186 Humboldt on Isothermal Lines, and the [March, 



isothermal lines. For example, in Belgium (geo. lat. 52°, isoth. 

 lat. 51-8°), and even in Scotland (geo. lat. 57°, isoth lat. 45-5°), 

 the winters are more mild than at Milan (geo. lat. 45° 28', isoth. 

 lat. 57-7°). Ireland presents one of the most remarkable ex- 

 amples of the combination of very mild winters with cold sum- 

 mers ; the mean temperature in Hungary for the month of 

 August is 71-6°, while in Dublin it is only 60-8°. These, and 

 many other instances which might be adduced, prove that the 

 isocheimal lines vary much more from the terrestrial parallels than 

 the isothermal lines ; in the climates of Europe the latitude of 

 two places, which have the same annual temperature, never differs 

 more than 8° or 9°, while there are places that have the same 

 winter temperature that differ in latitude 18° or 19°. 



The lines of equal summer heat, the isothermal curves, as we 

 may style them, follow an exactly contraiy direction to the iso- 

 cheimal. We find the same summer heat at Moscow and at the 

 mouth of the Loire, although the former is 1 1° further north than the 

 latter; a circumstance which is attributed to the radiation of the 

 earth in an extensive continent, without any considerable moun- 

 tains. With respect to the relation which subsists between the 

 temperature of winter and spring in different climates, it follows, 

 from what has been stated above, that the increase of vernal 

 temperature is considerable, and likewise much protracted, 

 wherever the distribution of the annual temperature among the 

 different seasons is very unequal, as in the north of Europe, and 

 the more temperate part of the United States ; that the vernal 

 increase is great, but short, in the more temperate parts of 

 Europe ; that it is small, but protracted, in islands ; and that in 

 the different bands of climate enclosed between the same meri- 

 dians, the vernal increase is smaller, and less protracted, in low 

 than in high latitudes. Many very important conclusions are 

 deduced from these facts respecting the effects of different 

 climates on the cultivation of various kinds of plants, depending 

 partly upon the absolute heat and cold of the summer and winter 

 respectively, partly upon their relation to each other, and partly 

 upon the transition from one season to the other. 



The southern hemisphere differs considerably from the 

 northern ; it is certainly colder ; but the degree of difference 

 between them has been very differently rated. The coldness of 

 the southern hemisphere has generally been ascribed to the sun 

 being a shorter space of time below than above the equator ; but 

 it probably depends more upon the greater proportion of ocean, 

 which gives to the southern temperate zone a climate approach- 

 ing to that of a collection of islands ; there is, therefore, a less 

 accumulation of heat during the summer, and a less radiation 

 from the land, in proportion to its less extent ; there is, conse- 

 quently, a less current of warm air from the equator towards the 

 south pole, which permits the ice to accumulate more round it. 

 Near the equator, and indeed through the whole of the torrid 

 zone, the temperature of the two hemispheres appears to be 



