THE MOST POWERFUL TELESCOPE. 343 



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 in- 

 genious method of balanced arms Mr. Common has contrived to sup- 

 port 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 considera- 

 tion. Mr. Common devised a new and most ingenious 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 favor the 

 plan, others seemed doubtful, and more than one were decidedly ad- 

 verse. 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 labors shows that he was correct in his 

 judgment. It would be impossible to describe the method of mount- 

 ing employed without the aid of several detailed drawings, but ref- 

 erence may be made to one ingenious point. As in all equatorial, 

 mountings, nearly the entire weight of the moving part of the tele- 

 scope (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 ingenious arrangement, supports the whole polar axis in mer- 

 cury, 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 with an ordinary train of clockwork, regu- 

 lated by the governor, which he had invented for his smaller tele- 

 scope. 



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 run- 

 ning on a circular railway. By means of a wheel on your left, you 

 can raise or lower yourself at pleasure, and observe with the telescope 

 in any position. The whole observatory only requires moving about 

 once in two hours, and can be moved with ease by one hand. 



Within a year of its being begun, the telescope was rapidly ap- 

 proaching its completion. The great speculum had been brought to 

 the right shape, and was partially polished, and every day the an- 



