VALUE OF WEATHER OBSERVATIONS 133 



pedition, 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 p.m., 

 1805), and the lowest was — 45° (sunrise December 17, 180 1 ) . Clearly 

 the expedition passed through a country of large annual and diurnal 

 ranges of temperature (east of the Eocky 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 r.]\r. The prevailing summer type of weather was fine, 

 warm or even hot days, witli 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 hesita- 

 tion 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 Eiver — 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 wind- 

 mills on the Great Plains for pumping water for stock and for irriga- 

 tion, 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 illus- 

 tration 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 Avheels, was blown along by the wind, the 

 boat's sails being set! Surely this account emphasizes the analogy be- 

 tween the winds of the ocean and the winds of the Plains. Both sweep 

 over a surface of little friction. Both attain high velocities in conse- 

 quence. 



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 



* It should be noted, however, that at the beginning of the table of observa- 

 tions, where the first data are given, for "Duboes, " January 1, 1804, it is 

 stated: "Thermometer on the north side of a tree in the woods." This surely 

 indicates careful attention to exposure, when possible. 



