348 Scientific Proceedings, Royal Dublin Society. 
with, and thus circumpolar motion is obtained for any object 
above the horizon. 
It appears to me, however, that this advantage (as, no doubt, 
it is), is gained at a disproportionate sacrifice, for even in the 
case of a small instrument the polar pillar becomes a stupendous 
size, for having no strut or support of any kind above the 
floor line it must entirely depend on its own stability, as a pil- 
lar placed out of the vertical, and there is clearly no mechanical 
means of increasing its strength, except by adding to the weight 
of metal in the casting. It appears also to me that if we can 
obtain cireumpolar motion for all objects below and to south of 
the zenith (for such latitudes as Great Britain), that we should 
have obtained our objects for at least 2ths of the objects usually 
coming under the observation of the astronomer. hus, I arrived 
at the present general form of the mounting in which the above 
conditions are obtained without any sacrifice to stability, for 
it is absurd to suppose that if an instrument can be made (as it 
has been) with its polar pillar projecting downwards and through 
the floor without any support and found to be sufficiently stable, 
that this form of instrument in which the polar pillar is compara- 
tively quite short (and supported too right under its neck) cannot 
be capable of being made with a maximum of stability. 
GENERAL DESCRIPTION OF INSTRUMENT.—Plate 24. 
A.A. Lower base casting bolted firmly to stone pier. 
B.B. Second base casting bolted to AA, but capable of azimuthal 
adjustment on same by screw 0. 
C.C. Upper base casting bolted to B, but capable of vertical ad- 
justment on same by means of levelling screws inside frame 
not visible in woodcut. 
Polar pillar, inside which revolves the polar axis. 
Stay or strut under polar pillar, giving direct support under 
the principal bearing of polar axis. 
D.D. Cross head, a hollow casting bolted to polar axis, and in 
which the declination axis revolves. On one end of this 
wn rd 
cross head is 
d.d. the declination circle eighteen inches diameter, and divided on 
gold alloy, and 
ee. the declination clamp, into which gears 
ff. a slow motion screw, worked from eye end by 
