distances and positions of 380 double and triple stars , &c. 13 
scription, accompanied with a plate, will be found in page 141 
of Vince’s Practical Astronomy. The axis is supported by 
strong piles of wood sunk deep into the earth ; and though 
not quite exempt from tremors, is sufficiently so for the per- 
formance (with due care) of the most accurate and delicate 
measurements. The telescope is furnished with a micrometer, 
the work of Troughton, similar in all respects to that of the 
five feet just described, with the exception of a peculiar ap- 
paratus carrying an additional moveable cross wire, for a 
purpose not connected with the present paper. The ordi- 
nary observing power employed with this telescope was 179, 
but occasionally a lower power of 105, and a higher one of 
273, were also used. The illumination of the field is effected 
by a lamp attached to the tube, and (as in the five feet), may 
be increased or diminished to any extent. 
The values of the parts of the scale of each micrometer 
were determined by separating the wires a certain known 
number of revolutions and parts, and having placed them in 
the direction of the meridian, measuring repeatedly the time 
occupied by the passage of an equatorial star, or other of 
known declination, from wire to wire. By this method, one 
part of the scale of the five-feet micrometer was ascertained 
to represent o' 7 . 3 1582, and of the seven feet, o".24044. The 
equality of the threads of the screw was proved by the same 
value resulting, whatever opening of the wires was em- 
ployed ; and the parallelism* of them in either micrometer 
was perfect. The position of the declination wire, when 
set to zero, was frequently examined by running a star 
* The wires employed in these micrometers, are spider’s lines of extreme tenuity, 
and were inserted by Mr. Simms. 
