64 



REPORT 1871. 



In nearly every case the spots seen during the first 60 hours of the luni- 

 solar day have increased durmg the day in visibility, i. e. they were seen less 



frequently during this group of 

 intervals than during the succeed- 

 ing sixty hours. This increase, 

 however, has not been regular, 

 which it would have been from 

 changes of illuminating angle 

 alone, some spots having been 

 seen, as before stated, more fre- 

 quently during the second group 

 of intervals, while others have de- 

 clined in visibility and not at- 

 tained their maxima until the 

 period 120 to 60 hours before sun- 

 set. The diurnal curves of spots 

 Nos. 14, 5, and 16 in the first 

 category, and those of Nos. 9 and 

 11 in the second, have already 

 been refeiTed to ; that of spot No. 

 22 (fig. 3) diff'ers from the others 

 by its showing an increase of visi- 

 bihty from sunrise to 120 hours 

 before sunset. The visibilities of 

 many spots are lower during the 

 last 60 hours of the luni-solar 

 day. 

 The 



curves of visibility 

 are 



durinj 



Diurnal Curves of Visibility. 

 Plato. 



Spots on 



the luni-solar day are essentially 

 difl^erent from the curves of visi- 

 bility as deduced from the obser- 

 vations of twenty-four lunations, 

 although both lead to the same 

 result ; and from both a very im- 

 portant conclusion may be drawn, 

 viz. that upon assuming other agen- 

 cies to be in operation than changes 

 of illuminating angle, such as present activity, the epochs at which such 

 activity was manifested varied to such an extent, and were so far separated 

 from each other in time, as to coincide, in the case of spots Nos. 14, 5, and 16, 

 with the period in the luni-solar day of 60 to 120 hours after sunrise, while 

 the activity manifested by spots Nos. 9, 11, and 22 occurred at a later period 

 of the luni-solar day, 120 to 60 hours before sunset. So far as the varia- 

 tions of visibility of spots Nos. 14, 5, 16, 9, 11, and 22 are concerned, 

 they do not appear to depend exclusively on changes of illuminating angle, 

 even if a certain intensity of solar light contributes generally to render the 

 spots visible. 



While the four craterlets Nos. 1, 3, 30, and 17 are visible during the whole 

 of the luni-solar day, the spots on their sites are seldom seen until the sun 

 attains an altitude of about 30°, and then they appear as " bright round 

 disks ;" and this characteristic attaches as well to the craterlets as to other 

 spots when the sun attains this altitude. With altitudes between 30° and 

 40° a different class of phenomena is manifested ; the sharp and distinct cha- 



