62 



Mr. W. Ellis on the tendency to Dispe)*sion of 



Table I. — Mean Amount of Cloud from all the Two-Hourly Observations 

 made from 1841 to 1847 at the Royal Observatory Greenwich. 



Greenwich 



Mean Amount 



Astronomical Time. 



of Cloud. 



h m 





23 20 



7-14 



1 20 



7*11 



3 20 



7-00 



5 20 



6-61 



7 20 



6-23 



9 20 



6-04 



11 20 



6-14 



13 20 



6-41 



15 20 



6-76 



17 20 



6-84 



19 20 



6-89 



21 20 



7-10 



Mean 



6-69 







It thus appears that, on the average of the whole period, the 

 mean amount of cloud at Greenwich diminishes from 23 h 20 m to 

 9 h 20 m , then increasing again to 23 h 20™, or the sky is more 

 clear by night than by day. Now, if the moon at full has any 

 peculiar power to disperse cloud, the mean amount of cloud at 

 night at full moon ought to be less than the mean amount at 

 night for the whole period, and still less than the mean amount 

 at night at new moon. Grouping together the observations 

 made (through the whole seven years) on the five consecutive 

 days of each lunation having full moon occurring on the middle 

 day, the two-hourly means of this collection of observations 

 were found; and similarly for new moon. The resulting num- 

 bers are contained in the following Table, to which those pre- 

 viously found are added for comparison. 



Table II. — Mean Amount of Cloud at Full Moon, for the whole period, 

 and at New Moon, 1841 to 1847. 



Greenwich 

 Astronomical Time. 



At Full Moon. 



For the 

 whole period. 



At New Moon. 



h m 



23 20 



1 20 



3 20 



5 20 



7 20 



9 20 



11 20 



13 20 



15 20 



17 20 



19 20 



21 20 



7-01 

 7-03 

 6-87 

 6-63 

 6-34 

 5-99 

 6-25 

 6-50 

 6-86 

 6-91 

 7-00 

 735 



7*14 

 711 



7-00 

 6-61 

 6-23 

 6-04 

 6-14 

 6-41 

 6-76 

 6-84 

 6-89 

 • 7-10 



7-45 



7'23 

 7-12 

 6-95 

 5-97 

 5-94 

 5-75 

 6-00 

 6-33 

 6-43 

 €•68 

 7-17 



Means 



6-73 



6-69 



6-59 







