VOL. LIX.] PHILOSOPHICAL TRANSACTIONS. 653 



instruments at the observatory ; the first was an achromatic tube of DoUond's, 

 29 inch focus, by which an image of the sun was formed, of about 6 inches 

 diameter, on a board covered with paper. The telescope being mounted on a 

 frame, by which it could be turned about as the sun moved, and the room pro- 

 perly darkened. This instrument was managed by Dr. Williamson and Dr. Reid, 

 at the west window of the room of the observatory, where the astronomical 

 clock stood ; the other two instruments were placed without, at the south and 

 north windows of the same room, one being a refractor of 13 feet, by which 

 Dr. Irvine observed ; the other a 12 inch reflector of Short's, by which Dr. W.'s 

 son observed. These two observers looked directly at the sun, having their in- 

 struments armed with smoked glasses ; another person stood at the clock, and 

 counted the seconds by coincident beats upon a piece of board, which he held in 

 his hand for that purpose, and who named every fifth second, so that all the 

 observers could hear him distinctly. The motion of the clock, made by Shel- 

 ton, was carefully adjusted by many transits of the sun and fixed stars, over the 

 meridian, both before and after the day of the transit ; the clock, by which Dr. 

 W.'s observations were made, was adjusted by Shelton's, by means of signals 

 made every hour, for some hours before and after the transit. Having made 

 these preparations, they thought they had nothing to fear but the clouds ; and 

 indeed the western part of the heavens was covered with thick clouds all the 

 afternoon, till a short time before the external contact ; but they drove away 

 towards the north, and left the sini perfectly bright, excepting that now and 

 then a cloud passed over him. But they soon found that the constitution of the 

 air was otherwise unfavourable to their observations ; the image of the sun on 

 the white board, made by the achromatic telescope, was bright enough ; but 

 there was a remarkable undulation in the limb, owing to the state of the air. 

 Besides this undulation in the sun's limb, there was also a considerable tremor 

 round the planet Venus, when she was seen on the sun's disk, and, in conse- 

 quence of this, an indistinctness in her limb, which made it impossible to mea- 

 sure her diameter by the object glass micrometer, or otherwise. After the 

 centre of Venus had passed the sun's limb, she appeared not to be circular, but 

 oblong, the longest diameter being that which passed through the sun's centre. 

 As the internal contact approached, Venus appeared to adhere to the sun's limb, 

 by a dark protuberance or neck, both the length and breadth of which varied 

 every moment by a constant undulation : neither did this neck break off instan- 

 taneously, but changed its colour from black to a dusky brown, till at last the 

 interval between Venus and the sun's limb appeared quite clear. Each of the 

 observers wrote down his observations on the spot. Dr. W. reduced them, to- 

 gether with his own, to apparent time, from the observations he had made on 

 the going of the clock, and they were as follow : 



