed inftruments. For fome years paft I 
have ufed a fingle filament of filk, without knowing that 
the fame was made ufe of by the European aftronomers, as 
I have lately found it is by Mr. Hirfchell. But this fub- 
ftance, though far better than wires or hairs of any kind, 
is ftill much too coarfe for fome obfervations. A fingle 
filament of filk will totally obfcure a {mall ftar, and that 
for feveral feconds of time, ifthe ftar be near the pole. I 
have lately with no {mall difficulty placed the thread of a 
{fpider in fome of my inftruments, it has a beautiful effect, 
it is not one tenth of the fize of the thread of the filkworm, 
and is rounder and more evenly of a thicknefs. I have 
hitherto found no inconvenience from the ufe of it, and 
believe it will be lafting, it being more than four months 
fince I firft put it in my tranfit telefcope, and it continues 
fully extended, and free from knots or particles of duft. 
N° XVIIL 
Account of a Worm in a Horfe’s Eye, by F. Hoe- 
KINSON, E/quire. 
Read Sep. AVING been myfelf a witnefs to the fol- 
pieloe lowing curious fa, I thought it fhould 
Aa 2 not 
