A HISTORY OF 



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 evapo- 

 ration ; arid it has been found, that the latter 

 by far exceeds the former. This calculation 

 was made by Mr. Marriotte. By him it was 

 found, upon receiving such rain as fell in a 

 year, in a proper vessel fitted for that pur- 

 pose, 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 millions of cubic 

 feet of this water, formed by evaporation. 

 But upon computing the quantity which pass- 

 ed 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 feet, 

 which is not above the sixth part of the for- 

 mer number. Hence it appears, that this ri- 

 ver 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 than sufficient for maintaining the great- 

 est rivers, and supplying the purposes also 

 of vegetation. 



In this manner, the sea supplies sufficient 

 humidity to the air, for furnishing the earth 

 with all necessary moisture. One part of its 

 vapours fall upon its own bosom, before they 

 arrive upon land. Another part is arrested 

 by the sides of mountains, and is compelled, 

 by the rising stream of air, to mount upward 

 towards the summits. Here it is presently 

 precipitated, dripping down by the crannies 

 of the stone. In some places, entering into 

 the caverns of the mountain, it gathers in 

 those receptacles, which being once filled, 

 all the rest overflows; and breaking out by 

 the sides of the hills, forms single springs. 

 Many of these run down by the valleys or guts 

 between the ridges of the mountain, and, 

 coming to unite, form little rivulets or brooks ; 

 many of these meeting in one common valley, 

 and gaining the plain ground, being grown 

 less rapid, become a river; and many of 



Phil. Trans. vol. ii. p. 128. 



these uniting, make such vast bodies of water, 

 as the Rhine, the Rhone, and the Danube. 



There is still a third part, which falls upon 

 the lower grounds, and furnishes plants with 

 their wonted supply. But the circulation 

 does not rest even here ; for it is again exha- 

 led into vapour hy the action of the sun ; and 

 afterwards returned to that great mass of wa- 

 ters whence it first arose. "This," adds Dr. 

 Halley, "seems the most reasonable hypo- 

 thesis ; and much more likely to be true, than 

 that of those who derive all springs from the 

 filtering of the sea-waters, through certain 

 imaginary tubes or passages within the 

 earth ; since it is well known that the great- 

 est rivers have their most copious fountains 

 the most remote from the sea." a 



This seems the most general opinion ; and 

 yet, after all, it is still pressed with great 

 difficulties ; and there is still room to look 

 out for a better theory. The perpetuity of 

 many springs, which always yield the same 

 quantity when the least rain or vapour is af- 

 forded, as well as when the greatest, is a 

 strong objection. Derham u mentions a spring 

 at Upminster, which he could never perceive 

 by his eye to be diminished, in the greatest 

 droughts, even when all the ponds in the 

 country, as well as an adjoining brook, have 

 been dry for several months together. In the 

 rainy seasons, also, it was never overflowed ; 

 except sometimes, perhaps, for an hour or so, 

 upon the immission of the external rains. He, 

 therefore, justly enough concludes, that had 

 this spring its origin from rain or vapour, 

 there would be found an increase or decrease 

 of its water, corresponding to the causes of 

 its production. 



Thus the reader, after having been tossed 

 from one hypothesis to another, must at last be 

 content to settle in conscious ignorance. All 

 that has been written upon this subject, af- 

 fords him rather something to say, than some- 

 thing to think ; something rather for others 

 than for himself. Varenius, indeed, although 

 he is at a loss for the origin of rivers, is by no 

 means so as to their formation. He is pretty 

 positive that all rivers are artificial. He 

 boldly asserts, that their channels have been 

 originally formed by the industry of man. 



b Derham Physico-Theol. 



