OBSERVATIONS OF LUNAR OBJECTS. 63 



Second maximum. Interval 7. 1870, March 13, 17 spots, 27 for the 

 whole interval, from 7 observations. 



Third maximum. Interval 12. 1870, May 13, 27 spots, 33 for the whole 

 interval, from 8 observations. 



Third maximum. Interval 12. 1870, Jan. 15, 22 spots, 33 for the whole 

 interval, from 8 observations. 



Fouith maximum. Interval 19. 1869, Dec. 20, 19 spots, 25 for the 

 whole interval, from 8 observations. 



Fifth maximum. Interval 22. 1870, Nov. 11, 13 spots, 20 for the whole 

 interval, from 6 observations. 



Sixth maximum. Interval 24. 1870, Sept. 14, 16 spots, 23 for the whole 

 interval, from 3 observations. 



When we take the mean numbers of spots seen at each intcn^al, the curve 

 is comparatively flat, rising but little above the mean line of 7-9 spots 

 visible at any interval, and this is about the mean number visible on any 

 evening. The flatness of the curve is not accordant with an increase of 

 spots dependent on an increase of solar altitude or greater angle of illu- 

 mination, otherwise the apex would be much more decided. We may, how- 

 ever, trace from the number of spots actually seen and contributing to 

 the maxima of the spot-curve, as weU as from the observations adduced 

 above, that the change of iUuminating angle does exercise an influence on 

 the appearance of spots, inasmuch as on a few occasions the largest number 

 of spots have been seen with higher illuminations. The actual curve, how- 

 ever, derived from two years' obsei-vations is not sufficiently decided to refer 

 the appearances of the spots to this agency. 



By dividing the whole of the twelve hourly intervals into six series or 

 groups, and taking spot No. 1 as the standard of comparison, we have the 

 data for computing Tables III. and IV. containing the visibilities of each 

 spot for each group of intervals : sunrise, or to 60 hours, altitudes 0° to 17° ; 

 60 to 120 hours, altitudes 15° to 34° ; 120 hours to meridian passage, alti- 

 tudes 31° to 42°; meridian passage to 120 hours, altitudes 42° to 31°; 120 to 

 60 hours, altitudes 34° to 15°; and 60 to hours, or sunset, altitudes 17° to 0°. 

 From the results in these Tables, Table V. has been formed, in which we 

 have a bird's-eye view of the visibilities during the luni-solar day. Gene- 

 rally the visibilities are low during the first 60 hours, i. e. compared with 

 spot No. 1, the smaller spots are but seldom seen ; and this is so far indica- 

 tive of solar light in some way developing or bringing the spots into visi- 

 bility. During the next 60 hours some spots have risen considerably in 

 visibUity, while others have been seen more frequently during the afternoon 

 hours of the luni-solar day. The numbers are, however, too irregular to 

 allow us to conclude that the smaller and less frequently seen spots are in- 

 fluenced in their visibility by further changes of illuminating angle beyond 

 theii- first development ; and this is very strikingly manifested by the curves 

 which these numbers furnish ; for example, the diurnal curves of spots 

 Nos. 5, 14, and 16 in fig. 2 generally agree in exhibiting greater visibilities 

 from 60 hours to meridian passage, while spots Nos. 9 and 11 are more 

 frequently seen from 120 to 60 hours before sunset. These, as well as the 

 peciiliarities of the other curves, strongly suggest that the variations of visi- 

 bility of certain spots are not to any great extent dependent upon an increase 

 of intensity of solar light, but rather upon some agency more particularly 

 connected with the spots themselves. It is important to remark that another 

 series of observations 7nay furnish totally diflferent diurnal curves, should 

 the variations in visibility depend upon local lunar action. 



