80 PHILOSOPHICAL TRANSACTIOKS. [aNNO 1770. 



XXXIX. On the Transit of Venus, and other Astronomical Observations made 

 at Cavan, near Strabane, in the County of Donegal, Ireland, by Appoint- 

 jment of the Royal Hociety. By Mr. Charles Mason, p. 454. 

 ■'The first of these is a series of observed equal altitudes of the sun and stars, 

 from April 3 till October 23, 1769, to regulate the clock. 2d. A like series of 

 apparent zenith distances of the sun, moon, and stars. 3d. A like series of me- 

 ridional zenith distances, for the latitude of the place, the medium of which is 

 54° 5l'40".8. 4th. Another series of the difference of right ascension between 

 the moon's limb and stars. 5th. Eclipses of Jupiter s satellites, also occultations 

 of fixed stars by the moon, and other phenomena, for the longitude. 6th. The 

 sun, moon, and stars passing the meridian, by a transit instrument. 7th. Also the 

 observations of the transit of Venus, June 3, 1769, and of the sun's eclipse the 

 next day. The times of the transit by the clock were as below : 

 wi(At 1 1** 17*" 53' the external contact of Venus and the sun. 

 11 35 30 internal ditto, judging by their peripheries. 

 1 1 36 8 ditto, when the thread of light broke out. 

 4 35 37.8 the sun passed the meridian. 

 Though the air at external contact was not quite so clear as sometimes seen, 

 yet the sun's limb appeared well defined, and the spots in the disk very strong, 

 their edges keen and distinct. At the internal contact, the air was much changed, 

 and the limb of Venus seemed to cohere to the sun's limb, by a protuberance that 

 appeared like a dark shade : which seemed to prevent seeing the thread of light 

 for about 40" longer than expected. 



When the planet was upon the sun's disk, there appeared 

 a faint light shade (having a gentle fluctuating motion) 

 round its periphery, and widest on that part farthest on 

 the sun's disk : it appeared as per fig., the black circle re- 

 presenting the periphery of Venus, and the dotted one that \\ 

 of the shade, which was very regular and well defined ; v ^^ /^t, 

 the upper, and m the lower part of the planet : and the 

 whole shade was apparently of equal brightness. 

 At 22'' IS'" 0« Cloudy with rain, 

 O Cloudy. 

 28 The eclipse of the sun began. 

 35 Very plain. 

 Cloudy with rain. 



O The clouds began to break ; and from this time to 23*' 54" Mr. 

 M. endeavoured with a micrometer (of Mr. Dollond's construction) to get 

 measurements for determining the digits eclipsed; but was so interrupted by 

 flying clouds, that nothing could be done with certainty; then cloudy with 

 rain till the end of the eclipse was past. 



