Royal Astronomical Society. 1 4- 1 



that, though pursuing different courses, the Earl of Rosse and Mr. 

 Lassell had each attained almost absolute perfection in figuring and 

 polishing their specula, and that the difficulties in mounting, &c. 

 were gradually being overcome by Lord Rosse, while they were 

 already nearly got rid of by Mr. Lassell in his comparatively small 

 instrument. 



Mr. Drew, who has lately built and furnished a very convenient 

 observatory at Southampton, adopts a collimating telescope for get- 

 ting rid of his error of collimation. To this latter telescope he has 

 attached a wire micrometer, which supplies the object to be viewed 

 by the transit. He also uses the wire-micrometer to measure the 

 intervals of his wires. The results are more readily obtained than 

 by slow moving stars, and he conceives with at least equal accuracy. 

 Specimens of the determination of the intervals by both methods are 

 given, which agree very nearly. 



Dec. 8, 1848.— Transit of Mercury, Nov. 8-9, 1848. By the 

 Rev, W. R. Dawes, at Cranbrook. 



" My attention was directed principally to the appearance of the 

 planet at its ingress, and to measurements of its diameter during the 

 transit. 



" The ingress was observed with my 8^-foot achromatic, the aper- 

 ture being limited to 4 inches, the eye-piece magnifying eighty- 

 seven times. So extremely undulating was the edge of the sun in 

 general, that no advantage seemed to arise from an increase of power. 

 Nothing remarkable was noticed till Mercury had advanced on the 

 sun's disc to about three-quarters of its jjwn diameter, when the 

 cusps appeared much rounded off, giving a pear-shaped appearance to 

 the jjlanet. The degree of this deformity, however, varied with the 

 steadiness and definition of the sun's edge, being least when the de- 

 finition was best. A few seconds before the complete entrance of 

 the planet, the sun's edge became much more steady, and the cusps 

 sharper, though still occasionally a little broken towards their points 

 by the undulations. At the instant of their junction the definition 

 was pretty good, and they formed the finest conceivable line. Mercury 

 appearing at the same time perfectly round. 



" The impression upon my mind was, that the distortion of the 

 planet arose entirely from the rounding off of the points of the cusps 

 by the tremor and diffusion of the image. I have repeatedly ob- 

 served precisely the same appearance at the ingress and egress of 

 the shadow of a satellite of Jupiter, when the edge of the planet has 

 been rather undulating and diffused. 



" For the measurement of the diameter of Mercury I had prepared 

 several different instruments. The filar micrometer was applied to 

 the 8^ foot equatorially mounted achromatic, the clock motion being 

 in use. A 5-foot achromatic by DoUond was furnished with one 

 of his spherical crystal double-image micrometers, and mounted on 

 a very stout floor-stand with an equatorial socket. An excellent 

 Gregorian reflector, of 5 inches aperture and 20 inches focus (the 

 large metal figured by Cuthbert), and furnished with its own di- 

 vided object-glass heliometer, was also employed. And lastly, j^ij.. 



