500 PHYSICAL GEOGRAPHY. 



Jamaica and through St. Domingo, descending across the Atlantic to the west coast of 

 Africa, which it cuts a little above Sierra Leone. It rises in that continent to the 

 northern tropic, runs closely parallel to it through Arabia to Hindostan, descends in that 

 peninsula, and cuts descendingly Siam and Cochin China, intersecting the group of the 

 Philippines to the south. This line forms the north boundary of the hot zone, coloured 

 deep carmine on the map, in which the warmth equator, distinguished by a deeper shade, 

 shows the districts of the greatest heat, which nave temperatures varying from 81 to 

 88. The warmth equator, it will be perceived, does not coincide with the geographical ; 

 but lies almost wholly to the north of it, attaining its greatest distance in eastern Africa. 

 It passes along the north coast of South America, intersects Africa from the Gulf of 

 Guinea to Abyssinia, and cuts the extremities of the southern peninsulas of Asia. The 

 minimum temperature, 81, occurs at sea in various places; the maximum, 8 7 -3, the 

 greatest heat hitherto observed, at Massowah in Abyssinia, much nearer to the northern 

 tropic than to the equator. 



The isothermal line of 70 cuts the west coast of America in the south of California 

 about latitude 25, falling below the tropic in the interior, and rising above it on the 

 opposite coast. It passes by the delta of the Mississippi, through East Florida, north of 

 St. Augustine, in about latitude 30, reaches the coast of Africa above the Canary Islands, 

 ascends towards Tunis about latitude 34, runs through the Mediterranean south of 

 Candia, enters Syria north of Beirout, traverses Asia south of the Hindoo-Koosh and 

 Himalaya mountains, descends in China nearly to the tropic. This line marks the 

 north boundary of the warm zone, coloured light carmine on the map, and is generally 

 the southern limit of the fall of snow at the sea level. 



The isothermal line of 50, central to the temperate zone, passes from Fort George at 

 the mouth of the Columbia river, on the west coast of America, latitude 46, descends in 

 the interior, and reaches the shores of the Atlantic near New York, latitude 41. It 

 then rises abruptly to latitude 56 in the north Atlantic, descends by Dublin, London, 

 and through Midland Europe to the mouth of the Danube, latitude 45, cuts the north of 

 the Black and Caspian Seas, falls in the interior of Asia, rising on the eastern coast. 



The isothermal line of 30, indicating an average temperature below the freezing point, 

 leaves the west coast of America in latitude 61, rises and declines in the interior, falling 

 to latitude 53 on the shores of Labrador. It then ascends abruptly to latitude 74 in 

 the Arctic ocean, and passing round the North Cape of Europe, as abruptly descends to 

 latitude 50 in the interior of Asia. This line marks the southern limit of permanently 

 frozen ground, which, in Asia, occurs in a latitude as low as that of London. 



The isothermal lines, indicating a lower degree of temperature, can only be traced with 

 certainty through portions of their course. 



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 isochcimal 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 

 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: 



