Page:Popular Science Monthly Volume 80.djvu/137

Rh, partly because they were made but twice a day, and partly because it is unlikely that the instrument was always well exposed. The highest temperature noted was 92° (July 31 and August 4, 4, 1805), and the lowest was –45° (sunrise December 17, 1804). Clearly the expedition passed through a country of large annual and diurnal ranges of temperature (east of the Rocky Mountains). The summer afternoon temperature rose to 70°–80°, and even 90°, while in winter the sunrise readings were as low as –20°, –30° and even –40°. The diurnal ranges are noted as having been extremely large among the mountains, and on one day a difference of 59° was noted between sunrise and 4 The prevailing summer type of weather was fine, warm or even hot days, with cooler evenings and nights; not infrequent thunder-storms, moderate to high winds, especially in the afternoons. The southerly winds are so often referred to that we have little hesitation in concluding that this is the prevailing direction in summer on the Great Plains, and we see at once, in our mind's eye, that great sweep of southerly and southeasterly winds, across the region west of the Mississippi River—the continental inflow of summer, in response to the pressure-gradient between the Gulf of Mexico and the interior of the continent. These winds are frequently described as of high velocity during the daytime, blowing the sand from sand-bars and river-banks. Within the past quarter-century these same winds have been harnessed for the service of man, and they are to-day driving hundreds of windmills on the Great Plains for pumping water for stock and for irrigation, for sawing wood and for grinding corn and wheat. Captain Lewis observed that "the winds blow with astonishing violence in this open country." We have learned since that their velocities are not only high, comparable with those along the seaboard, but that they are also very uniformly distributed through the year, and are "usable" for windmill purposes to a remarkable degree. But no more striking illustration of the wind velocities on the plains has ever been given than Captain Lewis's description of the occasion when one of his boats, which was being transported on wheels, was blown along by the wind, the boat's sails being set! Surely this account emphasizes the analogy between the winds of the ocean and the winds of the Plains. Both sweep over a surface of little friction. Both attain high velocities in consequence.

The frequent occurrence of rain in May and June emphasizes the season of maximum precipitation (the "Missouri Type" of Gen. A. W. Greely) which has since proved of such immense economic benefit