92 



currents, and that this prime mover, which controls our planet, 

 is equally available to guide us in our researches in sedimentary 

 rocks as it is in the fundamental series and mineral deposits. 



CHAPTER XIV. 



ON THE GENERAL NATURE OF THE DISTRIBUTION OF HEAT 

 OVER THE SURFACE OF THE GLOBE, AND THE ZONES OF 

 EQUAL TEMPERATURE. 



In order to avoid misconception with respect to the zones, and 

 not to suppose that they are considered here as parallel bands, 

 such as those mathematically drawn on our artificial globes, we 

 introduce these observations to explain their general character, 

 and their influence on the organic system. 



It is now well ascertained that zones of equal warmth, both 

 in the atmosphere and in the waters of the ocean, are neither 

 parallel to the equator nor to each other. It is also known that 

 the mean annual temperature may be the same in two places 

 which enjoy very different climates, for the seasons may be nearly 

 uniform or violently contrasted, so that the lines of equal winter 

 temperature do not coincide with those of equal annual heat. 

 The deviations of all these lines from the same parallel of lati- 

 tude are determined by a multitude of circumstances, among the 

 principal of which are the position, direction and elevation of 

 the continents and islands, the position and depths of the sea, 

 and the direction of currents and of winds. 



The amount of the immediate solar heat depends upon the 

 position of the sun in the ecliptic, the direction in which the 

 sun's rays strike the earth, and the variable degrees of intensity 

 occasioned by the atmospheric lens, illustrated in Plate XX. 

 In proceeding from the equator towards either of the poles, with- 

 out altering our height above the level of the sea, we must travel 

 a great distance before we find the mean annual temperature 

 reduced even a few degrees ; but by increasing our elevation, a 

 rapid change of temperature will be experienced, till we arrive 



