erschel had to
surmount many obstacles, to mount a telescope suitably, of which the
mirror alone weighed upwards of 1000 kilogrammes (_a ton_). But he
solved this problem to his entire satisfaction by the aid of a
combination of spars, of pulleys, and of ropes, of all which a correct
idea may be formed by referring to the woodcut we have given in our
_Treatise on Popular Astronomy_ (vol. i.). This great apparatus, and the
entirely different stands that Herschel imagined for telescopes of
smaller dimensions, assign to that illustrious observer a distinguished
place amongst the most ingenious mechanics of our age.

Persons in general, I may even say the greater part of astronomers, know
not what was the effect that the great forty-foot telescope had in the
labours and discoveries of Herschel. Still, we are not less mistaken
when we fancy that the observer of Slough always used this telescope,
than in maintaining with Baron von Zach (see _Monatliche Correspondenz_,
January, 1802), that the colossal instrument was of no use at all, that
it did not contribute to any one discovery, that it must be considered
as a mere object of curiosity. These assertions are distinctly
contradicted by Herschel's own words. In the volume of _Philosophical
Transactions_ for the year 1795 (p. 350), I read for example: "On the
28th of August 1789, having directed my telescope (of forty feet) to the
heavens, I discovered the sixth satellite of Saturn, and I perceived the
spots on that planet, better than I had been able to do before." (See
also, relative to this sixth satellite, the _Philosophical
Transactions_ for 1790, p. 10.) In that same volume of 1790, p. 11, I
find: "The great light of my forty-foot telescope was then so useful,
that on the 17th of September 1789, I remarked the seventh satellite,
then situated at its greatest western elongation."

The 10th of October, 1791, Herschel saw the ring of Saturn and the
fourth satellite, looking in at the mirror of his forty-foot telescope,
with his naked eye, without any sort of eye-piece.

Let us acknowledge the true motives that prevented Herschel from oftener
using his telescope of forty feet. Notwithstanding the excellence of the
mechanism, the manoeuvring of that instrument required the constant
aid of two labourers, and that of another person charged with noting the
time at the clock. During some nights when the variation of temperature
was considerable, this telescope, on account of its