Extent of Sky Clouded. 



evil 



228. Variation of the Extent of Clouded Ski/ with reference to the Moon's Position in Declination. — The 

 discussion has been performed for this argument also, and the resulting means are given Table XXXIX., 

 p. 66 ; and for gTOups of days, in Table 99. The two years do not agTee well. If the cloud depends upon the 

 heating influence of the moon we should expect the greatest value for the most northerly position of the moon ; 

 the result, however, would only indicate the excess due to the higher positions of full moon over the lower 

 positions, and as the latter occur in summer, the epoch of maximum cloud, the result becomes complicated with 

 other causes of variation. From the mean of both years the numbers indicate an equal extent of sky clouded 

 for the 14 days about the moon's farthest northerly, and for the 14 days about its farthest southerly positions. 

 When four periods, of seven days each, are considered, the extent of sky clouded is on the whole 0-20 less for 

 the mean of the groups for which the moon is near the equator than for either the northerly or southerly 

 groups. 



Table 100. — Diurnal Variation of the Estimated Extent of Clouded Sky, for each Astronomical 

 Season and for the Year, deduced from the Observations of the Years 1843-6. 



Mak. 



Nov. 



Feb. 



May. 



Aug. 





Mak. 



Nov. 



Feb. 



May. 



Aug. 





Mean 



Dec. 



March. 



June. 



Sept. 



Year. 



Mean 



Dec. 



March. 



June. 



Sept. 



Year. 



Time. 



Jan. 



April. 



July. 



Oct. 





Time. 



Jan. 



April. 



July. 



Oct. 





h. m. 



12 15 



-0-41 



-0-43 



-0-76 



-0-53 



-0-52 



h. m. 



15 



+ 0-59 



+ 0-53 



+ 0-49 



+ 0-57 



+ 0-57 



13 15 



-0-33 



-0-62 



-0-63 



-0-53 



-0-53 



1 15 



+ 0-52 



+ 0-60 



+ 0-38 



+ 0-69 



+ 0-55 



14 15 



-0-46 



-0-41 



-0-41 



-0-53 



-0-45 



2 15 



+ 0-58 



+ 0-56 



+ 0-20 



+ 0-41 



+ 0-45 



15 15 



-0-39 



-0-30 



-0-18 



-017 



-0-27 



3 15 



■fO-64 



+ 0-63 



+ 0-26 



+ 0-28 



+ 0-44 



16 15 



-0-41 



-0-49 



-0-21 



+ 0-08 



-0-28 



4 15 



+ 0-55 



+ 0-31 



+ 003 



+ 0-28 



+ 0-30 



17 15 



-0-14 



-0-23 



-0-18 



+ 0-28 



-005 



5 15 



-0-09 



+ 0-29 



-006 



+ 0-18 



+ 0-08 



18 15 



-0-23 



+ 0-02 



+ 0-22 



+ 0-32 



+ 0-09 



^ 6 15 



-0-38 



-0-04 



-0-15 



+ 001 



-0-14 



19 15 



+ 0-43 



+ 0-26 



+ 0-41 



+ 0-41 



+ 0-38 



7 15 



-0-57 



-0-33 



-015 



-0-31 



-0-34 



20 15 



+ 0-54 



+ 0-29 



+ 0-52 



+ 0-35 



+ 0-43 



8 15 



-0-40 



-0-57 



-0-34 



-0-90 



-0-56 



21 15 



+ 0-60 



+ 0-52 



+ 0-57 



+ 0-28 



+ 050 



9 15 



-0-43 



-0-56 



-0-33 



-0-78 



-0-53 



22 15 



+ 0-60 



+ 0-52 



+ 0'53 



+ 0-52 



+ 0-57 



10 15 



-0-86 



-0-75 



-0-36 



-0-78 



-0-69 



23 15 



+ 0-65 



+ 0-52 



+ 0-54 



+ 0-60 



+ 0-58 



11 15 



-0-63 



-0-29 



-0-48 



-0-63 



-0-52 



229. Diurnal Variation of the Extent of Clouded Ski/. — The variations in Table 100 have been obtained 

 from the detailed tables for each year in the manner already described for the other meteorological variations. 

 The following are the epochs of the maxima and minima, and mean extent of clouded sky for each quarter and 

 for the year : — 



Winter, 

 Spring, 

 Summer, 

 Autumn, 



Nov., Dec, Jan., 

 Feb., March, April, 

 May, June, July, 

 Aug., Sept., Oct., 



Year, 



Maximum. 



Mean. 



Minimum. 



Mean. 



9h A.M.— 3^ P.M. 



6^ 35" A.M. 



lOh P.M. 



5*^ 25™ P.M. 



9^ A.M. Z^ P.M. 



6^ 10™ A.M. 



101> P.M. 



6h 10m P.M. 



Qh A.M. 



5h 40m A.M. 



121> P.M. 



4:^ 35™ P.M. 



1'' P.M. 



3^ 55™ A.M. 



81^ P.M. 



6^ 15™ P.M. 



111^ 15m A.M. 



5h 35™ A.M. 



IQh 15m P.M. 



5^^ 35™ P.M. 



law of the amount of rain agrees with that of the extent of clouded sky. There is no doubt, however, that the way in which cloud 

 is generated by the solar heat must be different from that in which it is generated by the lunar heat, the former is due chiefly to 

 heating at the base of the atmosphere, the latter to heating in the upper region; in any case, however, it does not seem evident, from 

 the above considerations, that the lunar heat should generate more cloud than it dissipates. 



I may remark, in addition to the above, that the relation of the amount of rain to the amount of cloud must be chiefly a relation 

 to certain kinds of cloud ; those formed and dissipated in moonshine are not rain-clouds at all. May it not be for this reason, the 

 conversion of a certain portion of aqueous vapour into clouds which are not rain-clouds that the least rain falls at full moon, while at 

 new moon the same aqueous vapour is probably deposited below as rain-cloud ? The cirri, the highest of all clouds, are, I am per- 

 suaded, clouds of crystallization ; are they the least frequent in moonlight ? does the moon heat not tend to dissipate them, and to 

 convert them into watery cirro-cumulo-stratus ? 



I am strongly of opinion that the efiTect of the lunar influences in the upper regions of our atmosphere is of much greater im- 

 portance than might seem at all probable : previous investigations have shown that the laws of magnetic disturbance vary more with 

 the positions and age of the moon than with any other argument, and this is especially obvious when we regard the diurnal oscilla- 

 tions. It has also been shewn from the Makerstoun Observations for 8 years, that the frequency of the aurora borealis is greatest 

 near full moon. Scattered throughout the Makerstoun Observations, there will be found frequent reference to remarkable opera- 

 tions occurring in the upper regions of the air near full moon, chiefly among the cirrus, cirro-cumulus, and cirro-cumulo-stratus ; 

 this frequency, it is believed, is not wholly due to the better opportunity of observing these processes by moonlight, although that 

 may be partially the case. 



1 have used throughout the term " extent of clouded sky," because although there is a considerable probability that during a 

 large series of observations the extent of sky clouded will be a measure of the amount of cloud, yet this is not absolutely certain. 



MAG. AND MET. OBS. 1845 AND 1846. 



2d 



