VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 657 



reason to believe that the error of observation at Tornea is at the egress, where 

 indeed the two observers differ considerably. 



The parallax of the sun being thus found, by the observations of the internal 

 contact at the egress, = 8^52 on the day of the transit, the mean horizontal 

 parallax of the sun is = 8". 65. ,v »b (nmmjl . ?a 



I cannot help taking notice on this occasion, of a method employed by some 

 astronomers, of determining the diameter of Venus by the duration of the egress 

 over the sun's limb: for I am fully satisfied that the best eye, assisted by the best 

 telescope, and in the best and clearest air, could not see the very last contact of 

 Venus with the sun's limb, but must have lost sight of it several seconds before 

 she really had left the sun's limb; and this will the more plainly appear, when it 

 is considered that every second of the diameter of Venus took up about IQ* 

 of time in passing over the sun's limb. And to show this further, and in a 

 stronger light, I shall mention the following particulars. Mr. Canton measured 

 the diameter of Venus, and found it = 58'^, but by his duration of the egress, 

 the diameter of Venus is = 57''. 8: the same diameter was measured by myself at 

 Savile-house, and fonnd = 5Q", but by the duration of the egress observed 

 there, the diameter of Venus is = 58".6. Mr. Mason also at the Cape mea- 

 sured the diameter of Venus, which he found = 5g\-^, but by his duration of 

 the egress her diameter is found = 57 ".O: and therefore I must conclude that 

 the diameter of Venus, found by the duration of the egress, must be always less 

 than the true diameter for the reason given above. And since I am on this sub- 

 ject, I shall likewise mention the times of duration of the egress from several 

 diameters of Venus. If the diameter of Venus be supposed = 57^', then the 

 duration of the egress at London should have been = 18"^ Q^; if the diameter be 

 = 58'', then the duration at London will be found = }8™ 28^; and if the dia- 

 meter be = 5g% then the duration of the egress at London will be found = 

 jgm 47s 'j'l^Q diameter of Venus being 5g", and the diameter of the sun = 3l' 

 31'^, the duration of the egress at Stockolm is = 18"^ 43% at Paris = 18*" 45% 

 at the Cape of Good Hope = 18'" 6\ and at Rome 18"™ 38^ The duration of 

 the egress at this last place was observed = 18"^ 3V; and if we suppose the dia- 

 meter of Venus = 58% and the diameter of the sun = 31' 33% the duration of 

 the egress at Rome will be found = 18^" 18' ; which duration being less than the 

 observed duration, it therefore follows that the diameter of Venus was more 

 than 58'' on the day of the transit ; and the duration of the egress at Paris, ob- 

 served by M. de la Lande and P. Clouet, by Mr. Mallet and Mr. Bergman at 

 Upsal, by M. Chappe at Tobolsk, and by myself at Savile-house, prove the 

 same thing. 



VOL. XI. 4 P 



