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VIII. Inquiry as to whether the tendency to Dispersion of Cloud 

 under a Full Moon in any way depends on Lunar Influence. 

 By William Ellis, F.R.A.S* 



IN addition to some general belief in the power of the moon 

 to disperse clond, the tendency to disappearance of cloud 

 under a full moon is also spoken of by the philosophers Hum- 

 boldt, Herschel, and Arago. By Herschel (' Outlines of Astro- 

 nomy/ seventh edition, page 285) it is believed to be a conse- 

 quence of lunar action, and Arago also (Astronomie Populaire, 

 book xxi. chapter 33) appears to consider such influence pro- 

 bable. To question the existence of this power when supported 

 by such testimony may therefore seem presumptuous ; and yet I 

 cannot conceal from myself the fact that many years' experience 

 as an astronomical observer at the Royal Observatory at Greenwich 

 has impressed me with the idea that at that place the evening ten- 

 dency to clearance is not confined to the period of full moon, but 

 exists to an equal extent at all times, leading to the conclusion 

 that the phenomenon does not depend on lunar action. 



But there is also evidence that the disappearance of cloud is 

 not confined to the full-moon period. Admiral FitzRoy (Weather 

 Book, page 256) says that in fine weather there is a general 

 tendency towards a disappearance of clouds soon after evening, 

 " whether the moon is visible or not, whether full, or near any 

 other period." And the question is not whether the tendency 

 exists, but whether any portion of it at full moon is due to lunar 

 action — whether the tendency to dispersion of cloud in the 

 evening is greater at the time of full moon than at other times, 

 or greater than at the time of new moon. For if not greater, 

 then the added lunar influence at the time of full moon produces 

 no effect, or no lunar influence exists. 



I had often wished to investigate this matter for myself; and 

 it seemed to me that the observations made at the Royal Obser- 

 vatory, Greenwich, from the years 1841 to 1847, might be used 

 for such a purpose. I am not aware that they have before been 

 so employed. They were made continuously at intervals of two 

 hours, excepting on Sundays and a few other days; and the 

 series comprises in all 86^ lunations. The amount of cloud at 

 each observation was estimated, being supposed to represent 

 a sky free from cloud, and 10 one wholly cloudy. 



Now in the separate Greenwich volumes the mean amount of 

 cloud, at each two hours, is given for every month of the year. 

 Combining together these results for the whole of the seven 

 years, the following Table, showing the average diurnal in- 

 equality, is found. The observations were made at the even 

 hours of Gottingen astronomical time, for which I have substi- 

 tuted Greenwich time. 



* Communicated by the Author. 



