UNCERTAINTY OF MODERN METEOROLOGY. 25 



of freedom or obstruction of the flow of water over tLe surface, 

 the composition and density of the soil, its temperature,* the dry- 

 ness or saturation of the subsoil, the presence or absence of per- 

 forations by worms and small burrowing quadrupeds — in short, I 

 all the conditions upon which the permeabihty of the ground by 

 water and its power of absorbing and retaining or transmitting | 

 moisture depend — vary at comparatively short distances ; and ' 

 though the precipitation upon very small geographical basins and 

 the superficial flow from them may be estimated with an approach 

 to precision, yet even here we have no present means of knowing 

 how much of the water absorbed by the earth is restored to the 

 atmosphere by evaporation, and how much carried off by infiltra- 

 tion or other modes of underground discharge. When, therefore, 

 we attempt to use the phenomena observed on a few square or 

 cubic yards of earth, as a basis of reasoning upon the meteorol- 



ant phenomena in meteorology — I mean for watery vapor condensed and ren- 

 dered visible by cold. The Latins popularly expressed this condition of water 

 by the word vapor. For invisible vapor natural philosophers used vapar and 

 apiritus, bi^t with a looseness corresponding to the vagueness of their ideas ou 

 the subject, and Van Helmont was obliged to invent a word, gas, as a scien- 

 tific generic name for watery and other fluids in the invisible state. The mod- 

 ems have perverted the familiar meaning of the word vapor, and in science its 

 use is confined to express water in the gaseous and invisible state. When 

 vapor is rendered visible by condensation, we call it fog or mist if it is lying 

 on or near the surface of the earth or of water ; when it floats in the air we 

 call it cloud ; but between these tliree words there is no clearly established 

 distinction, for the condition of water in the "swirling cloud" is the same as 

 in the " misty wreath." They only express the form and position of the aque- • 

 ous aggregation, not the condition of the water-globules which compose it. 



The breath from our mouths, the steam from an engine, thrown out into 

 cold air, become visible, and consist of water in the same state as in fog or 

 cloud ; but we do not apply those terms to these phenomena. It would be an 

 improvement in meteorological nomenclature to restore vapor to its original 

 popular meaning, and to employ a new word, such for example as hydrogas, 

 to express the new scientific idea of water in the invisible state. 



* Temperature, as conditioning the capillarity of rock and the hygroscopicity 

 of earth, has not been much considered by physical inquirers, but it is cer- 

 tainly an element of very great importance. See Stoppani, Qeologia, I, 

 § 600, 607. The instance of the foundry, where, when its floor of earth had 

 received a great quantity of water by infiltration from a neighboring river at 

 flood, almost the whole of the moisture was absorbed by the portions of the 

 floor which had been heated by fires kindled upon it to dry the moulds, is very 

 instructive. 



