328 PHYSICAL GEOGRAPHY. 



to vary from fifty to sixty per cent, of the total rainfall. From England 

 and Ireland the amount of evaporation is estimated as two thirds ; from 

 the Ohio and upper Mississippi river basins, three fourths ; from the basins 

 of the Missouri, Arkansas, and Red rivers, five sixths of the whole rain- 

 fall.* The larger proportion west of the Mississippi is due to the preva- 

 lence of winds already reduced to dryness by passage over mountain 

 ranges, and still more inclined by the increased temperature of this 

 region to absorb rather than impart moisture. In the Ohio and upper 

 Mississippi basins the mean temperature is higher than with us, and the 

 air more drying throughout the year ; the surface, also, is far less favora- 

 ble for drainage than in our own state. In a comparison with the British 

 Isles, it is to be noted that our mean annual temperature is several 

 degrees lower than theirs, our winters sealed by frost against evaporation 

 while theirs are open, and our surfaces to a much larger extent forest 

 clad, producing probably with us a considerably lower proportion of 

 evaporation. 



The tendency to evaporation is of course less in proportion as the 

 winds which reach our area are charged with moisture. Our position on 

 the coast is favorable in this regard. It is not, however, the actual 

 amount of moisture which air contains that renders it moist, but the rela- 

 tion between that amount and the amount which is capable of being 

 held in suspension. Although the south-west storms which sweep over 

 us, laden with the evaporation of the Mexican gulf, have become de- 

 prived of much of their moisture before reaching us, they are still as 

 thoroughly saturated as at their start, having become cooled, and conse- 

 quently capable of holding less moisture in the same proportion as they 

 have given it up. As we have seen, this cooling process becomes more 

 rapid as these winds reach us, on account of our unusually low tempera- 

 ture, giving them great relative humidity, with the results of increased 

 rainfall and lessened evaporation. Thus the effect of our temperature is 

 to give us relatively moist instead of drying winds, making the loss by 

 evaporation from the surface a comparatively low proportion. 



The average summer temperature of New Hampshire is several degrees 

 lower than that of places of the same latitude farther west, while a still 

 greater difference is exhibited in a comparison with the middle and 



* Wells's Water Power of Maim, 1869, p. 52. 



