Ltd 
V. 
A NEW FORM OF RIGHT-ASCENSION SLOW MOTION FOR 
EQUATORIAL TELESCOPES, ILLUSTRATED BY THE 
DRIVING-GHAR OF THE CAPE TOWN EQUATORIAL. 
By SIR HOWARD GRUBB, F.R.S., Vice-President, Royal Dublin 
Society. 
[Prats I.] 
[Read, January 17; Received for Publication, Aprin 13; 
Published, SEPTEMBER 9,1 1905. ] 
In the older forms of equatorials, the slow motion necessary for 
the accurate setting of the object under observation on the 
micrometer-webs generally assumed the form of a tangent-screw 
and nut of some of the various well-known kinds, which, on being 
turned, moved the telescope itself independently of the driving- 
are by a small quantity. 
For very fine work it has been found practically impossible to 
move a large mass, such as a 24-inch telescope, which, with its 
axes, counterpoises, &c., weighs some tons, through even a very 
small quantity without giving rise to a slight swinging motion ; 
and this, though of little consequence in visual observations, is apt 
to leave a trail or wings to the image of the star in photographing. 
A slow motion of the form used in my automatic control, con- 
sisting of a set of differential wheels, is found to be practically free 
from this defect. 
This arrangement has the effect of increasing or diminishing 
the rate at which the driving-screw is driven without interfering 
with the rate of the clock itself. 
It is brought into action by stopping temporarily a loose disc 
which carries a pinion gearing across a pair of differential wheels. 
So long as this disc is allowed to revolve with the other wheels, it 
forms a coupling or clutch ; but when stopped it produces a slight 
differential rate between the two unequally-toothed wheels; and 
1 The publication of this paper has been unavoidably delayed. 
