THE MOST POWERFUL TELESCOPE IN EXISTENCE. 
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when the speculum is made of glass ; for it is possible to cast 
the speculum with grooves, projections, and recesses in its back, 
by means of which the task of supporting it is much simplified. 
With a glass speculum it is not practicable to have these aids, 
so that the back of the speculum is cast quite flat, and usually 
rests on a flat plate of metal. By an ingenious method of ba- 
lanced arms Mr. Common has contrived to support the speculum 
so that it is perfectly free from flexure. Thus the first point 
was secured. 
The second point, or the method by which the telescope 
should be mounted, was a problem which required long and 
serious consideration. Mr. Common devised a new and most in- 
genious method which, after long consideration, he thought would 
furnish a means of steadily supporting the telescope. In this 
steadiness is most essential, the slightest vibration, vibrations 
absolutely invisible to the eye, would ruin the performance of a 
telescope. The weight of the moving part of the telescope 
amounts probably to four or five tons, and this has to be kept 
in motion by a clock, yet it must not be liable to the least 
tremor or vibration. The difficulty of the problem is evident. 
His plan of a mounting was submitted by Mr. Common, for 
criticism, to several well-known astronomers, who might be 
supposed competent to advise on this subject. As might have 
been expected, very diverse opinions were expressed ; at most, 
one seemed to decidedly favour the plan, others seemed 
doubtful, and more than one were decidedly adverse. The 
result was to leave that matter much as it stood at first, so that 
Mr. Common decided to persevere in his original design. The 
success which has crowned his labours shows that he was cor- 
rect in his judgment. It would be impossible to describe the 
method of mounting employed without the aid of several detailed 
drawings, but reference may be made to one ingenious point. 
As in all equatorial mountings, nearly the entire weight of 
the moving part of the telescope (in the present telescope five 
tons) rests on the bottom pivot of the polar axis. This pivot, 
therefore, is exposed to enormous friction, and is a common 
cause of vibration. To obviate this, Mr. Common, by an inge- 
nious arrangement, supports the whole polar axis in mercury, 
thus taking off nearly the entire friction, and the whole instru- 
ment moves as if it were floating. By this means he is enabled 
to drive the whole telescope by means of an ordinary train of 
clockwork, regulated by the governor, which he had invented 
for his smaller telescope. 
The last two points specified above are obtained by making 
the observatory itself the ladder by which you approach the eye 
end of the telescope, and the whole observatory revolves on iron 
wheels running on a circular railway. By means of a wheel on 
