Page:Popular Science Monthly Volume 16.djvu/330

310 and the pressure ranging from 29*90 to 29*70 inches. It has long been known that such a "barometric trough," or stationary depression, forms over the northwestern portion of this continent every summer, when the soil is highly heated by the sun, and the air-strata above it become highly rarefied by terrestrial radiation; Mr. Buchan, on his isobaric charts, assigns to it in July a mean pressure of 29*80 to 29*70 inches. Now, by "the law of the winds," the effect of this barometric depression would be to set up an indraught, somewhat resembling that caused by an ordinary cyclone, around whose center, in our hemisphere, the air draws from right to left, and moves on all sides toward the vacuum. Necessarily, therefore, toward the central belt of this vast continental depression (which during last August covered the interior of our continent up to the Arctic Circle), as into an aerial hollow, the air would flow from the surrounding regions of high pressure, which in that month always lie to the southward in the Gulf of Mexico, and in the extratropical parts of the Pacific. Could the Signal-Service barometric observations have been supplemented in August by like simultaneous observations taken in central British America, so as to determine the extent and intensity of the low pressure there, the anomalous autumn of 1879 could have been in no small measure foreseen and forannounced. A "warm wave" was then rolling northward and likely to continue. Could the international system of reports have been extended to the upper valley of the Saskatchewan before this enormous barometric anomaly developed, the prevailing weather of last September and October could have been then measurably foreshadowed, with almost as much certainty as, when a "cold wave" from the north is moving over the Lakes any day in January, the Signal-Office indicates "cold weather" for the interior of the country.

But enough has been said on this part of our subject. The necessity of studying the atmosphere as a unit need not be further pressed; for, as Dove forcibly said, "it is, as its name shows, a great steam-apparatus, whose reservoir is the ocean, its furnace the sun, and whose condensing vessels are the higher geographical latitudes," so that only when viewed as a whole can its operations be clearly understood. The simultaneousness of the present international system insures the accuracy of the results that may be deduced. And the international chart acts as a sort of lens by means of which the scattered rays of meteorological light are concentrated in a focus upon the dark points of the science. It is but just to add that the credit of originating, organizing, and elaborating this simultaneous system, both of the ocean meteorology and that of the land, belongs to General Myer, who, in the execution of his plans, has been seconded by the indefatigable labors of his assistants in the Washington Weather Bureau, and sustained by the energetic cooperation of all foreign weather-services.

The extension of this research can not fail to afford the diligent investigator a magnificent view of the complicate and exquisite