Note 38 — STEPPES AND DESERTS.

p. 19.—“The scene in the Steppe is suddenly changed.”

I have endeavoured to depict the coming in of the rainy season, and the signs by which it is announced. The usual deep dark azure of the sky in the tropics arises from the more complete solution of the vapour contained in the atmosphere. The cyanometer indicates a paler blue as soon as the vapours begin to be precipitated. The dark spot or patch in the constellation of the Southern Cross gradually becomes indistinct as the transparency of the atmosphere diminishes, and this alteration announces the near approach of rain. The brightness of the Magellanic clouds, (Nubecula major and minor), gradually vanishes in a similar manner. The fixed stars, which before shone like planets with a steady, tranquil, and not trembling light, now scintillate even in the zenith, where the vapours are least. (See Arago, in my Relation hist. T. i. p. 623). All these appearances are the results of the increased quantity of vapour diffused in the atmosphere.