THE EARTH. 83 



than sufficient for maintaining the greatest 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 va- 

 pours fall upon its own bosom, before they arrive upon land. Another 

 part is arrested by the sides of mountains, and is compelled, bv t!ir 

 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 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 exhaled into vapour by the action 

 of the sun ; and afterwards returned to that great mass of waters 

 whence it first arose. " This," adds Dr. Halley, " seems the most 

 reasonable hypothesis ; 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 greatest rivers have their most copious foun- 

 tains the most remote from the sea."* 



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 afforded, as well 

 as when the greatest, is a strong objection. . Derhamt 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 ; ex- 

 cept 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, affords him rather something 

 to say, than something 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 

 <iave b^en originally formed by the industry of man. His reasons aro, 

 that when a new spring breaks forth, the water does not make itself 



* Phil. Trans, vol. ii. p. 128. f Derham Physico TheoL 



