( H3 ) 
VIII. Some (Remarks upon the Method of obferVmg 
the differences of Right Afcenfton and Declination 
by Crofs Hairs in a Telefcope, ©y Dr. Edm. 
Halley, A fir. Reg. R. S. S. 
T 'Hofe that are curious in obferving the Heavenly 
Motions, and particularly myfelf, whole Bu- 
hneis it is, are greatly obliged to the late Signior Cat- 
Jtm, for his Thought of applying Threads at half Right 
Angles in the common Focus of a Teiefcope, to deter- 
mine thereby the differences of Right Afcenfion and 
Declination of any two Stars, whofe fituation is I'uch, 
that oy their diurnal Motion they follow each other 
through the Aperture of the Telefcope, fo fixt as thac 
the firlt of them may pafs over the Centre of the Glals. 
and move exadly along one of the Threads whilft 
the interval of time between the Tranfit thereof, and 
that of the following Star, is exadtly meafured by a 
Pendulum Clock well adjutted to the mean motion of 
the fun, or elfe co the Revolution of the fixt Stars 
whereby the difference of Right Afcenfion is given; as. 
is the difference of Declination, by the time the foi- 
.owing Star takes to pafs from one diagonal Thread 
to the other. This manner ot obferving being long fince 
pubhlhed, will not in this place need any further Ex- 
plication; but it may not be amils to lay (omething 
of the lufhciency thereof, and of the exaflm Is of 
which an Inffrument of fo little charge and Appt~ 
rows is capable ; especially being at this time ob- 
liged co mane ule of it and the Micrometer onlv r 
