RIVERS. 43 



the average fall of rain on the surface which it drains probably 

 does not exceed 24 inches in a year. A far greater body of 

 water comes down the Tees, Swale, Ure, Wharf e, Aire, Calder 

 or Dun, though they drain smaller areas, because the annual fall 

 of rain in the districts which they drain is double that which 

 feeds the Derwent. The average annual fall on the whole area 

 of Yorkshire may be estimated at about 30 inches ; on the east 

 coast under 20, and on the extreme west under 50. 



The great body of water which flows through Humber to the 

 sea, has risen in vapour from the ocean and the land, ascended 

 to high regions of the atmosphere, collected into clouds, and 

 descended in snow-flakes, hail-stones, or rain-drops. There is 

 perhaps no more instructive history than that of a drop of rain. 

 The elements of its composition are diffused through the atmo- 

 sphere, but so sparingly that they constitute less than one hun- 

 dredth part of its bulk. Rain falls in Yorkshire to the average 

 depth of about 30 inches in a year ; and in some parts of Cum- 

 berland the annual fall of rain has been measured by Mr. Miller 

 to four times this amount ; yet at any one moment, upon the 

 average, the atmosphere probably contains of vapour only so 

 much as would yield over all the globe a depth of three inches 

 of water. And this quantity may be always nearly the same, 

 though almost every particle of it may be, or rather must be, 

 many times raised in evaporation, and thrown down in snow or 

 rain, in the course of a single year. The drop, gathered by ac- 

 cretion of minute particles, may be snow, ice, or water, according 

 to the circumstances of the place and time ; it may be collected 

 from elements which have floated from another quarter of the 

 globe, or have been exhaled from the surface immediately below. 

 On the ground, the drop is divided between two rivals the 

 earth and the air. The portion which enters the earth is again 

 demanded by two claimants ; vegetation, acting by the roots of 

 plants, carries upward much moisture to the air, and the porous 

 subsoil and rock conduct the residue to the hidden reservoirs of 

 springs. After seasons when rains are scanty, this residue is 



