14fi Royal Astronomical Society. 



spherico- prismatic crystal double-image micrometer was applied to 

 the 8^-foot equatoreal. Measurements were obtained with each of 

 these instruments ; but from the excessive tremor which usually 

 affected the image, the results were not very satisfactory. 



With the 8i-foot equatoreal and filar micrometer, power 163, aperture 



reduced to 2"84 inches, 



Polar diameter of Mercury =:9"'3694, six observations. 

 Same instrument and power, aperture 4'02 inches, 



Polar diameter =:9"'38!)0, six observations. 



The mean of the two sets =9"-393. 

 Same telescope, and spherico-prismatic micrometer, power 184, 



Polar diameter =8"'89, three observations. 

 With the 5-foot achromatic and the spherical micrometer, power 117, 



Polar diameter, by four observations =:9""02 "1 yen nf/.Qi 



Equatoreal diameter, by two do. =9"'36j * ' 



With the heliometer on the 20-inch Gregorian, power 115, 



Polar diameter, by four observations =8""89 \ ,•«. A'^Q1 



Equatoreal diameter, by ten do. =9'''20j ' 



" No difference is recognised in the Nautical Almanac between the 

 polar and equatoreal diameters of this planet ; yet my observations, 

 both with the 5-foot achromatic and the Gregorian, show a percep- 

 tible difference, and nearly to the same amount. And it was noticed 

 with each of the double-image micrometers that a satisfactory mea- 

 sure of the equatoreal diameter was always perceptibly too large for 

 the polar diameter, the images appearing slightly separated ; and that, 

 on the contrary, with a gQpd measure of the polar diameter, the images 

 overlapped when placed in the direction of the equator. The change 

 was repeatedly made from one to the other, and always with the 

 same result. The compression would thus appear to be about -^. 



" It will be remarked that no sensible difference was produced in 

 the apparent diameter by varying the aperture from 4"02 inches to 

 2'84 inches. The same darkening glass was employed with both 

 apertures ; and therefore, though the telescopic irradiation would be 

 least with the larger aperture, yet, the image being brighter with 

 that aperture, the ocular irradiation would be greater. Probably, 

 therefore, the two effects might counteract each other. 



" The measurements, though few, were taken with extreme care, 

 each of them having been repeatedly examined under the best views 

 before it was read off." 



By Mr. T. Dell, at Dr. Lee's Observatory, Hartwell. 



" The time was taken from the transit-clock, the error of which 

 was well known from observations on the 7th and 8th. The first 

 contact was not noted with any degree of certainty ; the interior 

 contact was well- observed. 

 Interior contact 14''18'"55'-3sid. time, or 23*" 3" 57' mean time at Hartwell. 



" My attention was directed by the Rev, Mr. Reade to a phseno- 

 menon described by the late Professor Moll (Mem. Ast. Soc. vol. vi. 

 p. 116), a recurrence of which we all observed, — Mr. Reade and his 

 assistant, with a Gregorian telescope, at Stone, and again with me 



