Page:Popular Science Monthly Volume 2.djvu/162

 halos are white, we may conclude that the particles are frozen, and expect cold weather. Crossed halos, mock moons, or highly-developed phenomena, indicate larger crystals of ice, and probably frost, hail, snow, or heavy rain, after three or four days, according to the season of the year. Similarly the laws of reflection of light indicate that the cause of a deep-purple morning or evening sky is the large amount of moisture present in the atmosphere. Another effect of the moon, when at the full, is to clear the sky of cloud, traceable, says Sir John Herschel, to a distinct physical cause, the warmth radiated from its highly-heated surface; though, why the effect should not continue for several nights after the full, remains, in the opinion of the same accurate observer, problematic. Other lunar prognostics, founded on arbitrary rules, as to the time of the day or night at which the changes or quarterings take place, are worse than useless, for they are calculated to mislead, and are generally included in almanacs or note-books intended for sale only, being in some cases attributed to an eminent meteorologist or astronomer—Sir W. Herschel or others.

It is of course far from our purpose to enter here into a disquisition on the theory of the trade and anti-trade winds, and their barometrical indications—subjects that can be usefully discussed only in a treatise on meteorology: we limit ourselves to the present position of weather prognostics, although it must be admitted that any advance yet made or likely to be made in prognosticating the weather arises from the study of such recurrent phenomena, the investigation being much aided by the highly-developed character of the laws of the expansion of gases, upon which laws the theory of the wind is founded. Thus we know that a rise in the barometer, together with a fall in temperature, as shown by the thermometer, indicates the approach of a cold, northerly current of air; while a fall of the mercury in the barometer, with a rise of that in the thermometer, indicates that a southerly or warm air-current is on its way. Northerly currents may include winds from the northwest and northeast, as well as from the north; similarly, southerly currents may include winds from the south, southeast, or southwest. When the barometer rises while a northeast wind is blowing, with prevalent hail, rain, or snow, there may be no change. Of barometrical indications alone, it is generally known that a rapid rise portends changeable weather; a slow rise, the contrary; a rapid fall, heavy wind, rain, and snow; while a fluctuating height of the column of mercury indicates unsteady weather. With a heavy gale of wind in the east or southeast, changing south, the barometrical column may fall until the wind shifts its quarter. Upon such observations did Admiral Fitzroy base his code of instructions, now to be found by the side of every barometer, his forecasts depending on the indications of the barometer and thermometer, with observations as to the direction and force of the wind with regard to time and place, and its previous course taken altogether. These indications are thus not