129 
{or Bain-band Spectroscope ) in Meteorology. 
Passing on now to the dry air-band at a, which, if intense, 
prognosticates a fine and dry period, extending from a few 
days to even weeks, Prof. Smyth chronicles an instance, in 
Yol. xiv. of the ‘Edinburgh Astronomical Observations,’ 
where an unusual darkness of this hand 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 £ is a low 
sun-band, v I have already remarked upon, e and b are 
constant solar lines, whicli 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 8 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 ram-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 
K 
