Thermal Circulation. 47 



waters of the ocean have attracted the attention of the scientific 

 observer, the resemblance between the phenomena which occur 

 in the gaseous envelope and those observable in the aqueous 

 envelope of our planet has frequently been pointed out. This 

 resemblance is the obvious result of a similarity of conditions 

 and an identity of natural laws which govern the internal 

 economy of the two envelopes. Both are composed of 

 fluids subject to the laws of gravity, and to the laws which 

 direct the movements of fluids in general, their expansion under 

 the influence of heat, their contraction under the action of cold. 

 The equilibrium of both is constantly disturbed in consequence 

 of the unequal distribution of solar heat between the Poles 

 and the Equator, and is as constantly restored through 

 the agency of currents, cold air and cold water unceasingly 

 flowing from high towards low latitudes, warm air and warm 

 water without intermission passing from the torrid zone 

 into the temperate and polar regions. As recent observations 

 have shown, in both, in the aqueous as well as in the aerial 

 ocean, the temperature rapidly decreases from the surface 

 towards the deeper strata (considering the stratum of the 

 atmosphere which is in close contact with the surface of the 

 earth as the virtual surface of the aerial ocean) ; and the 

 surface-stratum of both forms a stratum of maximum energy 

 which in the ocean extends to a depth of about 500 fathoms, 

 or half-a-mile (Plates 6 to 20), and in the atmosphere, to a 

 depth of 3000 fathoms, or three miles. Finally, the composition 

 of both fluids is altered under the influence of solar heat ; that 

 of air through an increase in the quantity of moisture held in 

 suspense, that of water by an increase in the percentage of salt 

 held in solution. 



Thermal Circulation. — It has been shown by Sir John 

 Herschel that the unequal exposure of the different zones of the 

 earth's surface to the rays of the sun must result in a system 



