Page:Popular Science Monthly Volume 80.djvu/139

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VALUE OF WEATHER OBSERVATIONS

and Captain Lewis was "astonished that more have not suffered in a similar manner."

Our general understanding of the essential climatic characteristics of the country through which the expedition passed, already reasonably accurate although only in outline, becomes clearer as we pick out other details which are noted in the journals. "The air is remarkably dry and pure in this open country. . . . The atmosphere is more transparent than I ever observed it in any country through which I have passed," Captain Lewis says, thereby bringing out very clearly one of the great climatic advantages of the region. The rapid evaporation, which has its disadvantages as well as merits, was frequently observed. Thus (September 23, 1804) on one occasion "in 36 hours 2 spoonfuls of water evaporated in a saucer," and elsewhere in the "Journals" we note that the rapidity with which Captain Lewis's ink dried up was recorded as furnishing a striking illustration of the dryness of the air. Surely that gives us a hint as to what can be done by a traveler who is alive to what is going on around him. The difficulty of making any accurate estimate of distances in the air of the mountain country, so much drier and purer than that to which he had been accustomed, struck Captain Lewis forcibly. Similar difficulty has been experienced by many persons whose eyes have become trained to estimate distances in turbid air near sea-level, and find, on mountain tops, that their whole scale of distances must be revised in order to allow for the greater clearness of the mountain air. Although the winter was spent on the Pacific coast, there was no lack of opportunity to observe frost and cold on the Plains and northern plateaus. Frost we find recorded as "white," "hard," "very hard." The thickness of ice frozen in a day is often recorded. On October 18, 1804, we note that water in vessels exposed to the air was frozen, as was "the clay near the water edge." And on another occasion (April 15, 1805) "the earth at the depth of about 3 feet is not yet thawed, which we discover by the banks falling in and disclosing a strata of frozen earth." It was recorded that snow fell on the mountains while rain fell at lower levels—a common phenomenon resulting from the lower temperatures aloft. The occurrence of nocturnal radiation fogs; the prevalence of cold northwesterly winds in the colder months (as contrasted with the warm southerly and south-easterly winds of summer); the depth of snowfall; the appearance of auroras, and of haloes and other optical phenomena; the migrations of birds; the coming of rains with northwesterly winds (this being a combination which is not very common in the eastern United States, but occurs more frequently in the west)—these are a few of the many instructive observations which have been picked out in a rather haphazard way from the very rich harvest in the "Journals." The occur-