Intelligence and Miscellaneous Article** 168 



just the same as Mr. Park Harrison, viz. a more elevated tempera- 

 ture about new moon ; this is also the opinion of the observers who 

 accompanied the late polar expeditions. 



The whole series gives, however, a sum in favour of the full 

 moon, just as it ought to be. 



You will agree with me that if the new moon really gives a more 

 elevated temperature at the surface of the earth than the full moon, 

 the question is still more interesting, because the opposite ought to be 

 the case, since the whole earth necessarily contains more heat about 

 the time of full moon than about the time of new moon. I have 

 thought, at the same time, whether the moon, when it has north 

 declination, imparts a more elevated temperature to our ther- 

 mometer than when it has south declination ; and I found it to be 

 so ; but the difference is equally variable and small. 



Now we all admit that other perturbations, a thousand times 

 as great, may totally eclipse for a considerable time the small influ- 

 ence of the moon ; but nevertheless it seems to me, when we have 

 135 years of continued observation, that those perturbations ought 

 to have annihilated themselves. 



Why is the mean result so small and uncertain, while we are so 

 certain of the heating action of the moon ? The explanation that 

 has occurred to me is this: — 



The moon acts partly directly, and by that part the thermometers 

 are most elevated the second or third day after full moon (the day 

 of the greatest north declination for our latitude) ; but it also acts 

 in some measure indirectly, by elevating the temperature of the 

 aqueous vapour in the atmosphere. By the latter action the clouds 

 and the air have a more elevated temperature, and contain more 

 heat, about and after full moon ; but some days must elapse before 

 those particles of air and cloud reach the surface of the earth. When 

 fourteen days elapse before they reach the surface, they have an 

 action wholly opposed to the direct action, and thus the two parts 

 of the action of the moon are necessarily neutralizing each other, 

 more or less. 



Hence it is that experiments at great elevations, such as those of 

 Prof. Piazzi Smyth at the Peak of Teneriffe, are of the greatest 

 value. It is one instance among many where they are indispensable. 



My letter is already too long to give you my numbers. I will 

 publish them, but would like very much to know your opinion before 

 doing so. Probably you have not time and leisure just now to give 

 your attention to this question. Perhaps you may think fit, how- 

 ever, to publish this letter of mine, with verbal correction, in the 

 Philosophical Magazine, together with a few remarks of your own. 



Excuse me for thus applying to you ; it is in order to fix your 

 attention on the subject xind to be instructed by your opinion. 



I am, 



Most respectfully yours, 



Buys Ballot. 



[In the autumn of 1861 I swept the heavens on several days and 



M2 



