182 Humboldt, on Isothermal Lines, and the [March, 



In advancing from the equator towards the noith pole, the 

 mean temperatures become less than that of the equator in the 

 following proportion : 



Lat. Temp. Temp. 



From 0°— 20° in the old contin. 35-6° in the new 35-6° 



20—30 39-2 .,' 42-8 



30—40 39-2 44-6 



40 —50 44-6 48-2 



50 —60 41-8 45-1 



—60 72-2 88-0 



Having traced the isothermal belts from Europe to the pro- 

 vinces of the New World, the next object will be to observe them 

 in North America itself. There are two chains of mountains in 

 (his continent— the Alleghanys, and the Rocky Mountains ; the 

 first running N.E. and S.W., the latter N.W. and S.E., making 

 nearly equal angles with the meridian, and enclosing the vast 

 plains of Louisiana, Tennesee, and the state of Ohio. This coun- 

 try possesses a milder climate than the parallel latitudes in the 

 Atlantic States, the winters being less severe, and the summers 

 less hot ; so that the isothermal lines remain parallel, or nearly 

 parallel, to the equator, from the coast of the Atlantic to the east 

 of the Mississipi, and the Misoury. Beyond the Rocky Mountain 

 the climate is still milder ; in New California, and along the 

 northern parts of the western side of the continent, the tempera- 

 ture appears to be very nearly the same with what it is in similar 

 latitudes on the western side of Europe. The isothermal lines 

 will, therefore, be bent upwards in this part. 



When we pass from the west of Europe eastward, the iso- 

 thermal lines are again curved downwards ; but the few accurate 

 observations which we possess render it very difficult to fix the 

 exact line ; of the general fact, however, there is no doubt. We 

 have hitherto found that, towards the north, the isothermal lines 

 are neither parallel to the equator nor parallel to each other; and 

 as the curve is the greatest in Asia and in America between 80° 

 west and 100° east longitude, it might be supposed that the 

 torrid zone of this part commences to the south of the tropic of 

 Cancer, or that its heat is less intense. This, however, does 

 not appear to be the case ; as we approach to the line, below 

 the parallel of 30°, the isothermal lines gradually become parallel 

 to themselves and to the equator. For some time the old conti- 

 nent was thought to be warmer between the tropics than the 

 new ; but more correct observations have shown that this is not 

 the case. The mean temperature of the equator may be fixed at 

 81-2.° 



The distribution of the temperature through the different parts 

 of the year differs in the same isothermal line ; this is the case 

 with respect both to the old and new continents ; in the former 



