llg PHILOSOPHICAL TRANSACTIONS. [aNNO 1764. 



with mine; but in order to make it tally with that of the Cape, it would be ne- 

 cessary to diminish this distance by 10 or 1 1% and it is not very likely that Mr. 

 Maskelyne should have committed such an error. His observation might like- 

 wise be brought to coincide with mine, by making a diminution of 40 or 45' in 

 the western longitude of St. Helena; whereas it would be necessary to take se- 

 veral minutes from that longitude in order to make the observation agree with 

 that of the Cape, which does not seem possible. Hence, though he does not 

 take Mr. Maskelyne's observation to be in itself absolutely decisive, yet he is 

 persuaded that it adds great weight to the exactness of his own. 



Fort St. George at Madras is, acwrding to Mr. Hirst, 13° S' north latitude, 

 and S'" 4^ of time eastward of Pondicherry, and consequently at least 5^ 12™ 54* 

 eastward of our observatory's meridian. The interval observed by him between 

 the two internal contacts, was 5'' 51"" 43', greater by 2"" 4Q'^ than that which 

 was observed at Tobolsk. This would give 9 ".56 for the sun's horizontal paral- 

 lax, a quantity which is about a medium between the Ca|)e observation and 

 Mr. Fs. 



It is to be observed, thai by increasing or diminishing by 10' the duration ob- 

 served at Madras, the question of the parallax will be decided conformably either 

 to the observation of Rodriguez, or that of the Cape. 



In the same vol.* are some observations of the same transit made at Abo and 

 atHernosand; the total duration was observed in both places; it may have been 

 lengthened somewhat beyond its limits; but these obseryations agree at least in 

 this point with all the others that were made in the north, viz. that being com- 

 pared with the Tobolsk observation, with regard to the duration of the transit, 

 they give above 10" for the horizontal parallax of the sun. 



Mr. P. had the communication of Mr. Rumowski's observation made at Se- 

 lenginsk in Siberia. The latitude of Selenginsk is 51° 6' 6". And he settled the 

 longitude to be 6^ 57"' 50* from the Paris meridian, from many observations. 

 This being supposed, the observation of Mr. Rumowski compared with Mr. P.'s 

 would give 10.1 for the horizontal parallax of the sun in the month of June, 

 and 1 0".26 for that parallax when the sun is at the mean distance. It is true, 

 the same observation, compared with that of Messrs Mason and Dixon, would 

 much reduce this parallax; and therefore it can be of no use to decide the ques- 

 tion, unless we had some from Africa or from the adjacent seas. 



Mr. P. observes, you will undoubtedly have observed a pretty remarkable dif- 

 ference between my observations of Venus, as I had the honour to send them to 

 you from Lisbon March 6, iyQ'l, and the same observations as I have related 

 them in my memoir. I imagined 1 had sent the correction to some one of the 



* P.ige 561 of vol. xi. 



