82 A HISTORy OF 



ram never penetrates the surface of the earth above sixteen inches. 

 From thencs he infers, that it is impossible for it, in many cases, to 

 sink so as to be found at such considerable depths below. Rain-wa- 

 ter, he grants, is often seen to mix with rivers, and to swell their cur 

 rents ; but a much greater part of it evaporates. " In fact," con- 

 tinues he, " if we suppose the earth every where covered with wa- 

 ter, evaporation alone would be sufficient to carry off two feet nine 

 inches of it in a year : and yet we very well know, that scarce nine- 

 teen inches of rain-water falls in that time; so that evaporation would 

 carry off a much greater quantity than is ever known to descend. 

 The small quantity of rain-water that falls is, therefore, but barely 

 sufficient for the purposes of vegetation. Two leaves of a fig-tree 

 have been found, by experiment, to imbibe from the earth, in five 

 hours and a half, two ounces of water. This implies the great quan- 

 tity of fluid that must be exhausted in the maintenance of one single 

 plant. Add to this, that the waters of the river Rungis will, by cal- 

 culation, rise to fifty inches ; and the whole country from whence they 

 are supplied never receives fifty inches in the year by rain. Besides 

 this, there are many salt springs, which are known to proceed imme- 

 diately from the sea, and are subject to its flux and. reflux. In short, 

 wherever we dig beneath the surface of the earth, except in a very 

 few instances, water is to be found : and it is by this subterraneous 

 water, that springs and rivers, nay, a great part of vegetation itself, is 

 supported. It is this subterraneous water which is raised into, steam. 

 by the internal heat of the earth, that feeds plants. It is this subter- 

 raneous water that distils through its interstices ; and there, cooling, 

 forms fountains. It is this, that, by the addition of rains, is increas* 

 ed into rivers, and pours plenty over the whole earth." 



On the other side of the question,* it is asserted, that the vapours 

 which are exhaled from the sea, and driven by the winds upon land, 

 are more than sufficient to supply not only plants with moisture, but 

 also to furnish a sufficiency of water to the greatest rivers. For this 

 purpose, an estimate has been made of the quantity of water emptied 

 at the mouths of the greatest rivers ; and of the quantity also raised 

 from the sea by evaporation ; and it has been found, that the latter 

 by far exceeds the former. This calculation was made by Mr. Mar- 

 riotte. By him it was found, upon receiving such rain as fell in a year, 

 in a proper vessel fitted for that purpose, that, one year with another, 

 there might fall about twenty inches of water upon the surface of the 

 earth, throughout Europe. It was also computed that the river Seine, 

 from its source to the city of Paris, might cover an extent of ground, 

 that would supply it annually with above seven billions of cubic feet 

 of this water, formed by evaporation. But upon computing the quan- 

 tity which passed through the arches of one of its bridges in a year, 

 it was found to amount only to two hundred and eighty millions of cubic 

 h;et, which is not above the sixth part of the former number. Hence, 

 therefore, it appears, that this river may receive a supply, brought to 

 it by the evaporated waters of the sea, six times greater than what it 

 gives back to the sea by its current ; and therefore, evaporation is more 



* Phil. Trans, vol. ii. p. 128. 



