SI 6 Biographical Memoir of Sir William Herschel. 
method, and on the 11th January 1787, he was rewarded by 
the discovery of two of the satellites of that planet, viz. the second 
and fourth. In 1790 and 1794, he discovered other four satel- 
lites, viz. the^r^, third , jtfth, and sixth, all of which have the 
remarkable character of moving in a retrograde direction, and 
in orbits nearly in the same plane, and almost perpendicular to 
the ecliptic. 
Not content with the instruments which conducted him to these 
magnificent discoveries, Dr Herschel resolved to construct tele- 
scopes of a still larger size. In 1781 he began a 30 feet aerial 
reflector, but his mirror, 36 inches in diameter, having at one time 
cracked in the cooling, and at another run into the fire, from a 
failure in the furnace, his project failed for a time. The plan of 
forming a telescope of extraordinary size having been submitted 
by Sir Joseph Banks to King George III., that munificent So- 
vereign instantly offered to defray the expence of it, and, under 
his august patronage, Mr Herschel began, about the end of the 
year 1785, to construct a telescope forty feet in focal length. 
The great speculum was 49^ inches in diameter ; its polished 
surface 48 inches, — its thickness inches, and its weight, when 
newly cast, 2118 lb. The tube was 39 feet 4 inches long, and 
4 feet 10 inches in diameter, and every part of it was made of 
rolled or sheet iron, united without rivets, by a species of sealing 
used in making the iron-funnels for stoves. The thickness of the 
iron was less than the 36th part of an inch, and a square foot 
weighed about 14 ounces. Hence, it was so light, that a wooden 
one would have exceeded it in weight at least 1000 pounds. This 
splendid instrument, which magnifies 6450 times, was completed 
on the 27th August 1789, and on the day following Dr Her- 
schel discovered a new satellite belonging to the planet Saturn. 
Soon afterwards, in the same year, he. discovered a second new 
Satellite of Saturn, and he found that both of them were situa-, 
ted nearer the planet than the five old ones, though he found it 
convenient to call them the Sixth and Seventh Satellites. On 
the 14th September 1789, Dr Herschel found that the equa- 
torial was to the polar diameter of Saturn as 11 to 10. He ob- 
served also belts parallel to the ring, and he concluded from 
this, and a change in the position of the spots, that the planet 
revolved round an axis perpendicular to the plane of the ring. 
