130 Determination of the Sun's Distance. 



one accomplished during the opposition of* Mars, in 1862, 

 the theoretical parallax being 8* 95, while that deduced from 

 the Mars observation here and at Greenwich was 8". 9 5. 



The near approach of Mars to the earth in its opposition 

 of 1862, offered a most favourable opportunity for a deter- 

 mination of the sun's distance, more especially as several 

 southern observatories had become possessed of graduated 

 instruments of the highest class, and their geographical 

 positions much more exactly known than at previous 

 favourable oppositions. 



Our Observatory, geographically speaking, was most 

 favourably situated to co-operate with the great European 

 observatories in this interesting undertaking, for between 

 here and Greenwich is included a base line of nearly twice 

 the earth's radius ; and fortunately a few months prior to 

 receiving a request from Europe to join in the work, our 

 beautiful transit circle arrived, which placed the Observatory 

 in the position of doing so on more equal terms with our 

 friends at the antipodes. 



The Observatory, too, at the Cape of Good Hope was well 

 situated, and ready with its magnificent instrumental 

 appliances to take the lead in the Southern Hemisphere. The 

 Observatory at Santiago, in Chili, under Dr. Moesta, also lent 

 its aid. Unfortunately, the Sydney Observatory had lost its 

 astronomer, in the retirement of Mr. Scott, or Australia 

 might have claimed even a greater share of . the honour of 

 helping in this great work. 



The observatories of Greenwich, Pulkowa, and Washington 

 were the principal northern ones which shared in the under- 

 taking. 



The correspondence from Greenwich, Pulkowa, and 

 Washington, in which our co-operation was asked, had 

 furnished the necessary directions to secure proper concert 

 in observing, so that at the commencement of the arranged 

 period all possible precautions to secure success at our Obser- 

 vatory had been made. The whole series of observations, 

 extending from August 25th to November 16th, were 

 obtained most satisfactorily. Out of the allotted period, 

 eighty-nine nights, Mars and its companion stars were 

 observed on sixty-six nights, which, considering the great 

 prevalence of cloudy weather during that portion of the year, 

 was more than was expected. The Cape Observatory was 

 less fortunate, and I believe that our Observatory obtained, 

 by a considerable amount, the largest number of observations. 



