26 Deceptive Figures. 



the apparent diameter would ee most probably not be sensibly 

 increased •" in tlie second, " the increase would be the angle 

 which the whole height of the atmosphere subtends at the 

 earth." Now it is clearly possible that the atmosphere of a 

 planet might in general exhibit one of these relations, but for 

 a season might present the other, either over the whole planet 

 or in certain latitudes only; and we have distinct evidence 

 that Saturn's atmosphere is of variable refractive power, for 

 whereas in nearly all pictures of Saturn, and notably in Mr. 

 De la Rue's admirable engravings, the disc is darker at the 

 edges than at the centre, so that this may be supposed to be 

 the general appearance of the planet, the contrary appearance 

 was presented in one instance to Bond II., and in another to 

 Chacornac. Now it is perfectly clear, that if for a season the 

 atmosphere over certain latitudes assumed the second state 

 described by Professor Challis, while the remaining part was 

 in the former — which we may perhaps be justified in calling 

 the normal state — an apparent irregularity of figure would 

 result. The outline of the disk would correspond in the 

 former latitude to the upper limits of the atmosphere, iu 

 the latter to the limit of the interior globe; and we may 

 suppose that in the intermediate latitudes the outline would 

 pass from one limit to the other by indefinite gradations. 

 There are reasons also for supposing that the "reflective 

 state" would be more commonly assumed in Saturn's tem- 

 perate zones than near his equator or poles. For the causes to 

 which our trade-winds are due are exaggerated in the case of 

 Saturn. Now Sir J. Herschel thus describes the circulation of 

 our atmosphere : — ' ' In each hemisphere inferior currents of 

 air run in on both sides towards the equator, and superior 

 ones set outwards, all around the globe, from the equator 

 towards the poles. It is quite clear that neither near the 

 poles nor near the equator, from or towards which these 

 opposite currents tend, is it likely that there will be that ten- 

 dency to stratification of the atmosphere into layers of variable 

 density which favours the "reflective state." On the other 

 hand, this state may be expected to occur — not commonly, 

 indeed, but occasionally — in the temperate zones, where these 

 currents attain their greatest velocity and steadiness of motion. 

 It need scarcely be remarked, that an apparent difference 

 of level of 500 or 600 miles in latitude 40° or 45°, would be 

 fully sufficient to account for Saturn's " square-shouldered" 

 figure; but there are reasons for supposing that the height of 

 Saturn's atmosphere in these latitudes exceeds the mean 

 height. For it is found that barometric pressure attains its 

 greatest value in the temperate zones ; and although this phe- 

 nomenon has never been fully accounted for, it appears highly 



