RIVER SYSTEMS OF NEW HAMPSHIRE. 325 



not dependent upon the caprices of a variable climate, which, by proving 

 unfavorable, would bring great inconvenience and loss. 



Lakes and reservoirs are also of great value in lessening the frequency 

 and violence of freshets. By their storage of water, in floods and snow- 

 meltings, they reduce the volume of streams at these times, causing a 

 comparatively moderate rise of surface. Manufacturing establishments, 

 built upon rivers subject to heavy floods, have to be constructed of great 

 strength and with much care to withstand the water, and, if so placed as 

 to be secure against the overflow of freshets, are often too high for 

 advantageous use at the ordinary stage of water. The equability of 

 volume in our rivers is not less important as insuring the security of 

 property invested, than it is in making the investment certain of a profit- 

 able return. 



A further consideration of importance is, that the water of lakes and 

 ponds is so warm in winter, that mills near them and fed by them expe- 

 rience no trouble from the formation of ice about their gate-ways and 

 wheels, even during the severest cold. Only their upper strata sink 

 below 39° in midwinter, the deeper portion being so protected by the 

 non-conducting mantle of ice, that during the longest winter the general 

 temperature remains far above the freezing point. 



The altitude of our lakes and ponds above the sea should also be 

 noticed. The supply which they are capable of furnishing in a drouth is 

 of equal benefit to all the water-powers situated below them on their path- 

 way to the sea. This enables a large number of interested companies to 

 enter into combination for the improvement of these natural storage 

 basins, most of which are so situated that the lowering of their outlets, 

 or the erection of dams at a comparatively small outlay, would double or 

 treble their value. The expense of such improvements would be divided 

 among a large number, each of whom would receive the full advantage of 

 the increased water supply. A large number of small ponds will also be 

 improved for the same object by proprietors on their outlets, thereby 

 increasing at the same time the capacity of the larger powers below. 



The average depth of reserve, that with a reasonable outlay could be 

 retained upon the surface of our lakes and ponds for use during the dry 

 season in each year, would probably not be less than eight feet, while the 

 natural low run of the streams would be more than doubled. Indeed, 



