30 Baron N. Schilling on the Constant Currents 



B 



air; so that by it, at all events in ordinary circumstances, a 

 breath of air, but no wind, can be produced. In a forest- con- 

 flagration, the enormous heat appears to cause a considerable 

 ascending current of air; but even then the inflow of air 

 thereby effected is perceptible only in the immediate vicinity of 

 the burning. 



Hence we believe that wind can mostly arise only from con- 

 densation of the aqueous vapours in the atmosphere, which 

 possess the property of very suddenly changing considerably 

 their degree of elasticity and the pressure resulting from it ; 

 this must, of course, exert a great influence on all atmospheric 

 phenomena. Certainly the elasticity of the aqueous vapour 

 stands in the closest relation with the temperature, which so 

 far, therefore, operates indirectly in the origination of wind. 

 Whether change of temperature is the only cause of the con- 

 densation of vapour, we know not : as, when the aqueous vapours 

 are condensed, electricity is always set free, perhaps conversely 

 the condensation may be caused by electricity. 



At any rate it is changes in the tension of the aqueous vapour 

 that produce strong winds ; and only in very peculiar cases can 

 the, by itself, slow expansion of the air develop stronger winds. 

 Thus, for instance, when the greater portion of a continent is 

 powerfully heated by the sun, the mass of air rising, though 

 only slowly, from the whole of the vast surface, would require 

 for its replacement (that is, for the restoration of equilibrium) 

 such a mass of air that the inflow must be much accelerated, 

 because it forms a stratum of little height in comparison with 

 the magnitude of the heated surface. An example of winds 

 thus produced is afforded by the monsoons. Over the ocean the 

 atmosphere can never be so much heated as over the land ; and, 

 besides, evaporation of the water of the sea and the elasticity of 

 the aqueous vapour are augmented as the temperature of the air 

 rises. If, therefore, the expansion of the air diminishes the 

 atmospheric pressure, on the other hand the augmented elasticity 

 and quantity of aqueous vapour will again increase it ; and it is 

 difficult to decide which of these two may exert the greater in- 

 fluence. Admitting that the diminution of the atmospheric 

 pressure by expansion of the air is greater than the rise of the 

 pressure by aqueous vapour, it yet appears to us self-evident 

 that the wind resulting from the heating of a continent must be 

 much stronger than that produced by the heating of the air 

 over the ocean, supposing both to take place over very consider- 

 able spaces. 



If, then, the recognized theory of the trade-winds were cor- 

 rect and they were produced by the ascent of the heated air, the 

 trade-winds would blow in summer towards the Sahara, since 



