(or Rain-band Spectroscope) in Meteorology. 129
Passing on now to the dry air-band at α, which, if intense,
prognosticates a fine and dry period, extending from a few
days to even weeks, Prof. Smyth chronicles an instance, in
vol. xiv. of the 'Edinburgh Astronomical Observations,'
where an unusual darkness of this band was accompanied by
very dry weather over the whole of Europe. On the other
hand, a very low percentage of it must not be taken as a sign
of rain: it simply denotes moisture, which may not be deposited
as rain.
Next in order is the principal rain-band at r, already partly
referred to at the commencement of this paper; this has had
more attention paid to it than any other part of the spectrum:
it is most conspicuous when well marked; and one can say
with absolute certainty, should 80 per cent, be shown, rain
will decidedly fall before long; and if 20 per cent, only be
visible, none will occur for at least six hours. At δ is a low
sun-band, v I have already remarked upon, E and b are
constant solar lines, which will be found useful in focussing
the spectroscope. At w is a moisture-band that is seldom
seen. The hydrogen line, f, is sometimes almost invisible
when the blue is very deep : this occurs more or less before
heavy rain ; but should it stand out clear and distinct in the
blue, which in such a case is lighter in tint, then fine weather
with or without wind will follow.
As regards the best hours for observing, 9 a.m. and 3 p.m.
will be found the most suitable. An observer when using the
spectroscope should be careful to shade the eyes in such a
way that all extraneous light may be excluded as much as
possible, and to adjust the focus and slit so that the lines
may be of the clearest definition. Elevate the instrument
about 13 degrees, and put down on a chart the percentage of
the principal rain-band for 8 points of the compass, then
direct it to the zenith; observe the other features in the
spectrum, especially any difference in tint of the colours :
this ought to be attended to first, as they will be found to
change somewhat after a time. Enter all these particulars
in a note-book, in order that they may be referred to at some
future time. By attending to these simple directions, with a