rOJj, LXI.] PHILOSOPHICAL TRANSACTIONS. 209 



difference of declination of the centres of the sun and planet at that time, 

 without any regard to the diameters of the sun or planet, or the error of adjust- 

 ment of the micrometer. And this would be a better observation than mea- 

 suring the difference of declination of the limbs of the sun and planet by 

 bringing them both in contact with the same wire parallel to the equator 

 described above ; as the measuring distances from the sun's east or west limb, in 

 lines parallel to the equator, is a better observation than measuring differences 

 of right ascension of the limbs by time. 



Xf By these two observations of distances, of an inferior planet from the sun's 

 limb, in lines parallel and perpendicular to the equator, its true place with 

 respect to the sun's centre may be accurately ascertained during any part of its 

 transit over the sun's disk; and consequently its nearest approach tb the sun's 

 centre, and the time of the ecliptic conjunction, may be deduced with great 

 exactness, though the middle of the transit should not be seen, and the sun 

 should be visible only for a small space of time sufficient for taking these 

 observations. 



.>,The following order of making the several observations with Dollond's 

 micrometer, in the late transit of Venus, was recommended to the observers 

 who went on the part of the Royal Society to the North Cape and to the South 

 Sea, which may serve to elucidate their observations. See Phil. Trans., vol. 59, 

 p. 266, 267, and this vol. 61, p. 397, 418. Instructions to the like effect were 

 also given to the other observers, sent by the Royal Society to Hudson's Bay and 

 the north of Ireland, on the same occasion. See Phil. Trans., vol. 59, p. 480 — 

 482, and vol. 60, p. 488. 



1st. Immediately after the first internal contact, you are to observe several 

 diameters of Venus, suppose 12, with O of the vernier placed alternately to the 

 right and left hand of the beginning of the divisions of the scale. 2d. You are 

 to observe several differences of declination of the northern limbs of the sun and 

 Venus, and the southern limbs of the sun and Venus alternately. 3d. If there 

 be considerable time left before the middle of the transit, you are to observe 

 distances of Venus from the sun's limb to the east and west alternately, in lines 

 parallel to the equator. 4th. If there still remain considerable time before the 

 middle of the transit, you are to observe several times the horizontal diameter of 

 the sun. 5th. You are to begin at least half an hour (an hour would be better) 

 before the middle of the transit, to measure the nearest distance of Venus from 

 the sun's limb, and the farthest distance of Venus from the sun's limb, alter- 

 nately. N.B. The same position of the micrometer will serve for both, without 

 turning it about. These observations are to be continued till the very middle of 

 the transit, when the distance will continue the same for a little space of time; 

 but it will be better to continue them for some time longer. 6th. The same 



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