1 



160 Rev. Dr. Robinson on the Melbourne Telescope. [Nov. 21, j 



a circle than the mover of such a huge mass. The clock is rather gigantic, \ 

 but does its work with great precision, the objects which I examined re- i 

 maining steady on the wire as long as I watched them ; and there is an 

 ingenious and new contrivance for suiting its speed to planets or the moon. ; 

 There remain but a few matters to be completed ; the second great specu- 

 lum is nearly polished, the glass small one is ready ; the micrometer and j 

 observing-chair are not commenced, nor the photographic apparatus and j 

 spectroscope. These two last are no part of Mr. Grubb's contract ; but 

 the Committee thought themselves justified by the correspondence in order- j 

 ing them, as their cost is small, and they will add greatly to the utility of \ 

 the telescope. In the fine sky of Melbourne it will, I trust, yield spectro- j 

 scopic results surpassing any that have as yet been obtained *. That it i 

 will realize fully the expectations of the people whose enlightened liberality I 

 has ordered its construction I am quite certain; but I am not so certain ] 

 that it will retain its present perfection very long if exposed without some 

 shelter. It is true that Mr. Cooper's great achromatic has stood exposed ■ 

 to the rain and wind of Connaught for more than thirty years, and is still 

 serviceable ; but besides its inferior size it is of coarser workmanship, and j 

 is provided with fewer of those beautiful contrivances which in this instru- | 

 ment make its movements so easy. At Melbourne the rain of Markree is ; 

 not to be feared ; but if one may judge from its position on the verge of a ' 

 great continent, and from the analogy of India and the Cape, another enemy | 

 is to be dreaded, the fine dust which winds from the interior will probably i 

 bring. This would find its way into all the bearings, and besides clogging j 

 their action would grind them out of truth. The danger of this induces j 

 me, after careful discussion with Messrs. Le Sueur and the two Grubbs, to | 

 lay before you my views, which (if you think them sound) you may hold i 

 it advisable to mention to the authorities of Victoria. \ 



Three modes occur to me of covering the telescope. In any case it must ' 



be surrounded by a wall, for the comfort of the observer and to prevent in- j 



trusion. This wall may support a moveable covering of such a kind as to I 



let the instrument be pointed to every part of the sky. ; 



The most usual form of this covering is a dome running on a circular ; 



railway, and with an opening or chase on one side reaching from its base ! 



to its summit, and closed by a sHding shutter. The disadvantages of this | 



plan are, that the performance of the telescope is somewhat injured by cur- ] 



rents of warm air rising through the chase, and that it is much heavier and -j 



more costly than either of the others. In this instance its diameter could j 



not be less than 56 feet ; and though that magnitude is not beyond the | 



resources of an accomplished engineer, yet it is not one to be encountered j 



without the prospect of some adequate advantage. The largest dome which ■ 

 I know (Sir James South' s, of 36 feet diameter) is a total failure ; but this 



does not weigh much with me — for, though planned by the celebrated ■ 

 Brunei, it transgresses against the elements of mechanical science. 



* I send a photograra of the instrument taken last week. i 



