40 Prof. Tyndall on the Relation of 



which dimmish far more speedily in density than the associated 

 strata of air, until finally our ascending body of vapour finds 

 itself lifted above the screen which for a time protected it. It 

 now radiates freely into space, and condensation is the necessary 

 consequence. The heat liberated by condensation is, in its turn, 

 spent in space, and the mass thus deprived of its potential energy 

 returns to the earth as water. To what precise extent this power 

 of aqueous vapour as a radiant comes into play as a promoter of 

 condensation, I will not now inquire ; but it must be influential 

 in producing the torrents which are so characteristic of the tropics. 



The same remarks apply to the formation of cumuli in our 

 own latitudes. They are the heads of columnar bodies of vapour 

 which rise from the earth's surface and are condensed to cloud 

 at a certain elevation. Thus the visible cloud forms the capital 

 of an invisible pillar of saturated air. Certainly the top of the 

 column, piercing the sea of vapour which hugs the earth, and 

 offering itself to space, must lose heat by the radiation from its 

 vapour, and in this act alone we should have the necessity for 

 condensation. The "vapour plane" must also depend, to a 

 greater or less extent, on the chilling effects of radiation. 



The action of mountains as condensers must, I think, be con- 

 nected with these considerations. When a moist wind encounters 

 a mountain-range it is tilted upwards, and condensation is no 

 doubt to some extent due to the work performed by the expand- 

 ing air; but the other cause cannot be neglected; for the, air 

 not only performs work, but it is lifted to a region where its 

 vapour can freely lose its heat by radiation into space. During 

 the absence of wet winds the mountains themselves also lose 

 their heat by radiation, and are thus prepared for actual surface 

 condensation. We must indeed take into account the fact that 

 this radiant quality of water is persistent throughout its three 

 states of aggregation. As vapour it loses its heat and promotes 

 condensation ; as water it loses its heat and promotes congela- 

 tion ; as solid it loses its heat and renders the surfaces on which 

 it rests more powerful refrigerators than they would otherwise be. 

 The formation of a cloud before the air which contains it touches 

 a cold mountain, and indeed the formation of a cloud anywhere 

 over a cold tract of land, where the cloud is caused by the cold 

 of the tract, is due to the radiation from the aqueous vapour. 

 The uniformly diffused fogs which sometimes fill the atmosphere 

 in still weather may be due to cold generated by uniform radia- 

 tion throughout the mass, and not to the mixture of currents of 

 different temperatures. The cloud by which the tract of the Nile 

 and Ganges (and sometimes the rivers of our own country) may 

 be followed on a clear morning is, I believe, due to the chilling 

 «of the saturated air above the river by radiation from its vapour. 



