126 PHYSICAL GEOGRAPHY. 



Merrimack valleys, where these currents converge and become united, 

 the cold is the most intense. Where the mountain slopes and valleys are 

 wooded, the flow of these cold currents is greatly impeded. In windy 

 and stormy weather there is, however, a gradual decrease of temperature 

 according to the height. This decrease, comparing the observations at 

 Hanover with those on the summit of Mt. Washington, is one degree 

 for every three hundred and fifty-four feet ; but observations continued 

 for a series of years might greatly modify this ; or, if we make the com- 

 parison at different seasons of the year, we find that the decrease, taking 

 the monthly mean, is one degree for every five hundred feet in January, 

 while it is the same in May for only two hundred and eighty-four feet. 



Charts and Diagrams. 



In order to present clearly the leading features of the climate of New 

 Hampshire, we have prepared several charts and diagrams. These are 

 chiefly the results of observations taken under the direction of the 

 Smithsonian Institution. 



On Chart I, we have traced the yearly isothermal lines. In the vicinity 

 of Manchester there is a small area where the yearly mean, 48°, is greater 

 than in any other part of the state. The observations extend over a 

 period of fourteen years ; hence, they ought to give at least an approxi- 

 mate average. An extended curve of 47°, of which Manchester is the 

 centre, lies some five miles beyond the first, and forms an entirely isolated 

 area. In contrast with this comparatively warm area, we find directly 

 west an island of cold with the isotherm of 42°, occupying Dublin, Nel- 

 son, Stoddard, and parts of the adjoining towns. The isotherm of 46° 

 begins at the state line in New Ipswich, runs northward, then turns 

 south of east, crosses the Merrimack at Thornton's Ferry, and strikes the 

 coast at Portsmouth ; thence it is deflected northward in a great curve that 

 passes above Lake Winnipiseogee, and returns to the coast at the mouth of 

 the Piscataqua river. The isotherm of 45° passes through Dover, runs 

 northward near the state line, and crosses into Maine from Effingham ; 

 the other end of it begins at South Charlestown, is deflected southward 

 through Francestown, then runs northward nearly parallel with the 

 Merrimack, passes around Newfound lake above Squam, thence through 

 Tamworth, Madison, and Eaton, connecting with the other part of it in 



