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XVIII. Description of a Forty feet Reflecting Telescope . By 
William Herschel, LL.D. F.R.S. 
Read June 1 i, 1 795. 
T[ he uncommon size of my forty-feet reflecting telescope will 
render a description of it not unacceptable to lovers of astro- 
nomy. I shall therefore endeavour to give as complete an idea 
of its construction as the limited compass of this paper will 
permit, and hope that, with the assistance of the annexed 
drawings, the mechanism of it will be sufficiently intelligible 
to such as have been in the habit of viewing machines and 
mechanical works. 
It will be necessary to mention a few circumstances that 
led the way to the construction of this large instrument, in 
the execution of which two very material requisites were ne- 
cessary : namely, the support of a very considerable expence, 
and a competent experience and practice in mechanical and 
optical operations. 
When I resided at Bath I had long been acquainted with 
the theory of optics and mechanics, and wanted only that ex- 
perience which is so necessary in the practical part of these 
sciences. This I acquired by degrees at that place, where in 
my leisure hours, by way of amusement, I made for myself se- 
veral 2-feet, 5-feet, 7-feet, 10-feet, and 2c-feet Newtonian 
telescopes ; besides others of the Gregorian form, of 8 inches, 
12 inches, 18 inches, 2 feet, 3 feet, 5 feet, and 10 feet focal 
