The universal geography : earth and its inhabitants

——^CLIMATE OF THE MISSISSIPPI BASIN. 253hundred feet. Even at El Paso the Rio Grande was completely dry for severalweeks in the year 18-31, though at other times it sends down a volume of manythousand cubic feet per second, flooding its valley with a copious stream.*III.Climate, Flora, and Fauna.A balance between the climate of the Atlantic slope and that of the NorthAmerican central basin is effected by insensible transitions. The trend of theAppalachian system, which is disposed parallel with the normal course of thewinds, facilitates the circulation of the aerial currents, which thus pass withoutabrupt deviation from one climatic zone to the other.In the Mississippi valley and along the eastern seaboard the oscillations oftemperature present analogous phenomena. Thus in both zones the deflections ofthe curves of equal temperature for winter and summer differ strikingly fromthe isothermal lines for the whole year. On both sides of the Alleghanies thesummers of the northern regions are warmer and the winters of the southernregions colder than are the same seasons respectively under corresponding isothermalsin regions such as France and the British Isles, which enjoy an insularclimate.In this respect the ccniral part of the United States presents equilibriaeven far superior to those of the Atlantic coastlands.tTowards the sources of the Mississippi, and on the Ileight of Land in Minnesotaand AVisconsin, the summer heats are greater than in the average of tropicalregions. Thus at Fort Snelling, near Saint Paul, records are reported of 118" Fahr.in the shade, which is scarcely infeiior to that of the Saharan furnace. On theother hand, the cold in these northern regions is often excessive, and far moreintense than that of Western Europe under the corresponding isothermals.According to Henry's researches the discrepancy shown between the real annualtemperatures and those yielded by the calculation based on the rotundity of theglobe, increases gradually in the direction from south to north. Near the mouthof the Iiio Grande under 20"^ north latitude the true and the calculated line coincide,whereas on the United States frontier towards Canada the abnormal lowering of themean temperature amounts to over 14 degrees Fahr. At the Mississippi-Missouriconfluence, which may be regarded as the meteorological centre at once of theMississippi basin and of the whole Union, the mean (53° Fahr.) of the annual• Lengrth of the Rio Grande, 1,800 miles; drainage area, l.)G,200 square miles; apprcximatodischarge per second, 26,400 cubic feet.t Mean and extreme temperatures in the central basin of the United States :MpanExtremM, -—i—Lat. temp. Sammer. Winter. Heat. Cold. Bange.Duluth (12 years) . . 4GM8' 40 F. 6.3' 10° 99° -38" 137°Saint Paul (12) . . .44°53' 44° 67° 9° 100° -39° 139°. . . 4r52' 49° 68° 22° 99° -l:i° 112°Chicago (12).Saint Louis (13) . . 38'37' 65' 74° 29° 106° —17° 123°Cairo (12) 37° 68' 76' 36° 103° —7° 110°Memphis (12) 3.5°8' 61° 78' 42° 98' +2° 96'. . .Virksburg (12) . . .32'24' e.'i' 81' S0° 101° + 10' 91°New Orleans (12) . . 29'o7' 69' 82° 58° 96' + I;".' 81'