EXPERIMENTS rn MAGNETISM. 181 
confined, the rod afterwards remained magnetical. All 
this is neverthelefs little more than conjecture, until con= 
firmed by further experiments. 
I am, dear fir, yours, &c. 
DAVID RITTENHOUSE. 
N° XVII. 
New Method of placing a Meridian Mark, in a Letter to 
the Rev. Dr. Ewi1nc, Provoft of the Univerfity. By 
D. RITTENHOUSE, E/quire. 
DEAR SIR, 
Read Nov. OME time ago I mentioned to you a new in- 
a vention I had for fixing a Meridian Mark for 
my Obfervatory. This I have fince executed, and as it 
anfwers perfectly well, I fhall give you a particular de- 
{cription of it. 
When my obfervatory was firft erected, I placed a me- 
ridian mark to the northward at the diftance of about 1200 
feet, my view to the fouth being too much confined by 
adjacent buildings, and that to the north was not diftant 
enough to have the mark free from a fenfible parallax. 
But laft fummer a new brick houfe was built dire@ly north 
of the obfervatory, and much too nigh for diftant vifion 
with the tranfit inftff'ument. Now though a fixed mark is 
not abfolutely neceflary where you have a good tranfit in 
ftrument, the pofition of which may be examined and ac- 
curately corrected, if neceflary, every fair day, by the 
paflage of the pole-ftar above and below the pole, it is ne- 
verthelefs very convenient, faves much trouble, and may 
fometimes prevent miftakes. We have an inftance in the 
obfervations of the Aftronomer Royal at Greenwich. His 
Aa mark 
