. These streaks are again reinforced farther
out. These parallel rays must therefore not be considered as two streaks,
but as two series of streaks, the components of which are placed end to
end."

Thus, according to Professor Pickering, we must no longer regard the rays
emanating from the Tycho region and other centres as continuous, but as
consisting of a succession of short lengths, diminishing in brilliancy
but increasing in width, till they reach the next crater lying in their
direction, when they are reinforced; and the same process of gradual
diminution in brightness and reinforcement goes on from one end to the
other.

The following explanation is suggested to account for the origin of the
rays:--"The earth and her satellite may differ not so much as regards
volcanic action as in the densities of their atmospheres. Thus if the
craterlets on the rim of Tycho were constantly giving out large
quantities of gas or steam, which in other regions was being constantly
absorbed or condensed, we should have a wind uniformly blowing away from
that summit in all directions. Should other summits in its vicinity
occasionally give out gases, mixed with any fine white powder, such as
pumice, this powder would be carried away from Tycho, forming streaks."

The difficulty surrounding this very ingenious hypothesis is, that
though, assuming the existence of pumice-emitting craters and regions of
condensation, there might be a more or less lineal and streaky deposition
of this white material over large areas of the moon, why should this
deposit be so definitely arranged, and why should these active little
craters happen to lie on these particular lines?

The confused network of streaks round Copernicus seem to respond more
happily to the requirements of Professor Pickering's hypothesis, for here
there is an absence of that definiteness of direction so manifestly
displayed in the case of the Tycho rays, and we can well imagine that
with an area of condensation surrounding this magnificent object beyond
the limits of the streaks, and a number of active little craters on and
about its rim, the white material ejected might be drawn outwards in
every direction by wind currents, which possibly once existed, and,
settling down, assume forms such as we see.

Nasmyth's well-known hypothesis attributes the radiating streaks to
cracks in the lunar globe caused by the action of an upheaving force, and
accounts for their whiteness by the o