PERIODICITY OF TEMPERATURE. 409 



area of ocean, the sea presenting a medium readily and citaptek 

 equally heated, and less susceptible of local influences on -^-^VIIL 

 its temperature than the land. Even then, however, Temperahire 

 currents exercise a considerable influence, and occasional "' "'<^ <"=<'''"• 

 very marked inflexions of the isothermal zones occur 

 within the area of the ocean. 



But in addition to the results which these observations Pcriodicitj- 

 involve as to the variations in the distribution of heat '.''re™^'"'^ 

 over the earth's surface, they also lead to the considera- 

 tion of another remarkable class of phenomena connected 

 with the unequal distribution of heat over the year. In 

 the cold meridians, as is well known, a brief, but very 

 hot summer, and a cold winter occur, whereas in south- 

 ern latitudes of Europe and America, the cold of win- 

 ter is greatly moderated, and in the whole warmer meri- 

 dians of the globe, the difference between the tempera- 

 ture of summer and winter is greatly diminished. The 

 full importance of these observations of isothermal phe- importance 

 nomena is now fully appreciated ; but it is only by a °{,se^a{j™^' 

 very extensive and combined series of observations that 

 these comprehensive bearings will be made fully known, 

 and the direct clue obtained to the great lavs-s which 

 they involve. The meteorological observations which 

 are now established in so many parts of the world are 

 accumulating much valuable data for this purpose. 

 Private obsei-vers may also do much, and have already 

 furnished some important contributions to the same end, 

 so that the great scientific traveller to whom we owe the 

 foundation of these remarkable disclosures regarding the 

 laws that govern the universe, lias the satisfaction ot 

 himself observing a noble host of scientific pioneers ad- 

 vancing on the path which he had jiointed out, and fol- 

 lowing up his suggestive thoughts by the patient and 

 laborious analysis of scientific investigation. 



But Humboldt has also taken into consideration other otiicrsonrces 

 sources of heat besides that which the earth derives from o'''»"'f- 

 the sun. He even reasons on the possil)ility of the earth 

 being a mere solid shell, inclosing a fluid mass, the heat 



