I have said that the barometer has no fair weather standard—the mean of 30 inches at the level of the sea being an average of the fair weather elevations and the foul weather depressions. Its fair weather position, it would seem, must be above the mean, therefore, and as much above as its foul weather depressions are below. But this is not precisely true. Its extreme fair weather range is 31 inches, and it rarely reaches that; while its lowest storm range is down to 28, and is the most often reached of the two. My barometer stands about 40 feet above ordinary high-water mark. It is not a “wheel,” but an open, “scale” barometer, and a perfectly good one. Its most reliable fair weather standard is about 3030⁄100 inches. It is its most common summer, set fair position, but that position is often at other and different elevations, at other periods of the year, during fair weather. The reader must observe for his own locality, and satisfy himself what the most common set fair position for the barometer is, at the different periods of the year, where he resides. When he has ascertained this, he may apply the following principles to illustrate its exceptional action, and in judging of the future of the weather:
1st. As to its rise before storms.—Supposing it to have been stationary, at or about a set fair position, for the period, and for one or two or more days, a very gradual and moderate rise is an indication of continued fair weather; and a sudden and considerable rise is indicative of a storm. If the sudden and considerable rise occurs in the latter part of spring, summer, or early autumn, it indicates a storm of the first or third classes described in Chapter X., if in winter, a storm of the first class only. If the elevation is very sudden and considerable, the storm will probably be severe. The philosophy of this, according to my present apprehension of it, is, that these storms present an extended easterly front—settle very near the earth—and have a rapid progress—thus accumulating the atmosphere somewhat, in advance of them.
2d. As to its fall before storms without previous rise.—This is always very regular before the second class of storms, or polar belts of showers and storms. It is very fairly exemplified in the table from Reid, on page 329. The barometer, so far as I have opportunity to observe, does not rise from a stationary position on the approach of this class of storms. At the commencement of heated, summer, dry terms, my barometer has most frequently ranged at about 30.30, and gradually, but slowly, fallen below 30 inches before the belt of showers arrived, and the term closed. The fourth rule of Dalton (Meteorology, page 183) indicates a similar law in England. It is as follows:
“In summer, after a long continuance of fair weather, with the barometer high, it generally falls gradually, and for one, two, or more days, before there is much appearance of rain. If the fall be sudden and great for the season, it will probably be followed by thunder.”
3d. It falls frequently and considerably without rain.—This is owing to the fact that all regular, periodic efforts at condensation do not result in rain. The second, third, and fourth classes of storms described, may not (as we have said) be sufficiently active to precipitate, although the series of phenomena (including the fall of the barometer) may be, in other respects, perfect. Such an instance may be found in Reid’s table, on page 329, and on the 11th of the month. But the fall in such cases is not as great, unless the wind be violent.
4th. It rises during considerable gales.—But these are of the kind so often alluded to—viz., the N. W., in the northern hemisphere, and the S. W., in the southern; and the philosophy of it has been explained, and is observable.
With these explanations, the reader will be able to understand, and practically apply, the barometric changes, in connection with the other phenomena, in forming an opinion of the weather.
The thermometer is also an auxiliary. It rises, during the winter half of the year, in the advance portion of the storm, and falls when it passes off again; and the reverse is true, as we have seen, when its range is very high in summer. It is, therefore, to some extent, a useful auxiliary, although of minor importance.
The hygrometer is of less importance still. It is not in general use as a practical guide to the changes of the weather, and does not deserve to be.
A question, which has been much mooted, deserves a passing notice in this connection—viz., whether our climate has gradually become ameliorated and milder on the eastern part of our continent, since its settlement. I have not space left for its discussion. Humboldt (Aspects of Nature, page 103) is of opinion that there has been no material change. He says: