THE ROYAL ARTILLERY INSTITUTION. 
353 
In conclusion, the Sub-Committee would venture to suggest that this 
valuable instrument should, together with the rest of the Observatory, 
be placed under the charge of a small Sub-Committee, composed of 
officers conversant with the use of instruments; and that a competent 
person—N.C. officer or civilian—be placed in direct charge of the 
instrument, the necessary sanction to be obtained for his residence 
in the building, and the removal of the two Institution orderlies. 
W. H. KIN Gr-H ARM AN, 
Capt. R.A., 
Secretary , 
Woolwich, 
May 5, 1873. 
Tlie general form of the 7-in. equatorial recently erected for the Eoyal Artillery 
Institution, Woolwich, is the same as that of the 15-in. equatorial erected for the 
Eoyal Society at Dr. Huggins’ Observatory (Tulse Hill, S.W.), and the 15-in, 
and 12-in. equatorials now in course of construction by the same maker for the 
Lord Lindsay and the University of Oxford respectively. It has the advantage of 
combining great steadiness with circumpolar motion from horizon to zenith. 
The objective is 7 ins. aperture; but as the components are cemented, the 
light transmitted,is about equal to a 7J- uncemented objective. The focus is 
about 9 ft. 
The telescope is supplied with two finders of 2| ins. aperture; bifilar micro¬ 
meter;- transparent position circle, and arrangements for dark and bright field 
illumination of micrometer wires, as described in the following paper, which was 
read before British Association, 1872, by Mr. Howard Grubb; also convenient 
arrangements for final balancing of tube, &c.:— 
“ The rack and pinion tube carrying the eye-piece or micrometer revolves freely 
in the casting which forms the lower end of the telescope tube, and carries a brass 
plate (all cast in one piece), on which is cemented a flat ring of plate glass, muffed 
on back, and in front varnished with an opaque varnish. Through this varnish 
the divisions are cut, so that on being illuminated from behind, the divisions appear 
bright upon a black ground. The vernier is similarly treated, and the whole of 
this circle being covered with a cap, with a glazed window only sufficiently large 
to expose the vernier and about 15° of the circle, is protected from possible injury 
and is read most conveniently through this window, being illuminated by a beam 
of light constantly directed upon it from a lamp hanging on end of the declination 
axis, as will be afterwards explained. 
“ Between the fixed casting which forms the end of the telescope tube and that 
which revolves in it is another metallic circle cut into 360 teeth on edge, and with 
90 holes drilled accurately on face : into the teeth on edge is geared a screw which 
is mounted on fixed casting, one revolution of which is of course equal to an 
angular movement of 1°. 
" In the other (outer) moveable brass circle is mounted a steel pin working up 
and down in a small cylinder; this pin, being pressed down by a small spiral spring, 
enters into one or other of the 90 holes in the intermediate circle, and thus clamps 
the whole eye-end to the intermediate circle, in which condition a slow motion is 
obtained by the endless screw. When it is desired to move the eye-end through 
