1867.] Rev. Dr. Uobinson on the Melbourne Telescope. 161 



A much simpler plan is the sliding roof. In this case the walls are rec- 

 tangular, enclosing a space rather broader than the instrument, and about 

 three times as long. The longer sides carry two rails, on which runs a 

 kind of house long enough to cover the instrument and pier, and high 

 enough to clear the latter. That end which at Melbourne will be its north 

 is closed by doors, which are opened at the time of observation, and the 

 roof is wheeled away, leaving all in the open air. It will be the cheapest 

 and least bulky of the three. Its defects are, that the open end presents 

 some engineering difficulty, that the roof will hide about 1 2° under the 

 pole, and that the whole machinery is exposed to any dust that may be 

 stirring during the hours of observing. 



That which appears the best is the revolving roof. Its vertical part is 

 a prism of sixteen sides, six feet high, springing from a ring of cast iron, 

 which revolves by rollers on a circular rail borne by the wall. The top is 

 nearly flat, with a chase large enough to let the telescope work freely, 

 which can be covered by sliding shutters. The tube, when in use, would 

 project through the chase, and be essentially in free air, at other times 

 could be lowered and completely sheltered ; while the other parts would 

 be as well protected as under a dome. In this case the internal diameter 

 should be about 46 feet, with a chase 1 6 feet wide. These dimensions would 

 give complete command of the heavens, and such a roof would give less 

 hold to a high wind than either of the others. I enclose a rough sketch 

 of its framing. The panels and the three girders at the top to be of angle- 

 iron, light but strong, and these covered with tin plate. If it were adopted, 

 I suppose the frame would be made here, sent out in pieces, and put to- 

 gether and covered on its arrival. The weight would be about 5 tons. 

 As to its cost, no estimate can be given, as labour costs more at Melbourne 

 than with us ; but in Ireland it would be about a61200. 



I will conclude this long letter by telling you how much I am satisfied 

 with our selection of the astronomer who is to work this glorious instru- 

 ment. He is not a mere mathematician ; such a one might be very help- 

 less when he came to the practical details of observing, but he is tho- 

 roughly versed in its optical and mechanical requirements, and in the daily 

 work of an observatory. For this last he has been trained by Professor 

 Adams during the past year ; one of the Committee, Mr. Warren De la 

 Rue, the first of celestial photographers, has instructed him in the mys- 

 teries of that surprising art ; and for the last three months he has been 

 constantly in Mr. Grubb's works, studying all the mechanism of the tele- 

 scope (of which I see he has acquired full command), and taking an active 

 part in the polishing of the great specula. He seems fully to understand 

 this most delicate process ; and it is my opinion that, if repolishing becomes 

 necessary, he is fully competent to do it successfully. 



I may therefore congratulate you in full hope on the inestimable harvest 

 of discovery and triumph which will soon crown this magnificent enter- 

 prise. Yours ever, 



(Signed) T. R. Robinson. 



