CELESTIAL MEASURINGS AND WEIGHINGS. 193 



peat the explanations there given. The use which may 

 be made of Venus as a stepping-stone on the way 

 towards the great centre of our system, however, is 

 there rather alluded to than explained ; so that a few 

 words on this subject will not be out of place here. 

 The interval between Venus and the earth when near- 

 est, is not more than one-fourth of the sun's distance, 

 and its angular displacement when seen from opposite 

 extremities of a diameter of our globe, therefore, four 

 times as great as the sun's. Venus, however, is invisible 

 in this situation, except on those very rare occasions 

 (occurring at intervals alternating between eight years 

 and upwards of a century) on which it passes between 

 us and the sun, and is seen as a round black spot on its 

 disc. In this state of things the face of the sun itself 

 serves as a screen on which the planet is seen projected ; 

 and its circular outline serves as a celestial line of refer- 

 ence, across which the planet is seen to " transit," as it 

 would across wires fixed in the focus of a telescope ; or 

 rather as it would across the circular outline of what 

 astronomers call a " ring micrometer." The sun itself is 

 thus transformed for the time into an astronomical in- 

 strument of that description, freed by nature from all 

 the sources of fluctuation and instability which affect our 

 instruments. And the whole observation is reduced to 

 determining the precise moments of time at which the 

 foremost and hindmost borders of the planet cross this 



479, col. I, line 52, of this last-mentioned Lecture as printed ori- 

 ginally in "Good Words" For 16,071, read 6,071. All the other 

 numbers are right, as they there stand. 



N 



