EXPLANATION OF THE ISOTHERMAL CHARTS. 105 



As already pointed out, the position of the isothermal curves is intimately con- 

 nected with the hypsometric features of the country, and this direct dependence 

 has consequently been made the basis of the above division, greater or less eleva- 

 tion constituting the principal cause of their deflections. This appears, for instance, 

 conspicuously in the isothermal of 52°, depending on the direction of the Apalachian 

 range, and in the isothermal of 44°, depending on the directions of the Rocky 

 Mountains, the Cascade range, and the Sierra Nevada. 



In the eastern part of the United States, the distribution of heat appears normal, 

 as indicated by the isothermals between 44° and 68° which follow, with no great 

 departures, parallels of latitude ; in the loestern part, on the contrary, it is altogether 

 more irregular, and the pure solar climate is apparently subverted, the distribution 

 of temperature on the Pacific shore being governed by a system almost at right 

 angles to that in the eastern part, and possessing an intermediate system of distri- 

 bution at the head of the Gulf of California. 



In the winter months, the proximity of the Gulf stream to the Atlantic sea-board 

 has the eff"ect of elevating the temperature in the vicinity of the ocean, the amount 

 being 0° in Florida, about 4° in North Carolina, and about 8° or 10° in Massachu- 

 setts; in the summer months, the effect is reversed, as shown by the isotherals 

 curving southwards ; this is due to the cold current running southwards between 

 the coast and the gulf stream, and the depression produced would be still greater 

 but for the circumstance of the prevalence of westerly winds which carry the heated 

 air to seaward. The depressing efi'ect, however, in amount, is less than one-half 

 that given for the opposite season. It would appear that in summer nearly the 

 whole of Florida enjoys an almost equal temperature, barely rising above 80° Fah. ; 

 with this we connect the fact that in Florida summer constitutes the rainy season. 



On the yearly average the vicinity of the Atlantic is apparently without any 

 direct efi'ect on the temperature of the coast. 



Passing now to the infiuence of the great lakes we shall find it similar, viz. : a 

 loarmitig effect in ivinter, rising to about 10°, and a cooling effect in summer, 

 depressing about 5°, whereas, during the year the presence or absence of this body of 

 water would seem to be of no particular consequence as regards mean temperature. 



The coldest region is in northern Minnesota and northeastern Dakota, the iso- 

 cheimal of 4° appearing along the low elevations near Red Lake in Minnesota. 

 It is near these regions that the extremely cold waves, which occasionally sweep 

 over the eastern and southern states during the winter appear to enter the United 

 States. 



In the western part of the country we recognize as the most remarkable feature, 

 the great uniformity of the distribution of temperature along the Pacific coast as 

 exhibited in the isothermal of 52°, skirting the coast for about 650 miles between 

 San Francisco and the northwestern part of Washington Territory; the same 

 featui-e is indicated by the direction of the isocheimals, approximating to parallelism 

 with that of the coast and again in the isotheral of 60°. The direct influence of 

 the Pacific Ocean on the climate of the western states (west of 100° longitude) is 

 heightened by the presence of a cool current running southward close along the 

 coast. The presence of the cool ocean, together with the prevailing westerly winds, 



14 February, 1S75. 



