12 BROWN : 



half a minute at least for the horizontal parallax of Venus 

 from the sun 



(VSV" in the diagram) 



and consequently Venus' motion will be accelerated ^ of a 

 minute at least by that of parallax while she passes over the 

 sun's disk in such elevations of the pole as are near the Tropic 

 and still more so near the Equator. For Venus will at that 

 time accurately enough describe within the sun's disk 4 min- 

 utes an hour and consequently at least eleven minutes of time 

 (by which the duration of this eclipse will be contracted by 

 reason of parallax ) answer to ^ of a minute of arc, and by 

 this contraction alone we might safely determine the parallax, 

 provided the sun's diameter and Venus' latitude were accu- 

 rately given, which as yet we cannot possibly bring to a cal- 

 culation in a matter of such great subtlety. 



" We must, therefore, have another observation if possible 

 in places where Venus possesses the middle of the sun at mid- 

 night, namely, under the opposite meridian, that is of 6 hours 

 or 90 '^ more westerly than London and where \'enus enters the 

 sun's disk a little before his setting and goes off a little before 

 his rising, which will happen in the same meridian in about 

 56° of N. latitude, that is at Nelson Harbour in Hudson's 

 Bay. 



"And indeed I would have several observations made of 

 the same phenomenon in different parts, both for further con- 

 firmation and lest a single observer should happen to be disap- 

 pointed by the intervention of clouds from seeing what I know 

 not if those of the present or following age shall ever see 

 again and upon which the certain and adequate solution of 

 the noblest and otherwise most difficult (sic ) problem depends. 

 Therefore again and again I recommend it to the curious 

 strenuously to apply themselves to this observation. 



" By this means the surf's parallax may be discovered to 

 within its five hundredth part : which will doubtless seem 



