APPLICATIONS OF THE MICROSCOPE. 131 



as the laws of gravitation unerringly direct the 

 heavenly bodies in their orbits ? If Lieutenant 

 Maury's theory can be maintained, even this 

 seemingly hopeless achievement has been so far 

 effected. To illustrate thoroughly his process 

 of reasoning, we must at some length state his 

 views, and after all, from lack of the diagrams 

 represented in the Physical Geography of the 

 Sea, perhaps very imperfectly succeed in ex- 

 plaining the theory. 



9. Currents of air or wind arise, it is well 

 known, from causes which tend to disturb the- 

 equilibrium of the atmosphere, such as changes 

 of temperature, or amount of aqueous vapour 

 contained in it. Two adjoining regions being 

 unequally heated, an upper current of air will 

 proceed to the colder region, and an under cur- 

 rent in the opposite direction. The difference 

 of temperature between the polar and equatorial 

 regions, amounting to upwards of 82, of neces- 

 sity produces a constant exchange of air between 

 them. The cold and dense air of the polar re- 

 gions seeks to replace the warm rarefied air of 

 the equator, while it again ascends and forms a 

 current towards the poles, in order to restore the 



