ssibly the deepest of the large lunar depressions.

"It is about 280 miles long from north to south, and 355 miles wide from
east to west, but, owing to its position, the width is seen from the
earth very much foreshortened, so that it really looks nearly twice as
long as it is wide. It contains an area of about 75,000 square miles,
thus being as large as the combined area of Scotland and Ireland, and
the five largest northern counties of England. It is surrounded by
mountains, some being over 11,000 feet high, reckoning from the dark
floor."

I drew their attention to Proclus--a ring-mountain on the eastern side
of this sea--which is about eighteen miles in diameter, and the second
brightest of the lunar formations. "From its neighbourhood several
bright streaks diverge in different directions, two extending a long way
across the dark area, and there is a longer one striking towards the
north and another towards the south at an angle of about 120 degrees
with each other.

"Seen through the telescope, these ray-streaks often appear very
brilliant under a high sun, looking in fact very like electric
search-lights; though I notice that the Rev. T.W. Webb has rather
curiously remarked that these particular streaks are not very easily
seen. Similar ray-streaks, many enormously longer than these, are found
in various parts of the lunar surface, but their exact nature and origin
has never yet been definitely settled. They only come into view when the
sun is beginning to be high up in the lunar sky, and the higher the sun,
the brighter the rays appear. Some of the shorter ones are ridges, but
this is evidently not the case with the others, for they cast no
shadows, as ridges would when the sun is low. Very many radiate from a
large ring-mountain called Tycho, in the southern hemisphere; and one of
them extends, with some breaks, nearly three thousand miles, passing
northward over the Sea of Serenity and finally disappearing on the
moon's north-western edge, or 'limb,' as it is termed.

"Professor Pickering assumes that these rays were caused by volcanic
dust or other light reflecting material emitted from a series of small
craters, and states that they are really made up of a series of short
rays placed or joined end to end. What I have observed myself seems to
bear out this latter statement; but the opinion I have formed as to
their origin differs from the theory of Professor Pickering. It seems to
me more probable that t