384 



ANNUAL OF SCIENTIFIC DISCOVERY. 



Average Cloudiness 



Full moon 

 First day after full 

 Second day after 

 Third day after 

 Fourth day after 

 Fifth day after 

 Sixth day after 

 Third quarter 

 Sixth day before new 

 Fifth day before 

 Fourth day before 

 Third day before 

 Second day before 

 First day before 



6.8 

 6.7 

 6.1 

 6.9 

 7.0 

 6.9 

 6.9 

 6.9 

 6.6 

 6.5 

 6.3 

 6.6 

 6.6 

 6.4 



Average Cloudiness 



Day of new moon 

 First day after new 

 Second day after 

 Third day after 

 Fourth day after 

 Fifth day after 

 Sixth day after 

 First quarter 

 Sixth day before full 

 Fifth day before . 

 Fourth day before 

 Third day before . 

 Second day before 

 First day before 



6.b 

 6.6 

 6.6 

 6.6 

 6.8 

 65 

 6.4 

 6.7 

 6.6 

 7.0 



f* r* 



b,.o 

 6". 5 

 6.8 

 7.6 



These numbers indicate but slight deviation from 6.7, which is the 



^5 



average cloudiness of the whole period, (10 representing a sky per- 

 fectly overcast.) In other words, exactly two-thirds of the sky at 

 Greenwich is, upon an average, covered,, with clouds. The greatest 

 departure from the mean, occurs on the second day after full, when 

 the average is 6.1 This might be suspected to indicate a law of na- 

 ture ; but such a conclusion is discountenanced by the fact that the 

 amount of cloudiness on the different years was very unequal. In 

 1844, the average cloudiness for the second day after full was only 

 4.7, which is 2.0 below the average result of the same year. In 1842, 

 the average for the second day after full was 5.7 or 1.0 below the gen- 

 eral mean. The average of the remaining five years is 6.5, which is 

 only 0.2 below the general mean for the entire period. 



If we divide the whole month into four parts, in such a manner that 

 the middle of the intervals shall correspond to new moon, first quarter, 

 full moon and last quarter, we shall obtain the average cloudiness at new 

 moon 6.6, first quarter 6. 7, full moon 6.7, and last quarter 6. results 

 which may be pronounced entirely identical, and seem to demonstrate 

 that Sir John Herschel's meteorological fact is unmingled moonshine. 



It may possibly be objected that my mode of discussing these ob- 

 servations, is calculated to conceal the fact claimed by Sir John Her- 

 schel ; inasmuch as I have employed the average cloudiness of each 

 entire day, whereas the full moon is only claimed to exert an influence 

 when she is above the horizon. I do not admit the force of the objec- 

 tion ; for if the full moon, when above the horizon, exerts an influence 

 to dissipate the clouds, such an influence ought to appear in the aver- 

 age cloudiness of the twenty-four hours. This conclusion can only be 

 avoided, by supposing that the full moon, when below the horizon, 

 exerts a positive influence upon the clouds, contrary to what is pro- 

 duced when she is above the horizon, and the one influence ought to 

 be just as palpable as the other, and ought not to have escaped the 

 notice of so shrewd an observer as Sir John Herschel. 



In order, however, not to leave any room for cavil, on this point, I 

 have compared the observations made at midnight, on the days of new 

 moon, first quarter, full moon, and last quarter, for the entire period 

 of seven years. 



According to these observations, the average cloudiness of midnight 



