4 BROWN : 



at the distance seven from Venus. The dimensions of the 

 Karth, also, are known with great precision, though at the 

 time referred to the measurements were not quite so accurate. 

 We have not yet computed the diameter of the sun, but we 

 do know from elementary geometry that whatever his distance 

 from us, his diameter bears a certain proportion to that dis- 

 tance. This proportion is approximately ascertained by angu- 

 lar measurement ; and it is found that the distance of the 

 Earth from the sun is about one hundred and eight times his 

 diameter. By way of illustration : — A round bit of board 

 one foot in diameter will exactly cover the sun if held at the 

 distance of one hundred and eight feet from the eye. 



Suppose now that two observers on the same or nearly the 

 same meridian, but in widely diiferent latitudes, observe a 

 Transit of Venus. The path of the planet across the sun's 

 disc, as seen by the southern observer will be to the north of 

 the path observed from the northern station, and the zone 

 marked out by these two paths upon the sun will have a cer- 

 tain width in miles dependent upon the known distance of the 

 observers from each other, and not at all upon the distance of 

 the Karth from the sun. For were the Karth moved in ima^^i- 

 nation nearer to the Karth or farther off from it, Venus also 

 would have to be moved nearer to the Karth or farther from it 

 in order U) preserve the proportion mentioned above of 25 to 7. 



In this little diagram let K be the Karth and S the sun. A 

 and B are two observers ; A near the south pole of the Karth 



