1881.] 



President's Address. 



49 



of two threads at one second of arc apart when seen projected on the 

 sun with a power of 150. The times corresponding to the internal 

 contacts should be denned as follows : — 



At Ingress. — " The time of the last appearance of any well-marked 

 and persistent discontinuity in the illumination of the apparent limb 

 of the sun near the point of contact." 



At Egress. — " The time corresponding to the first appearance of any 

 well-marked and persistent discontinuity in the illumination of the 

 apparent limb of the sun near the point of contact." 



It is a point of primary importance that all the observers shall, as 

 far as possible, observe the same kind of contact ; and it is therefore 

 desirable that the times recorded for contacts should refer to some 

 marked discontinuity in the illumination of the sun's limb about which 

 there cannot be a doubt, and which may be supposed to be recognisable 

 by all the observers. If a pure geometrical contact is alone seen, 

 there can be no doubt about the time which should be given ; but, if 

 haze is noted, it should be haze nearly as dark as the outer edge of 

 the planet ; and if a ligament is seen, it should be nearly as dark as 

 the outer edge of the planet. 



A further proposal was made to establish a Central Bureau in Paris 

 to receive and discuss the observations, and to enter upon other work 

 more or less directly connected with the determination of solar 

 parallax. But, as this question was not contemplated in the instruc- 

 tions given to our representative, and indeed exceeded the powers of 

 the British Committee, no definitive resolution was parsed on the 

 subject. 



On the subject of the longitude of a point in Australia, to which I 

 made allusion in my address last year, as important for the observa- 

 tions of the Transit of Venus, I have lately received a letter from 

 Mr. Todd, of the Observatory, Adelaide, from which the following is 

 an extract : " With regard to the determination of Australian longi- 

 tudes : as it is understood that Lieut. -Commander Green, U.S.N., 

 will call at Port Darwin to determine its longitude by signals from 

 Singapore on the one side, and with the Adelaide Observatory on the 

 other, I have taken no further steps for going to Port Darwin as 

 previously arranged. I shall take all the necessary observations here, 

 and exchange signals with Lieut.- Commander Green over my over- 

 land telegraph ; and, in conjunction with Messrs. Ellery and Russell, 

 make fresh determinations of the difference of longitude between 

 Adelaide, Melbourne, and Sydney." 



Since our last anniversary, Sir George Airy, the late Astronomer 

 Royal, having completed his eightieth year, and .nearly half a century 

 of office, has retired. Of his services to science, and to this Society as 

 President, and in other ways, the time to speak has happily not yet 

 arrived. His great intellectual powers are in fact in no way impaired, 



VOL. XXXIII. e 



