326 PHYSICAL GEOGRAPHY. 



the proportion of reservoir water used by large manufactories is in many 

 cases eighty or ninety per cent, of the whole amount during the drier 

 season of the year. It is easy to see, therefore, how great an advantage 

 our water-power has in comparison with any which is obliged to rely 

 upon the natural flow of the streams, or, if supplied with reservoirs at all, 

 has to depend on artificial constructions of comparatively great cost and 

 limited capacity. 



Rainfall and Evaporation. 



A previous chapter of this report has been devoted to the general con- 

 sideration of the climate of New Hampshire : reference must be made to 

 this for statistics and meteorological tables. It is proposed to consider 

 briefly in this place the rainfall and temperature of the state, as related 

 to the volume and constancy of our rivers. 



The rainfall of New Hampshire, as we have already noticed, is consid» 

 erably in excess of the normal amount. From a comparison of the 

 records of the Smithsonian Institution, it appears that the average rain- 

 fall for the same latitude westward to the Missouri river is not more than 

 three fourths of the average amount which we receive. A like compari- 

 son with places south of us along the Atlantic slope shows that their 

 annual rainfall also is less than that of New Hampshire. The difference, 

 however, is small ; but the comparison is remarkable, since these places 

 are entitled upon general principles to a considerably larger rainfall than 

 our state. This deviation from the general law of distribution becomes 

 intelligible when the local conditions are taken into account, namely, the 

 cold ocean currents north-east of us and along our shores, the consequent 

 low temperature, and its influence upon the storms which sweep over us. 

 The precipitation of moisture throughout the year is also remarkably 

 uniform, the total amount being almost equally divided between the four 

 seasons. The practical consequence of this distribution of rainfall is, 

 that our state enjoys comparative security from the two great obstacles 

 to the successful employment of water-power, freshet and drouth. 



These favorable conditions are still further promoted by our tempera- 

 ture. In no other part of the Northern Hemisphere, except north-eastern 

 China, does the isothermal line, which represents the mean temperature 

 of our state, sink so near to the equator,— that is, our average temper- 



