74
frequently continue their course with an apparent disregardof all changes of level. Their origin may perhaps be moreplausibly attributed to the cracking of a shrinking surface,than to any other known cause. About 1,000 of them havebeen observed, some being as much as 200 or 300 miles inlength.
When the Moon is full, or nearly so, and its aspect isin many other respects comparatively uninteresting, severalsystems of long rays, or light streaks, are seen to originatein certain districts, or mountain formations. Some of themare 600 or 700 miles long, and one appears to extendto a distance of at least 2,000 miles, which is considerablymore than |-th of the Moon ’s equatoreal circumference. Themost noticeable are those which radiate from the mountainTycho; the position of which we have pointed out in ourdescription of Mr. llutherfurd’s photograph. There are, how-ever, others which are very remarkable. They seem to be ofsuch a nature that, when the sunlight is shining fully uponthem, they reflect more of it than the surrounding soil or rock,and consequently appear exceptionally bright. They pass on-wards, like many of the clefts, or rills, up hill and down valley,over crater, ridge, and mountain; evidently showing that theirorigin has, in some cases, been subsequent to that of theother formations which they thus traverse.
The hypothesis has been suggested that they arise from theAiling up, by the exudation of volcanic matter from depthsbelow, of cracks radiating from the site of some vast explosion.When, however, we remember the very great distances to whichthey extend, it is hard to imagine how any catastrophesufficiently terrible to have produced them can have occurred.Nor do we see other effects remaining which such a suppositionwould lead us to expect. No really satisfactory theory of theirorigin has yet been put forward. We can only say that the way inwhich they radiate from various centres is strongly suggestiveof an explosive cause.
One of the most puzzling problems of the lunar surface is toexplain the origin of such vast circular , or approximately circular,formations as we have previously described. Nothing at all