128 
The Use of the Hygro-Spectroscope 
the evening, about 7 o’clock, the storm did come, much to 
the astonishment of a great many, including the friends with 
whom I was staying. 
It must be remembered that the spectroscope is quite 
independent of any threatening appearance of clouds, or the 
opposite; frequently in the former case it will tell you—no 
precipitation will occur ; and in the latter we must expect 
some before long. I will now advert to the dry and moist 
air-bands, and principal constant solar lines, the position of 
which I have delineated on the chart of the spectrum (Fig. 8). 
Fig. 3. — In the above chart the relative positions of the permanent 
lines and bands of the spectrum are represented, but not their actual 
appearances as seen in a spectroscope. ( From want of space the chart 
has been cut in two , and the second half placed below the first). The 
principal rain-band is figured by means of the two lines to the red side 
of D ; with greater dispersion these are split up into numerous lines of 
varying intensities. To the blue side of D is shown the position of the 
line that is unusually prominent and defined previous to the falling of 
snow, when this part of the spectrum is partly, if not wholly, cleared 
from vapour-lines. The instrument used in the construction of this 
diagram was a Grace’s spectroscope, with attached micrometer. 
Commencing at the red end we have first a ; in spectroscopes 
of moderate dispersion this permanent line is seldom seen, 
unless the spectrum at this part is unusually clear ; meteoro¬ 
logically speaking, I have not found it of much value. 
At a is the position of a moist air- or rain-band, and at b a 
dry air-band; these three, a, a, and b, are best studied by 
looking direct at the sun. The band at c is probably due to 
water vapour. At c' is a small rain-band. 
