500 PHYSICAL GEOGRAPHY. 



North America it is almost coincident with the parallel of 30 or 31 of latitude on the 

 eastern coast ; but, in the old continent, we find the same plants and a corresponding 

 temperature at a greater distance from the equator. The curve reaches latitude 37 in 

 the south of Spain, Sicily, Asia Minor, and Syria ; but in Persia it descends to 30. It is 

 still lower in passing round the table-land of Tartary, and probably does not extend 

 beyond 30 in China. 



The line of 59 of temperature passes through Raleigh, in North Carolina, in latitude 

 35 ; but in Europe it ascends to 44, and cuts Italy between Rome and Florence. The 

 curve is here near the northern boundary of the olive and the fig, and if traced by these 

 plants, it will be found following the range of mountains north of the Mediterranean, 

 extending north of the Black Sea to the Caspian, south of Astrachan. On the east of 

 Asia it descends probably to latitude 35 ; but on the western coast of America it ascends 

 to 37 or 38, and perhaps still higher. 



The line of 55*4 of temperature, passes to the north of Bourdeaux in latitude 45. On 

 the east of Asia, it is near Pekin in latitude 40. On the west of America it ascends to 

 45, near Cape Foulweather, to the south of the embouchure of the Columbia, descending 

 on the east coast to latitude 39, passing near the city of Washington. 



The line of 50 of temperature nearly corresponds in Midland Europe with the limit 

 of the advantageous cultivation of the wine-grape, or the parallel of 50 of latitude, which 

 passes over the confluence of the Rhine and the Moselle. More to the west it reaches a 

 higher latitude, but in going eastward from Germany it continually descends, and is as 

 low as 40 in Asia. It strikes the eastern coast of America south of Boston about latitude 

 41, ascends and descends in the interior, and strikes the western coast in latitude 50, 

 north of the Columbia. 



The line of 41 of temperature is nearly coincident with the northern boundary of the 

 oak and wheat. This is in latitude 63 on the coast of Norway, 58 in Russia, and lower 

 in Siberia. In North America the line is found at the Bay of St. George in Newfoundland 

 in latitude 49, and generally at 50 on the eastern coast, but it rises higher on the 

 western. 



The line of 32 of temperature, which marks the average for the year as at the freezing 

 point, passes to the south of Nain, a settlement on the coast of Labrador, in latitude 55. 

 It descends from thence so as to strike across the lower extremity of Hudson's Bay in 

 latitude 53, and then tends northward towards the Great Slave Lake in latitude 60. 

 In Europe the curve is a little to the north of Umea in Sweden, in latitude 64, where 

 it makes the remarkable inflexion shown in the diagram, ascending as high as the North 

 Cape in latitude 71. It then returns southward, and attains a very low latitude on the 

 east of Asia. 



It has been already stated that places where the mean annual heat is the same vary 

 considerably in their mean summer and winter temperatures. Hence isocheimal lines, or 

 lines of equal winter temperature, and isotheral lines, or those which show equal summer, 

 unite points which are upon different isothermal curves. Thus Belgium on the isothermal 

 line of 51 8', Scotland on that of 45 5', and Milan on that of 55 8', under widely 

 different parallels of latitude, are on the same isocheimal line. Moscow also, and the 

 mouth of the Loire, are on the same isotheral line, notwithstanding a difference of 11 of 

 latitude ; and London and Pekin are on the same isothermal curve, though on widely 

 different lines of mean summer and winter temperature. 



Isogeothermal lines are curves connecting points where the temperature of the ground 



I is equal, at or below the surface of the earth. The interior temperature of the earth is 



| measured by that of subterranean excavations, natural or artificial, and by that of springs. 



It appears to decrease, as might be expected, from the plains to the tops of mountains : 



