295 
of the Georgian planet. 
be useful to cast an eye upon the action of a power which 
is become so essential. Its advantages and its inconveniences 
must equally be objects of consideration. 
A very material inconvenience is that mirrors, which must 
be large in order to grasp much light, must also be of a great 
focal length ; and that in consequence of this, we must sub- 
mit to be incumbered with a large apparatus, which will 
require an assistant at the clock and writing desk, and also 
an additional person to work the necessary movements. The 
machinery of my twenty feet telescope is however so com- 
plete, that I have been able to take up the planet at an early 
hour in the evening, and to continue the observations of its 
own motion, together with that of its satellites, for seven, 
eight, or nine hours successively. 
The forty feet telescope having more light than the twenty 
feet, it ought to be explained why I have not always used it 
in these observations. Of two reasons that may be assigned, 
the first relates to the apparatus and the nature of the instru- 
ment. The preparations for observing with it take up much 
time, which in fine astronomical nights is too precious to be 
wasted in mechanical arrangements. The temperature of 
the air for observations that must not" be interrupted, is 
often too changeable to use an instrument that will not easily 
accommodate itself to the change : and since this telescope, 
besides the assistant at the clock and writing desk, requires 
moreover the attendance of two workmen to execute the 
necessary movements, it cannot be convenient to have every 
thing prepared for occasional lucid intervals between flying 
clouds that may chance to occur ; whereas in less than ten 
