Page:Popular Science Monthly Volume 22.djvu/254

242 or absence of rain for very want of material to make it; and the band has, therefore, been called, shortly, "the rain-band." Thus, also, "rain band spectroscopes" have been specially constructed by several most expert opticians in size so small as to be carriable in the waistcoat pocket, but so powerful and true that a glance of two seconds' duration through one of them suffices to tell an experienced observer the general condition of the whole atmosphere. Especially, too, of the upper parts of it, where any changes—as they take place there almost invariably earlier than below—enable such an observer to favor his friends around him with a prevision of what they are likely soon to experience.

As an example of what may be done, and done easily, after a certain amount of experience and understanding of the subject has been acquired, I append, from a lady's meteorological journal, her notes of the mean temperature of the air and the intensity of the rain-band for each of the first fifteen days of the present month, and in a final column have entered the amount of rain-fall measured at the Royal Observatory, Edinburgh, on each of those days. The darker the rain band the larger is the figure set down for it, and it will be seen pretty plainly, on running the eye down that column and the next one, that with an intensity of either or 1 no rain follows, or, we might almost say, can follow; but with an intensity of 2 rain-fall begins, and with 3 it may be very heavy. All these rain-band notes have been made with a spectroscope no larger than one's little finger, purchased some six years ago, and taken on many voyages and travels since then:

But, if so much can be done by so small a spectroscope, the question may be well asked whether more still might not be accomplished with a bigger and more powerful one, especially seeing that the dispersive powers of both chemical and astronomical spectroscopes have