46 ROUGH WAYS MADE SMOOTH. 



It must be remembered that an inexperienced observer 

 of the sun might readily mistake a spot of unusual roundness 

 and darkness for a planet's disc. The practised observer 

 would perceive peculiarities at once indicating the object as 

 a spot on the sun ; but these peculiarities would escape the 

 notice of a beginner, or of one using a telescope of small 

 power. Again, an inexperienced observer is apt to mistake 

 the change of position which a spot on the sun undergoes on 

 account of the diurnal motion, for a change of place on the 

 sun's disc. At noon, for instance, the uppermost point of 

 the sun's disc is the north point ; but in the afternoon the 

 uppermost point is east of the true north point. Thus a 

 spot which at noon was a short distance below the highest 

 point of the sun's disc would at two or three be considerably 

 to the west of the highest point, though it had undergone in 

 the interval no appreciable change of position on the solar 

 disc. Suppose now that at two or three in the afternoon 

 clouds come over the sun's face, and he is not seen again 

 that day. On the morrow the spot may have disappeared, 

 as solar spots are apt enough to do. The observer, then (as- 

 suming him to be inexperienced like most of those who 

 have described such spots), would say, I saw at noon a small 

 round spot which in the course of the next three hours 

 moved over an appreciable arc towards the west (the right 

 direction, be it remembered, for a planet to cross the sun's 

 face). An experienced observer would not make such a 

 mistake. But let one point be carefully noted. An expe- 

 rienced astronomer would be very apt to forget that such a 

 mistake could be made. He would take it for granted that 

 the observer who described such a change in a spot's posi- 

 tion meant a real change, not a change due to the diurnal 

 motion. 



Therefore, although Leverrier, Moigno, Hind, and other 

 men of science, have adopted Lescarbault's account, I hold 

 it to be absolutely certain that that account is in some 

 respect or other erroneous. Newcomb goes even farther. He 

 says, it is very certain that if the disturbance of Mercury is 



