TERRESTRIAL HEAT. 



51'44 does not differ by so much as two- tenths of a degree from 

 the true mean temperature of that place. 



Observation, however, has been hitherto so limited, both as to 

 extent and duration, that this thermal character has been deter- 

 mined for a very limited number of places. Indications, never- 

 theless, have been obtained sufficiently clear and satisfactory to 

 enable Humboldt to arrive at some general conclusions, which we 

 shall now briefly state. 



9. In proceeding successively along the same meridian from the 

 equator towards the pole, the mean temperature decreases generally, 

 but not regularly nor uniformly. At some points it even happens 

 that the mean temperature augments, instead of decreasing. These 

 irregularities are caused partly by the varying character of the 

 surface, over which the meridian passes, and partly by the atmo- 

 spheric effects produced by adjacent regions, and a multitude of 

 other causes, local and accidental. As these causes of irregularity 

 in the rate of decrease of the mean temperature, proceeding from 

 the equator to the poles, are different upon different meridians, it 

 is evident that the points of the meridians which surround the 

 globe, at which the mean temperatures are equal, do not lie upon 

 a parallel of latitude, as they would if the causes which affect the 

 distribution of heat were free from all such irregularities and 

 accidental influences. 



If, then, a series of points be taken upon all the meridians 

 surrounding the globe, having the same mean temperature, the 

 line upon which such points are placed is called an isothermal line. 



Each isothermal line is therefore characterised by the uniform 

 mean temperature which prevails upon every part of it. 



10. Isothermal zones. The space included between two iso- 

 thermal lines of given temperatures is called an isothermal zone. 



The northern hemisphere has been distributed in relation to its 

 thermal condition into six zones, limited by the six isothermal 

 lines, characterised by the mean temperatures, 86, 74, 68, 59, 

 50, 41, and 32. 



The first zone is a space surrounding the globe, included between 

 the equator and the isothermal line, whose temperature is 74. 



The mean temperature of the terrestrial equator is subject to 

 very little variation, and it may therefore be considered as very 

 nearly an isothermal line. Its mean temperature varies between 

 the narrow limits of 81| and 821. 



11. If, iipon each meridian, the point of greatest mean tempe- 

 rature be taken, the series of such points will follow a certain course 

 round the globe, which has been designated as the thermal equator. 

 This line departs from the terrestrial equator, to the extent of ten 

 or twelve degrees on the north, and about eight degrees on the 



68 



