640 PHILOSOPHICAL TRANSACTIONS.*'' [aNNO I769. 



spheroid, till he lost sight of the sun by its being intercepted by a dark cloud, or 

 rather fog-bank, some time before the calculated time of sun -set. Though these 

 last phenomena are entirely optical deceptions, owing to the state of the air at 

 that particular time, or to horizontal vapours, yet Mr. H. begs it may be here 

 very carefully remarked, that, at the times of the contacts, the air was perfectly 

 clear and favourable, so that the observations were then certain, and not sub- 

 jected to any fallacy in vision. The following circumstance is a proof of this 

 assertion. The first warning which he had of the near approach of Venus to the 

 sun's external edge was, as before said, by the sudden appearance of a violent 

 corruscation, ebullition, or agitation of the upper edge of the sun, 5 or 6 seconds 

 before the edge of Venus broke in upon the sun ; where alone he observed the 

 violent agitation, the edge on each side remaining perfectly quiescent, as a, b, 

 fig. 3. If this appearance had been owing to the state of our atmosphere only, 

 then would the edge of the sun be universally fluctuating or trembling; but as 

 this was not the case, the undulation must be imputed to some other cause, not 

 improbably to an atmosphere about Venus. Mr. H was the more minute on 

 this circumstance, because Mr. Maskelyne, in a late publication,* had taken 

 notice, that when Mr. H. took the observation of the transit of Venus at Ma- 

 dras, in the year 1761, he saw " a kind of penumbra or dusky shade, which 

 preceded the first external contact 2 or 3 seconds of time, and was so remark- 

 able, that he was thereby assured the contact was approaching, which happened 

 accordingly." 



On the foregoing paragraph Mr. H. remarks, that in the transit of this pre- 

 sent year, he did not take notice of the same phenomenon as he did of the transit 

 of Venus in India, in the year 1761 ; but he here again insists on it, that such 

 penumbra or dusky shade he then actually saw; but he did not recollect that he 

 then saw the least undulation, ebullition, or corruscation, as happened in the 

 transit of this present year. Yet both phenomena were conducive to the same 

 purpose, and served to give him notice of the near approach of the planet Venus, 

 to the solar disk, the event, in both instances, justifying the presage; and both 

 appearances might be the consequences of the same cause; which cause, as be- 

 fore observed, might be nothing less than the atmosphere of Venus. He says 

 might be, for he would not be understood to assert here any thing dogmatical, 

 preserving at this time the same diffidence in expression as he made use of when 

 he observed the transit of Venus in India, where he was apprehensive, that " to 

 be able to discern an atmosphere about a planet at so great a distance as Venus, 

 may be regarded as chimerical ;" yet he might venture to say, that his observa- 



* Instructions relative to the observations of the ensuing transit of the planet Venus over the sun's 

 disk, on the 3d of June 1769. By the Rev. Nevil Maskelyne, astronomer royal, p. 3'^.— Orig. 



