50 PHILOSOPHICAL TRANSACTIONS. [aNNO 1763. 



expected from such small differences in the duration, which must in some mea- 

 sure be influenced by the necessary and unavoidable errors in observation. If 

 the quantity of the sun's diameter, and the least distance of the centres were 

 very exactly known, the sun's parallax might safely be determined, by comparing 

 the duration of the transit, as observed at different places, with the duration as 

 supposed to be seen from the earth's centre. According to this method, sup- 

 posing the least distance of the centres to be p' 294-", which is a mean between 

 the Greenwich, Shirburn, and Paris observations, and the difference of the se- 

 midiameters of the sun and Venus = Ql6".4, the duration as observed at To- 

 bolski was more than 10 minutes shorter, than if seen without parallax; at 

 Tomea°, at Stockholm, at Cajaneburg, at Astracan, and indeed in almost every 

 part of Europe and Asia, the duration was considerably shortened; and if a 

 number of good observations made in several of those parts were procured, the 

 quantity of the sun's parallax might be well enough ascertained, as the difference 

 in duration, for a difference of one second in the sun's parallax, will be found 

 very considerable. Though this method should not be practised, unless the 

 necessary requisites for the computation be known with some degree of preci- 

 sion, Mr. H. has ventured to compare the durations observed chiefly in the 

 northern parts of Europe, and some in Asia, with the duration as seen from the 

 earth's centre, = 5^ SQ"" IQMO mean time, or 5^ 59'" ld^64 apparent time, 

 and calculated from the elements above mentioned. 



, Observed Duration without Differ- of Diff. for 1" Q'spa- 



hlaces. durations. parallax. duration of parallax, rallax. 



Tobolsk! 5" 48" 53'.25 6" 69" l6'.6i 10" 23'.39 6i\C9 9".726 



Cajaneburg 5 49 54 9 22.64 57.32 9.815 



Tornea". 5 50 09 - 9 17-64 56.83 9.636 



Xornea" 5 50 21 8 55.64 56.83 9.425 



Abo 5 50 09 9 07.64 55.12 9.935 



XJpsa'l 5 50 26 8 50.64 53.67 9888 



Upsal 5 50 07 9 09.64 53.67 1»).241 



Upsal 5 50 02 9 15.64 53.67 10.352 



Hernosand 5 50 26 8 50.6* 53.65 9.890 



Stockholm 5 50 42 8 34.64 53 62 9579 



Stockholm 5 50 45 8 31.64 53.62 9-523 



Calcutta 5 50 36 8 40.6* 53.31 9.766 



Madras 5 51 43 7 33.64 46.36 9.785 



The 4th column contains the difference between the observed and calculated 

 duration; in the 5th is given the difference in the duration for a difference of 1* 

 in the sun's parallax, and the 6th column is obtained by dividing the 4th by the 

 5th. The mean of all the results is Q".ei2: and if we reject two of the observa- 

 tions at Upsal, which differ most in excess, the sun's parallax is Q".7'2,4, agreeing 

 very nearly with the quantity resulting from a comparison of some of the ob- 

 served durations with the shortest observed at Tobolski and Cajaneburg. 



