98 Things not qenerally Known, 



y u y 



another reflector, with a speculum six feet in diameter 

 feet long ! and this magnificent instrument was completed early 

 in 1845. The focal length of the speculum is fifty-four feet. It 

 weighs four tons, and, with its supports, is seven times as heavy 

 as the four-feet speculum of Sir William Herschel. The specu- 

 lum is placed in one of the sides of a cubical wooden box, about 

 eight feet wide, and to the opposite end of this box is fastened 

 the tube, which is made of deal staves an inch thick, hooped 

 with iron clamp-rings, like a huge cask. It carries at its upper 

 end, and in the axis of the tube, a small oval speculum, six 

 inches in its lesser diameter. 



The tube is about 50 feet long and 8 feet in diameter in 

 the middle, and furnished with diaphragms 6 feet in aperture. 

 The late Dean of Ely walked through the tube with an umbrella 

 up. 



The telescope is established between two lofty castellated 

 piers 60 feet high, and is raised to different altitudes by a 

 strong chain-cable attached to the top of the tube. This cable 

 passes over a pulley on a frame down to a windlass on the 

 ground, which is wrought by two assistants. To the frame are 

 attached chain-guys fastened to the counterweights ; and the 

 telescope is balanced by these counterweights suspended by 

 chains, which are fixed to the sides of the tube and pass over 

 large iron pulleys. The immense mass of matter weighs about 

 twelve tons. 



On the eastern pier is a strong semicircle of cast-iron, with 

 which the telescope is connected by a racked bar, with fric- 

 tion-rollers attached to the tube by wheel work, so that by 

 means of a handle near the eye-piece, the observer can move 

 the telescope along the bar on either side of the meridian, to 

 the distance of an hour for an equatorial star. 



On the western pier are stairs and galleries. The observ- 

 ing gallery is moved along a railway by means of wheels and a 

 winch ; and the mechanism for raising the galleries to various 

 altitudes is very ingenious. Sometimes the galleries, filled with 

 observers, are suspended midway between the two piers, over 

 a chasm sixty feet deep. 



An excellent description of this immense Telescope at 

 Birr Castle will be found in Mr. Weld's volume of Vacation 

 Rambles, 



Sir David Brewster thus eloquently sketches the powers of 

 the telescope at the close of his able description of the instru- 

 ment, which we have in part quoted from his Life of Sir Isaac 

 Newton. 



We have, in the mornings, walked again and again, and ever with 

 new delight, along its mystic tube, and at midnight, with its distin- 

 guished architect, pondered over the marvellous sights which it dis- 



