Effect of the EartlCs Light upon the Moon. 65 



9. On the Effect of the Earth* s Light upon the Moon. — It is 

 entirely to this terrestrial light reflected upon the moon that 

 the authors attribute, according to the explanation of Leonardo 

 da Vinci, which is generally admitted, the faint lustre which the 

 obscure part of the moon presents at the commencement and 

 termination of each of her periods, a lustre which renders the 

 whole disk visible, and which has been named the ash-coloured 

 light. The observations of Messrs Beer and Madler completely 

 confirm this explanation. The light, they say, diminishes so 

 soon as the phase of the earth notably diminishes in regard to the 

 moon, and disappears completely when the earth appears to the 

 moon like a small crescent. It should appear brighter in pro- 

 portion as the moon is near the sun, if the twilight of our atmo- 

 sphere had not a qualifying effect. A maximum of visibility 

 of the obscure portion of the moon results from a combination 

 of the two causes just alluded to ; and this occurs from two 

 and a half to three days before and after the new moon. The 

 visible spots exhibit precisely the same relative degree of clear- 

 ness as when they are illuminated by the sun during the full 

 moon. But, as our atmosphere is commonly not sufficiently 

 clear for observations of this kind, as they must be made near 

 the horizon, it follows, that the visibility and the glimmering 

 light of these spots are but rarely witnessed ; and this has led 

 to the assertion, that volcanos have sometimes been seen in the 

 moon, and that they are seen to burn in its dark portion. For 

 ourselves, we have never been satisfied that we have witnessed 

 any thing of this sort. It is common to speak of observations 

 of this kind having been made t)y Herschel, an account of which 

 may be found in the Philosophical Transactimis for 1783. By 



earth, or if the whole effect of the libration is only apparent. He likewise 

 demonstrates the utility of the knowledge of the moon's relief in the calcula- 

 tion by anticipation in solar eclipses and the oscillations of stars, of the 

 moon's profile, and the effects of her inequalities upon the period of the com- 

 mencement and the termination of these phenomena. In No. 338 of the same 

 Journal, the author gives the result of the obseiTations made during the 

 eclipse of October 13. 1837, upon sixteen spots, which have given him an 

 augmentation of the semi-diameter of the shadow equal to l-54th part. 



VOL. XXV. NO. XLIX. JULY 1838. E 



