VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. 57 



scopes of 10 and 22 feet saw the contact 24' later, Mr. H. supposes it to have 

 happened at 21*^ 4"" 58'. At Florence, the internal contact was observed with a 

 reflector of more than 4 feet, at 21'' 4™ 28' by Father Ximenes. The longitude 

 of this place is 34™ 48" to the east of Paris, according to the table in the Cou- 

 noissance des Mouvemens Celestes, or 35™ 58' according to the table in the 

 Elemens d'Astronon)ie by M. Cassini. By taking a mean of both, Florence is 

 44™ 40' to the east of Greenwich. The longitude of St. Peter's at Rome is 49™ 

 54' according to the French astronomers. The internal contact was observed to 

 happen at 21*" Q™ 36'. But as it is not said where this observation was made, the 

 longitude given above will be found to be somewhat inaccurate. 



Observations were also made at Madrid and Lisbon : at the former, the inter- 

 nal contact happened at 20^ 6™ 56' apparent time ; and at Lisbon at IQ*" 44™ 26'. 

 The longitude of Madrid as given in the Philos. Trans, is certainly erroneous ; 

 being more than a minute and a half too little, if the observation of the transit 

 can be depended on. At Lisbon, the longitude of the place was not determined 

 by M. Ciera, who observed the transit, when Mr. Pingrc, from whom Mr. H. 

 had taken the observation, left it in his way from Rodrigues. From the best 

 accounts that he can collect, particularly from the 385th number of the Philos. 

 Trans., and from an account of some observations by Mr. Short, Lisbon is about 

 36™ 26' to the west of Greenwich. 



Now in order to deduce the sun's parallax from the observations related above, 

 Mr. H. proceeded in the following manner. Having subtracted the difference of 

 longitude between Greenwich and the Cape, = l" 13™ 28', from 21'' SQ™ 52' the 

 mean of the observed times at the Cape, and compared the remainder with the 

 observed time at Greenwich, he finds that the internal contact was observed 7™ 

 24' later at the Cape than at Greenwich on account of the parallax. He then 

 calculated what would be the effect of parallax at each place, supposing the sun's 

 parallax to be 9 seconds ; and found that the time of the internal contact would be 

 accelerated 1™ l6».63 at Greenwich, and retarded 6"" 3 1^.9 at the Cape : the whole 

 effect of parallax therefore is 7'"47'.72. But the difference in time as found by 

 observation, is only 7™ 24' : and therefore the difference by calculation is to the 

 difference by observation, as the assumed parallax is to the true parallax on the 

 day of the transit, which by this observation is 8 '.543. The parallax resulting 

 from each observation will be found in the following table, which will be sufficiently 

 explained by the foregoing example. 



VOL. XII. 



