Royal Astronomical Society. l-l-S 



here. This is a considerable grayish spot on the disc of Mercury, 

 very indefinite, but gradually shading off from the brightest point in 

 the centre to the blackness of the rest of the planet. I have at- 

 tempted to give some idea of this appearance in the drawing annexed, 

 as seen with a power of 240 ; with a less power we could not di- 

 stinguish it." 



By the Rev. Mr. Reade, at his observatory, Stone. 



Mr. Reade has sent a drawing of the gray spot observed in Mer- 

 cury, which agrees with Mr. Dell's. The observations consist of a 

 numerous series of angles measured from Mercury to spots on the sun, 

 from which M. Fazell has made an elaborate chart of the path of 

 Mercury over the sun's disc. 



By Mr. Hartnup, at the Observatory, Liverpool. 



Equatoreal, 8^ inch achromatic ; power, 134. 



Internal contact 23** 6™ 54*-4 Greenwich mean time. 



"The instant is noted at which the sun's light was first seen to 

 surround the planet completely." 



Description of a Machine for Polishing Specula. By Mr. Lassell. 



" The twelfth volume of the Memoirs of the Royal Astronomical 

 Society contains a description of a Newtonian Reflecting Telescope, 

 of 9 inches aperture and 112 inches focus, equatoreally mounted in 

 a revolving dome of 14^ feet diameter. 



" Several years' experience in the use of this instrument so well 

 convinced me of its general efficiency, and especially of the con- 

 venience and stability of its mounting, that I determined, two or 

 three years ago, to carry out precisely the same principle on a much 

 larger scale. 



" With a view of informing myself what degree of perfection is 

 attainable in figuring surfaces of larger mirrors than can be wrought 

 by hand, and also of ascertaining the proportion of aperture to focus 

 which it would be most desirable to adopt, I visited Birr Castle ; 

 and, by the kindness of the Earl of Rosse, enjoyed the opportunity 

 of two nights' observations with the 3-foot telescope erected by his 

 lordship. 



" I was also favoured with an examination of the whole of the 

 machinery employed in grinding and polishing the great speculum : 

 and I returned so well satisfied with all I had seen, that I very 

 shortly resolved to cast a speculum of 2 feet diameter and 20 feet 

 focus. 



"The mode of casting the large speculum which I employed 

 involved the principle, discovered, I believe, and first published, by 

 Lord Rosse, of casting the speculum on what is technically called a 

 chill, i. e. an iron base, slightly warmed, which causes the speculum 

 to cool upwards in horizontal strata. 



" Principally, however, from the difficulty of forming it, I did not 

 employ a base constructed with iron hoops placed edgewise, and 

 turned to the gauge, as Lord Rosse recommends, but, instead of it, 

 a disc of cast iron, with its upper surface convex, according to the 

 required radius of curvature, and a rebate formed on the edge of its 



