CVl 



Geneeal Results of the Makeestoun Obseevations. 



of the minimum in the diurnal variation ; they are also the night-hours during which the effect of the full moon 

 must be greatest : upon the whole, this mode of determining the fact, from a short series of observations, seems 

 open to the fewest objections. The following Table contains the means for groups of 3 or 4 days. 



Table 99. — Variations of the Extent of Clouded Sky for the Six Observation Hours 9*" p.m. to 2^ a.m. 

 with reference to the Moon's Age and Declination for the Years 1844-5. 











After 









Moon's 









Moon 









Age. 



1844. 



1845. 



Mean. 



farthest 

 North. 



1844. 



1845. 



Mean. 



d. d. 









d. d. 









14—16 



+ 0-16 



+ 0-55 



+ 0-35 



27— 1 



+ 0-63 



-0-37 



+ 013 



17—20 



+ 0-58 



+ 0-59 



+ 0-58 



2— 5 



+ 0-21 



-0-56 



-017 



21 — 24 



-0-33 



-0-51 



-0-42 



6— 8 



-003 



-014 



-008 



25—28 



-0-33 



-0-51 



-0-42 



9—12 



-0-28 



-012 



-0-20 



29 1 



-0-29 



-0-39 



-0-34 



13—15 



+ 0-44 



+ 0-42 



+ 0-43 



2— 5 



+ 0-22 



-0-62 



-0-20 



16—19 



-0-23 



-006 



-015 



6— 9 



+ 0-51 



+ 002 



+ 0-26 



20—22 



-0-85 



+ 0-80 



-002 



10—13 



-0-52 



+ 0-76 



+ 012 



23—26 



+ 0-12 



+ 0-01 



+ 006 



227- The values for each year indicate that the extent of sky clouded was greatest about full moon, and least 

 about new moon ; this is shewn with greatest distinctness in the means for 1845. We obtain the same result 

 if we take from Table XXXIX., p. 66, the means for the 15 days with full moon in the middle, and for 15 days 

 with new moon in the middle ; these are, for 1844, 672 and 637 ; for 1845, 7'10 and 6-23 respectively. 



For the vears 1844 and 1845 [ *^^ °'^''" ^^ ^^^^ ''^°''* ^""^^ ™°'''' = ^'^^ 

 •' \^ the mean 15 days about new moon = 6-30. 



It may be a question still, how far error of estimating the extent of clouded sky in the presence and in the 

 absence of the moon may enter into the production of this result. It is conceived that the effect of error in 

 estimation must be nearly constant : in dark nights the extent of clouded sky was estimated by the space shew- 

 ing clear stars ; and it is not improbable that the extent of cloud might be rather over than under estimated 

 during the absence of moon-light ; an error which could only have diminished the distinctness of the result 

 obtained. Before we refer the result to the heating effect of the moon, there are other co-ordinate facts to be 

 considered with reference to the motion of the atmosphere. (See No. 200.) We may inquire, however, how 

 fiir it agrees with the heating effect of the sun, thus ; — the extent of clouded sky appears greatest in summer, 

 and least in winter, it appears greatest near noon, and least near midnight ; apparently, therefore, the heating 

 effect is to increase the amount of cloud, and, by analogy, we should have the greatest amount of cloud about 

 full moon* 



* Since the previous investigation was performed, I have met with a passage in Sir John Herschel's very excellent " Outlines 

 " of Astronomy," page 261, in which he supposes that the lunar heat is extinguished in the upper regions of the atmosphere ; and adds, 

 " Some probability is given to this by the tendency to disappearance of clouds under the fall moon, a meteorological fact (for as such 

 " we think it fully entitled to rank) for which it is necessary to seek a cause, and for which no other rational explanation seems to 

 " oiFer." He adds as a note to the parenthesis, — " From my ovrn observation, made quite independently of any knowledge of such 

 " tendency having been observed by others. Humboldt, hovrever, in his personal narrative, speaks of it as well known to the pilots 

 " and seamen of Spanish America (H)." 



Sir John's observations were probably purely qualitative not quantitative. I have much diflSculty in making any objection to 

 the conclusions of so accurate an observer, at the same time if his observations were not of comparative measurement, I must point 

 to the previous conclusions from two years' estimations, and add my own qualitative observation for a considerable period, that the 

 clouds are 'both formed and dissipated under the influence of full moon, and that they are chiefly cirro-cumuli, or of that kind which 

 I have termed cirro-cumulo-stratus (See No. 217), noticed frequently during the existence of the aixrora borealis as the growing and 

 dissipating cloud. Whether the resultant is an excess or defect of cloud during full moon, as compared with other periods, I have 

 no impression, and think it extremely difficult to have any. Sir John refers, in an addendum, page xv. of his " Outlines," to what 

 he conceives a fact confirmatory of his conclusion, thus : — " M. Arago has shown, from a comparison of rain registered, as having 

 " fallen during a long period, that a slight preponderance in respect of quantity falls near new moon over that which falls near the 

 '•' full. This would be a natural and necessary consequence of the preponderance of cloudless sky about the full, and forms, therefore, 

 " part and parcel of the same meteorological fact." It will be seen, No. 235, that this result has also been obtained from the 

 Makerstoun Observations, but it may still be a question whether it is confirmatory of Sir John's conclusion. When we compare the 

 annual extent of sky clouded with the annual fall of rain, we do not find any direct connection : I do not know whether the diurnal 



