724 Reviews. [Oct., 



which to illustrate other presumed analogous facts. The same may 

 be said with regard to the employment of the Nebular hypothesis, as 

 though, instead of being what it is, a highly probable supposition, it 

 were an established certainty. But the second kind of evidence, 

 that derived from observation, is the one on which the opinions of Mr. 

 Proctor ought to find their securest support, and yet here their most 

 serious difficulties are experienced. The author seems to feel this, 

 and labours earnestly to disprove the existence of a cloud-bearing at- 

 mosphere, the presence of which, if it could be established, would 

 satisfactorily account for many of the phenomena, and be contradictory 

 to none, and would therefore render his hypothesis unnecessary. Yet 

 it seems to us that, besides the phenomena mentioned, all of which 

 either support or are not directly antagonistic to the hypothesis, there 

 are others which, while they are readily explicable on the supposition 

 of an atmosphere enveloping the rings, will, on any other supposition, 

 remain strange. Such, for example, is the absence of any perceptible 

 shadow on the planet's disc when the plane of the ring passes through 

 the Sun. Now if the ring had even the least thickness ever ascribed 

 to it, viz. 40 miles by Bond, it would be sufficient to produce a total 

 eclipse of the Sun on Saturn's equator, if no qualifying agency operated, 

 as it would subtend an angle more than double that subtended by the 

 disc of the Sun as seen from the planet. As no such eclipse is pro- 

 duced, we must look for some cause competent to prevent it. Such a 

 cause is readily found in a somewhat dense atmosphere, which, if it 

 existed, would probably produce just the modification described, for the 

 light, by refraction through it on both surfaces of the rings, would 

 reduce the shadow to a slight and undistinguishable penumbra.* On 

 the same supposition also another phenomenon, viz. the visibility of the 

 ring system, when its unenlightened side is turned towards the Earth, 

 is readily accounted for, while on Mr. Proctor's hypothesis these seem 

 to us to meet with no explanation. Moreover, in support of his opinions, 

 the author considers the greater visibility of the dark ring in modern 

 times to be due not to improvements in the telescope, but to the fact 

 that its formation is only of recent date. Yet it seems to us by no 

 means certain that this ring was not observed nearly a century and a 

 half ago. In the number of the Koyal Society's Abstracts for April 6, 

 1720, the following notice appears of a paper read by Hadley : — 

 " Within the ring he discerned two belts, one of which crossed Saturn 

 close to its inner edge and seemed like the shade of the ring upon the 

 body of Saturn, but when he considered the situation of the Sun with 

 respect to the ring and Saturn, he found that the belt could not arise 

 from such a cause." And again, in a later notice, — " The dusky line 

 which in 1720 he observed to accompany the inner edge of the ring 

 across the disc continues close to the same, though the breadth of the 

 ellipse is considerably increased since that time." This was written 

 in 1722. 



In the chapter devoted to the discussion of Saturn's habitability 

 the reasoning appears somewhat deficient in force. After expressing 



*Vide a paper by Rev. R. Dawes, in 'Monthly Notices, Astr. Soc.,' vol. for 1861. 



