34 Astronomy and Geology compared. PT. i. 



Earth ; but this does not occur, because the orbit of 

 Venus is not in the same plane with that of the 

 Earth, but is, as it were, tilted up on one side and 

 down on the other, to use a very unscientific mode 

 of expression ; or, as astronomers would say, the 

 orbit of Venus is inclined at a certain angle to the 

 orbit of the Earth. When, therefore, Venus, in her 

 revolution, passes between the Earth and the Sun, 

 she does so, to use the same popular and unscientific 

 mode of expression, either at a height above, or at 

 a depth below the exact line between the centres of 

 the Sun and of the Earth ; Venus only passes 

 directly between the Earth and the Sun on those 

 rare occasions when the planes of the two orbits 

 intersect each other, so that a line drawn from the 

 centre of the Earth to the centre of the Sun would 

 pass through the centre of Venus. Now, in con- 

 sequence of the angle of inclination of these orbits 

 to each other, such a result happens but rarely ; it 

 only takes place at intervals of 105 and of 8 years, 

 two transits following each other at the short in- 

 terval, and then the long period of 105 years 

 elapsing before another takes place. Now the proof 

 of the great exactness and uniformity in the motions 

 of both these planets is found in the fact that these 



