446 THE MOUNTAIN. 



just above the latitude of 60 and 75 respectively, ascend abruptly 

 surrounding the magnetic pole, and falling short of the geographical 

 one." 



Of the isothermal curves in the interior valley, Dr. Drake 

 indites : 



"The curves of equal mean temperature, which traverse the In- 

 terior Valley, cannot yet be delineated, for the want of a sufficient 

 number of observations. In the west, from the cooling influence of 

 the Cordilleras of Mexico, and the Rocky Mountains, extending into 

 the polar circle, and to the east from a similar though smaller influ- 

 ence of the same kind exerted by the Appalachian chain, from the lati- 

 tude of 33 to 48 or 50 north we know that the curves of equal 

 mean temperature cannot lie parallel to the lines of latitude, except 

 for a certain distance in the middle of the valley. East of the Mis- 

 sissippi, as they approach the Appalachian Mountains, they must 

 bend to the south ; west of that river, as they ascend the great in- 

 clined plane, they must come in the same direction, and, on reaching 

 the Rocky Mountains, must of necessity extend along their slopes, 

 rising gradually as the latitude lessens, but not attaining the sum- 

 mits of these mountains until we come within the tropics. It results 

 from these data that the isothermal lines of the valley are nearly 

 parallel to those of one side of a compressed ellipsis or long oval with 

 their eastern curved extremities much shorter than their western. 

 Where they intersect the trough of the Mississippi they have their 

 highest latitude." 



It has been clearly demonstrated that the climate of the 

 State of Pennsylvania, in its thermometric oscillations, is 

 the most equable of the whole Atlantic range. This comes 

 from its mediate latitudinal position, also its being saddled 

 over the two planes of the Atlantic and Ohio water-sheds, 

 and over the Appalachian range, its territory extending from 

 the lakes to the arms of the ocean ; one cause of its dis- 

 tinctive climate thus being the well-known equalizing influ- 

 ences of large bodies of water. A region of agreeable 

 equilibrium of climate, that happily-located State enjoys a 

 most desirable temperature ; removed from the chilling north- 

 ern and northeastern Atlantic and lake sweep of currents, 

 also from the hotter southwestern and southeastern streams 

 of air, it exhibits less of the excessiveness of climate de- 



