258 CLIMATOLOGY. 



The preceding pages contain the temperatures of the 

 districts through which the Expedition travelled, wherever 

 I have been able to ascertain them, and also data for extend- 

 ing the lines of mean annual heat (isothermal), mean summer 

 heat (isothceral), and mean winter heat (isocheimenal) across 

 the continent. By comparing the sea-coast temperatures 

 in Table II. and those of the shores of the great lakes in 

 Table I. with those of places in France and Italy lying 

 between the same parallels of 42° — 45° north latitude, we 

 perceive that the mean annual heat of Europe is from 8° 

 to 15° Fah. greater than that of America at the same 

 distance from the equator, while the summer heats differ 

 only from 2° to 6°. * The inferior mean heat of America 

 is therefore due principally to excessive winter colds, and 

 this is decidedly the case in the interior. As the summer 

 heats, however, regulate the culture of the cerealia and 

 the growth of deciduous plants generally, the severe 

 winters of America do not cause a scanty vegetation. 

 From the 50th parallel northwards the trees are frozen 

 to their centres in winter ; and, consequently, the de- 

 velopment of buds and other vital processes which go on 

 in the temperate climate of England, even in the coldest 

 months, are completely arrested. This hybernation of 

 plants increases in length with the severity and duration 

 of winter which, generally speaking, augment in the in- 

 terior of America with the latitude. The summer heats 



* Dove's table in the Report of the Brit. Association for 1847 

 has furnished the means of making this comparison. The places 

 compared were Alais, Aries, Bourdeaux, Dax, Manosque, Marseilles, 

 Montpellier, Pau, Puy, Tarascon, Toulon, Toulouse, Perpignan, Alba, 

 Bologna, Cascina, and Lucca. Oleron of Beam has the same altitude 

 •with Lake Superior, and Mount Louis is 4,900 feet high. All of them 

 lie within the parallels of 43° — 45° except the last, which is in 

 42° 50' N. The maps of isothermal lines of this author express the 

 general results of the study of the table referred to. 



