134 THE POPULAR SCIENCE MONTHLY 



over this great region, for on the whole the most rain falls when it is 

 most needed for agriculture. A different seasonal distribution of pre- 

 cipitation would banish agriculture from thousands of acres of land 

 which are to-day giving our farmers good crop returns. The fact did 

 not escape the watchful eye of Captain Lewis that the rainfall of the 

 warmer months over the region which he crossed is essentially spas- 

 modic and " patchy " in character, i. e., is of the shower or thunder- 

 storm type, as contrasted with the more general and widespread rains 

 and clouds of the large storms which characterize the winter months 

 over the country as a whole. It is this very peculiarity of " patchiness " 

 of the warm-season rains which renders them disappointing to the 

 farmers whose crops are suffering from drought. A half-hour shower, 

 covering perhaps a very small portion of a state, is a terribly exaspera- 

 ting occurrence to those whose lands are " scream^ing for water," but 

 are outside of the limited area covered by the rain. Further, the fact 

 tbat there is " very little rain or snow either winter or summer," is a 

 sufficient emphasis on the general decrease in the rainfall to the west 

 of the Mississippi Eiver, which is so marked a feature on our mean 

 annual rainfall maps. Captain Lewis paid particular attention to 

 thunderstorms, in which, probably because of their violence, he seems 

 to have been much interested. On April 1, 1805, he wrote, " I have 

 observed that all thunder clouds in the western part of the continent 

 proceed from the westerly quarter, as they do in the Atlantic states." 

 This is perhaps the first specific mention of this important meteorolog- 

 ical fact. On May 18, 1805, the record states: ''We have had scarcely 

 any thunder and lightning; the clouds are generally white, and ac- 

 companied with wind only." This we may take to indicate that the 

 season of maximum thunderstorm activity had not begun, the clouds 

 were doubtless our typical summer cumulus clouds, which, being best 

 developed when the wind is strongest, i. e., in the warmer hours, are 

 often called " wind clouds." A thunderstorm which occurred on June 

 27, 1805, receives special mention. This storm lasted two hours and a 

 half, and was accompanied by hail about the size of pigeons' eggs, which 

 covered the ground to the depth of 1^ inches. Some of the hail-stones 

 rebounded from the ground to a height of 10 or 12 feet. Several of the 

 men were knocked down and bruised; some got under the canoe for 

 protection, and others covered their heads. One hail-stone weighed 

 3 ounces and measured 7 inches in circumference. The stones were 

 generally round, and perfectly solid. Captain Lewis adds : " I am con- 

 vinced that if one of these had struck a man on his naked head it 

 would certainly have fractured his skull." On July 6, 1805, another 

 thunderstorm brought hail which covered the ground and was near the 

 size of musket balls. One blackbird was seen to be killed by the hail, 



