148 Royal Astronomical Society. 



supporting plates being thus in a great measure relieved from tlie 

 pressure of the speculum, must, by their elasticity, tend to distort 

 the metal by pressure at its back ; and when the telescope is moved 

 towards the zenith, the plates yield again by the weight of the spe- 

 culum, while the lower edge, still in hard contact at the points of 

 support, is unduly borne up there, and the equilibrium is destroyed. 

 To remedy this evil I have slung the speculum in a hoop of thin 

 iron, equal in length to half its circumference, the ends of the hoop 

 being attached to swivels fixed in each of the two horizontal brackets, 

 aiid the lower part of the hoop being thus quite at liberty to rise and 

 fall with the plates. 



" This has nearly, if not entirely, removed all perceptible distor- 

 tion ; yet in some positions, and under some circumstances, vestiges 

 of it are to be perceived. I have devised a plan of supporting the 

 metal laterally by an equal tension on the several points of support, 

 and think it may probably be useful ; but I have not yet had leisure 

 to carry it into effect, 



" Instead of a plane speculum I usually employ a prism, which 

 transmits a pencil of two inches in diameter, made for me by Messrs. 

 Merz and son, of Munich. I am persuaded, from repeated experi- 

 ments, that the prism has an obvious advantage in light over a spe- 

 culum, and the material is so fine, and the surfaces so exquisitely 

 wrought, that no perceptible injury of the image exists. The only 

 care necessary in the use of the prism is to preserve it from dew, 

 which it is extremely liable to collect ; this I have remedied by ha- 

 ving a chamber made in the mounting of the prism, which receives 

 a cube of cast iron enveloped in thick felt : this, being moderately 

 warmed and placed in the chamber, effectually prevents the deposi- 

 tion of dew for at least some hours, while the extremely slow radia- 

 tion through the felt does not produce any sensible disturbance in 

 the formation of the image. The prism is rather small ; for though 

 it transmits the entire pencil, there is scarcely anything to spare ; 

 and had it been easy to obtain a sufficiently good one half an inch 

 larger, I should have procured it." 



A short notice of the Equatoreal of the Liverpool Observatory. 

 By Mr. Hartnup. 



As the Astronomer Royal will probably give some account of this 

 instrument, which has been constructed on his recommendation and 

 entirely under his superintendence, Mr. Hartnup states, in a few 

 ■words, that it is of the English construction ; that is, the telescope 

 is a transit supported at each end, between two long supports 

 which form the polar axis. The telescope is by Merz of Munich, 

 8| inches in aperture, and 12 feet focal length. The circle and 

 declination-circle are each 4 feet in diameter, divided by Mr. 

 Simms upon his "self-acting circular dividing engine*." The 

 hour-circle revolves independently of the instrument, and is carried 



* Described in vol. xv. of the Memoirs. The new altitude and azimuth 

 instrument at Greenwich, which was divided on the same engine, is consi 

 dered by Mr. A'ry to he exceedingly well divided. 



