lid form. They
consist, in fact, of tiny crystals of ice, not of little drops of
water. If the precipitation has been hasty, the crystals will, though
all small, be of many sizes jumbled together, and in that case the
beautiful optical phenomenon with which we are now dealing will not
occur. But if the opposite conditions prevail (which they do on rare
occasions), if the vapor had been evenly distributed, and if the
precipitation took place slowly, then will the crystals in any one
neighborhood be little ice crystals of nearly the same form and size,
and from one neighborhood to another they will differ chiefly in
number and size, owing to the process having gone on longer or taken
place somewhat faster, or through a greater depth, in some
neighborhoods than others. This will give rise to the patched
appearance of the clouds which prevails when this phenomenon presents
itself. It also causes the tiny crystals, of which the cloud consists,
to grow larger in some places than others.

Captain Scoresby, in his "Account of the Arctic Regions," gives the
best description of snow crystals formed at low temperatures with
which I am acquainted. From his observations it appears--(a) that
when formed at temperatures several degrees below the freezing point,
the crystals, whether simple or compound, are nearly all of
symmetrical forms; (b) that thin tabular crystals are extremely
numerous, consisting either of simple transverse slices of the
fundamental hexagon or, more frequently, of aggregations of these
attached edgewise and lying in one plane; and (c) that, according as
atmospheric conditions vary, one form of crystal or another largely
preponderates. A fuller account of these most significant observations
is given in the appendix to this paper.

Let us then consider the crystals in any one neighborhood in the sky,
where the conditions that prevail are such as to produce lamellar
crystals of nearly the same thickness. The tabular plates are
subsiding through the atmosphere--in fact, falling toward the earth.
And although their descent is very slow, owing to their minute size,
the resistance of the air will act upon them as it does upon a falling
feather. It will cause them, if disturbed, to oscillate before they
settle into that horizontal position which flat plates finally assume
when falling through quiescent air. We shall presently consider what
the conditions must be, in order that the crystals may be liable to be
now an