object had been observed over two months before. The recur- 

 rence of some of the markings on Jupiter at the same places 

 where similar forms had already been observed previously, 

 although quite a rare phenomenon, is not, however, without 

 precedent, as appears from my observations of this planet. Tor 

 instance, the angular marking on the southern border of the 

 equatorial belt described above, and near which the red spot 

 made its appearance, very strikingly illustrates the principle of 

 recurrences, since this spot, so familiar to me, has disappeared 

 and reappeared three times in a period of observation embrac- 

 ing a little over one year." There seems to be something 

 curious about this remark, for this particular marking is now 

 visible, and has been ever since the appearance of the red spot. 

 It is true that the angularity is now on the eastern side of the 

 red spot, but this is attributable to a very gradual change, oc- 

 curring as described below, and is due neither to a motion 

 westwards of the red spot, nor to a sudden alteration of the 

 belt. A similar angularity was noticed on the eastern side of 

 the spot shortly after Mr. Trouvelot made his remarks, and it 

 looked then as if the great red spot was simply an oval patch 

 of the southern dark belt that had got separated and moved 

 southwards, a huge bight being formed in the belt immediately 

 north of the spot, of which the two angular formations pre- 

 viously noticed formed the edges. After a time a white streak 

 was noticed almost all along the middle of the southern dark 

 belt, and this gradually spread over that portion of the belt 

 immediately west of the spot, until now the dark belt at this 

 part of the planet is reduced almost to two streaks, the nor- 

 thern (corresponding to the north edge of the remainder of 

 the belt) being very dark, and the southern so faint as to be 

 almost invisible unless the definition is good. 



Then, with regard to the periods of visibility of the red 

 spot. In August, 1879, Professor Pritchett, in referring to 

 Mr. Trouvelot's remarks, gives several dates on which the red 

 spot ought to have been seen if it were visible, but mentions 

 that, although he looked carefully for it, he was unable to see a 

 sign of it. These dates are 1878, July 10, August 15, 31, Sep- 

 tember 10, 20, 22, October 2, and November 2. Added to these 

 we have the days when Mr. Trouvelot looked for and failed to 

 see it, viz., September 6, 10, 15, 20, 22. But, on the other 

 hand, Mr. Dennett observed it on July 27, and at the Adelaide 

 Observatory it was first noticed on August 4, and was again 

 seen on the 9th and 13th of the same month. Professor 

 Pritchett goes on to say : — " I find by numerous trials that this 

 spot is plainly visible in my telescope with a power of 275 for 

 more than two hours before and after passing the middle point 

 of its visible arc of rotation. Here, then, are at least eight 



