34$ Dr. Herschei/s Description of a 
length. My way of doing these instruments at that time, 
when the direct method of giving the figure of any of the 
conic sections to specula was still unknown to me, was, to 
have many mirrors of each sort cast, and to finish them all as 
well as I could ; then to select by trial the best of them, 
which I preserved ; the rest were put by to be repolished. In 
this manner I made not less than 200, 7-feet; 150, 10-feet; 
and about 80, 20-feet mirrors ; not to mention those of the 
Gregorian form, or of the construction of Dr. Smith's re- 
flecting microscope, of which I also made a great number. 
My mechanical amusements went hand in hand with the 
optical ones. The number of stands I invented for these tele- 
scopes it would not be easy to assign. I contrived and deli- 
neated them of different forms, and executed the most pro- 
mising of the designs. To these labours we owe my 7-feet 
Newtonian telescope-stand, which was brought to its pre- 
sent convenient construction about 17 years ago; a descrip- 
tion and engraving of which I intend to take some future 
opportunity of presenting to the Royal Society. In the year 
1781 I began also to construct a 30-feet aerial reflector ; and 
after having invented and executed a stand for it, I cast the 
mirror, which was moulded up so as to come out 3b inches in 
diameter. The composition of my metal being a little too 
brittle, it cracked in the cooling. I cast it a second time, but 
here the furnace, which I had built in my house for the pur- 
pose, gave way, and the metal ran into the fire. 
These accidents put a temporary stop to my design, and as 
the discovery of the Georgian planet soon after introduced 
me to the patronage of our most gracious King, the great 
work I had in view was for a while postponed . 
