st Comes i eae: aera 
ie ~ ee ete 
= 
23 
Oe cee ts ee ee, ee SP a BER Sr SE et a a a a a i ea 
a ‘ : Spec e 
S ete Se = 3 ee ay Bae 
Astronomy. 433 
time required by the moon to move oyera space equal to its diameter, 
and we have but to compare with this, the time of the star’s disappear- 
ance. This observation has been repeated a thousand times, and the 
result is, that the two intervals are almost identically the same. Fora 
_ long time it was considered doubtful whether there was any apprecia- 
ble difference ; but astronomers are now generally inclined to the opin- 
ion that there is a difference of a few seconds, This would indicate 
that the moon’s atmosphere does refract light ; but the effect is exceed- 
ingly small. The refraction produced by the earth’s atmosphere is 
More than a thousand times greater than that of the moon. The pres- 
Sure of the moon’s atmosphere would be balanced by a column of mer- 
cury one forty-fifth (1) part of an inch in height. The best French 
air-pumps are warranted to rarefy air to one twenty-fifth (4) part of 
 aninch of mercury. * * If we use language with the utmost pre- 
cision, we must say the moon has an atmosphere; but to avoid being 
misunderstood, we should add, that it is more rare than any we can 
Produce with our best air-pumps. 
2. On the Projection of a Star on the Dark Limb of the Moon just 
before its Occultation ; by Prof. Stevetty, (Rep. Brit. Assoc., 1845, p. 
5.)—This the Professor considered to be a result of diffraction. Sir 
Isaac Newton having observed the shadow of a hair placed in a strong 
_ beam of sunlight to be broader than the hair itself, was led to investi- 
gate the course of a ray as it passed by the edge of a body, like the 
edge of a knife placed across a hole in the. window-shutter, through 
r Which a sunbeam is admitted. Beyond a certain distance the rays pro- 
ceeded in their usual straight courses; at that distance they were bent 
towards the edge ; but the courses of the nearest rays were bent away 
- from the edge, so as to form curves conyex towards it. The undulato- 
ry theory enables us to trace these curves, and they are known to be of 
‘the nature of the hyperbola, with asymptotic branches extending on- 
Wards from the diffracting edge. Prof. Stevelly conceived the dark 
| _ limb of the moon to be such a diffracting edge to the slender beam of 
_ light which reached us from a fixed star; and that as the curve was, at 
the last moment the light was allowed to pass, convex towards the 
; Moon, the portion of the ray which last enters our eye before the star 
disappears, being the direction in which we should then see the star, if 
: : Produced backwards, would meet the moon om her dark surface. 
8. The Central Sun of the Universe.—Prof. Madler of Dorpat has 
_ §0nounced that he has discovered the central sun about which our sun 
With its attendant planets performs its circuit. His conclusions are de- 
: ed from a comparison of catalogues of stars since the time of Brad- 
, “Srcoxp Sznres, Vol. II, No. 6.—Nov., 1846. 56 
