1868.] Description of the Great Southern Telescope. 457 
hoop hanging from A, and on which the speculum is hanging, but 
with this peculiarity that it is equally effective in all positions; and 
again at the upper side the speculum is free from the ring B, while 
at the lower side the ring B is free from A. Therefore the specu- 
lum is not confined in any exact position or prevented from taking 
a proper bearing on its back supports. This is just what is 
required, and so effective is this apparatus that on no occasion 
could any evidence of strain be detected by the most critical 
observer. 
The driving clock is, a8 may be readily supposed, of rather 
ponderous dimensions, but is still, considering the size of the 
telescope it has to move, as compact as possible, fitting in a niche 
2 feet square and 3 feet high. Its regulating power is that of a 
governor, the balls of which, when the desired speed is attained, fly 
out and bring two leather pointed screws into contact with a cir- 
cular disc, which serves by the friction thus produced to prevent any 
acceleration of rate. Several driving clocks have been constructed 
on this principle, but owing to the omission of some very impor- 
tant details, nearly if not all of them have proved quite insufficient 
for their work, and consequently clocks of this kind are looked upon 
by many with distrust and incredulity of their efficiency, but it 
has been found that when this clock is constructed with proper 
attention to its details it presents some very important advantages 
for driving large equatorials. No clock is able to produce any 
correction in speed until after an error has been committed. 
Consequently the question becomes, What clock is it that corrects 
these errors most efficiently and most instantaneously? That this 
clock corrects errors in speed efficiently is proved by the fact 
that doubling the driving-weight (viz. that weight which will just 
keep the friction screw in contact) produces an acceleration in speed 
of only =1, part, whilst the great vis inertia of the balls themselves 
(weighing about 20 lbs. each) prevents any sudden oscillation, so 
to speak, in rate; and that this regulating action takes place more 
instantaneously than in other clocks may be readily understood by 
inspection of the construction, when it will be seen that the actual 
movement required in any of the members of the governor to correct 
an excessive difference of power like the above amounts to something 
probably under -+,',, inch, instead of having to move by some 
considerable quantity a complicated system of link-work connected 
with fans, breaks, water regulators, air regulators, &c., &c., whose 
name is legion, and whose principal effect is that if one of them 
gets out of order all the rest are powerless. The general effect of 
these complicated systems is that when any error is made the 
ensuing correction is too great, which necessitates a counter- 
correction in an opposite direction, and so an oscillating effect is 
