206 The North-West Lunar Limb. 



As in this region many instances occur of one of the pecu- 

 liarities of the lunar conformations — the principle of parallelism, 

 by which the details of whole districts are for the most part 

 arranged in lines lying in the same direction, a caution appro- 

 priately given by B. and M. ought to be taken into considera- 

 tion by the student. There are many cases in which an appear- 

 ance of this kind is merely an optical illusion. Towards the 

 limbs, the foreshortening of all objects in breadth, while their 

 length remains unchanged, must naturally produce this effect ; 

 and all over the disc ridges lying N. and S. are more visible, 

 and therefore apparently more numerous, than those running 

 E. and W., from their power of casting shadows along their 

 sides ; and hence again in the polar regions, where the direc- 

 tions of the foreshortening and of the illumination act in oppo- 

 sition to each other, there is much less perceptible parallelism 

 than near the equatorial limbs. But notwithstanding these 

 causes of illusion, some portions of the moon are characterized 

 by an appearance of this kind, the reality of which cannot be 

 denied ; and though of course it is more distinctly traceable, 

 and more free from the possibility of deception, in proportion 

 as we approach the centre of the disc, yet even in remoter 

 situations there are districts where its presence cannot be 

 overlooked. One of these we have now before us. B. and M. 

 refer to the meridian position of the ridges in the interior of 

 Gauss and Messala* (a large fiat ring-plain near Struve in the 

 direction of Geminus) as instances of it, and add, that ' ' though 

 the first glance for the above-mentioned reasons can determine 

 nothing, yet by a more accurate examination we find many 

 remarkable indications of such a bearing. Thus many of the 

 craters and ring-mountains to be found here are either quite 

 open or made more accessible by a saddle-like depression 

 exactly at the most northern or southern point, more usually 

 at the latter, sometimes at both. Most of them show espe- 

 cially at these points some kind of peculiarity — an encroaching 

 crater, a peak like a key-stone, a cleft, an excurrent ridge, a 

 central point of union for branching heights, etc., and if, be- 

 sides the optical ellipticity, a real one also occurs anywhere, 

 it always lies in this direction. A force therefore has here 

 operated under the surface of the moon in the line of the 

 meridian ; but did its action proceed from the equator towards 

 the pole, or the reverse ? Probably the former" (why, is not 



* As a specimen of the way in which B. and M. have occasionally treated the 

 observations ofSchroter, it may be remarked that in describing this spot they 

 have blamed him for his hasty and defective representation of it in his first 

 volume, without the slightest mention of the additional details contained in two 

 supplementary diagrams in his second volume, among which are included some 

 which they state are very difficult to be seen. ^ 



