HIS FORTY—FOOT TELESCOPE. 277 
lite, 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 
manceuvring 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 tele- 
scope, on account of its great mass, was always behind- 
hand with the atmosphere in thermometric changes, 
which was very injurious to the distinctness of the 
images. 
Herschel found that in England, there are not above a 
hundred hours in a year during which the heavens can 
be advantageously observed with a telescope of forty feet, 
furnished with a magnifying power of a thousand. This 
remark led the celebrated astronomer to the conclusion, 
that, to take a complete survey of the heavens with his 
large instrument, though each successive field should re- 
main only for an instant under inspection, would not 
require less than eight hundred years. 
Herschel explains in a very natural way the rare oc- 
currence of the circumstances in which it is possible to 
make good use of a telescope of forty feet, and of very 
large aperture. 
