SPRING. 



B quarter of its water, or even a half, and not be too dl^ 

 for fuppnrting vegetation. 



Clay, jiilt dug out for the purpofe of making bricks, 

 was tried in the fame manner. It gave the fame fpecific 

 gravity as the earth, and yielded not much lefs water. 



Thefe experiments and obfervations prove, that M. de 

 la Hire's conclufions, drawn from the vegetation of plants 

 in a given quantity of luil, precluded from any communi- 

 cation with the earth at large, are erroneous, or at lealt 

 unwarranted ; as it does not thence appear, that the eva- 

 poration for the whole year exceeds the rain in the year, 

 whatever it may do for a month or two in fiimmer. 



The origin of fprings may if ill, tlierefore, be attributed 

 to rain, till fome more decifive experiments appear to the 

 contrary. 



Upon the whole it (hould feem, that at the commence- 

 ment of fpring, the ground is nearly faturated with water 

 for five or fix feet in depth, as the rains and dews in autumn 

 and winter far exceed the evaporation. There are then five 

 or fix inches of water at leaft to be raifcd up again to the 

 furface in cafe of exigence in the fpring and (ummer. If 

 this happen to be fo, then it is at the expence of fprings ; 

 for we find the generality of fprings become languid, or 

 entirely ceafe to flow at the end of a long drought. As to 

 the few fprings that feem to be little aftefted by dry or wet 

 feafons, they form exceptions, which it would not be diffi- 

 cult to account for. See Evaporation. 



Dr. Halley refers the origin of iprinirs merely to vapours 

 raifed by the heat of the fun, or of fubtcrrancous fires, 

 from the fea, lakes, rivers, &c. 



Now, to fhew that vapour is a fufficient fund to fupply 

 all our fprings, rivers, &c. the fame excellent author makes 

 an eftimate of the quantity of vapour raifed from the Medi- 

 terranean fea, by the aftion of the fun : for the refult of 

 which, fee Vapour. 



Yet the quantity of water thus raifed, great as it is, is 

 only the remains of what is raifed another way ; viz. by 

 the winds, which fometimes fweep the water off much faft er 

 than the fun takes it up. 



To find, now, the quantity of water the Mediterranean re- 

 ceives, allow the moil confiderable rivers it receives, viz. 

 the Iberus, Rhone, Tiber, Po, Danube, Neifter, lioryf- 

 thenes, Tanais, and the Nile, each to furnifh ten times as 

 much water as the Thames ; not that any of them are in 

 reality fo great, but fo to allow for the Icffer rivulets ; but 

 the Thames is found, by calculation, to evacuate 20,300,000 

 tons of water daily. All the nine rivers above-mentioned, 

 therefore, will only evacuate 1827 milhons of tons in a day; 

 which is little more than a third of what is raifed in that 

 time in vapour. The whole Mediterranean, according to cal- 

 culation, is computed to contain 160 fquare degrees, yield- 

 ing at leaft 5280 millions of tons. (Phil. Tranf. N° 189. 

 or Abr. vol. li. p. 1 10.) For a more correft ellimate by 

 Mr. Dalton, fee River. , 



The prodigious quantity of water remaining, Dr. Hal- 

 ley allows to rains, which fall again into the fcas, and for 

 the purpofes of vegetation, &c. As to the manner in 

 which this water is coUefted, fo as to form refcrvoirs for 

 the different kinds of fprings, it feems to be this : the tops 

 of mountains, in general, abound with cavities and fubtcr- 

 raneous caverns, formed by nature to ferve as refervoirs ; 

 and their pointed fummits, which feem to pierce the clouds, 

 ftop thofe vapours which fluftuate in the .itmofphere, and 

 being condenfed by them, they precipitate in water, and 

 by their gravity cafily penetrate through beds of fand and 

 lighter earth, till they are (lopped in their defcent by more 

 denfe (Irata, as beds of clay, Hone, &c., where they form 



a bafon or cavern, and work a paflage horizontally, and 

 ill'ue out at the fides of the mountains. Many of thefe 

 fprings difcharge water, which running down between the 

 ridges of hills, unite their itreams, and form rivulets or 

 brooks ; and many of thefe uniting again on the plain, be- 

 come a river. 



After all that can be faid in favour both of rain and Ta- 

 pour, it mull be owned, they are both ilill encumbered with 

 great difficulties ; though by no means infuperable. 



The perpetuity of divers fprings, always yieldiHg the 

 fame quantity of water, when the leail rain or vapour \s 

 afforded as well as the greateft, is a llrong objection to 

 both. Dr. Dcrham mentions one in his own parilh of Up- 

 minfter, which he could never perceive by his eye to be 

 diminiflied in the greateft droughts, even when all the ponds 

 in the country, as well as an adjoining brook, had been dry 

 for fevcral months together ; nor ever to be increafed in the 

 moft rainy feafons, excepting perhaps for a few hours, or 

 at moft for a day, from fudden and violent rains. Had 

 this fpring its origin from rain or vapour, there would be 

 found an increafe and decreafe of its water correfponding 

 to thoic of its caufes ; as we aftually find in fuch tempo- 

 rary fprings as have undoubtedly their rife from rain and 

 vapour : add to this, another confiderable thing in this Up- 

 miniter fpring, and thoufands of others ; viz. that it breaks 

 out of fo inconfiderable an hillock, or eminence, as can have 

 no more influence in the condenfation of the vapours, or 

 Hopping the clouds, than the lower lands about it have. 

 The very higheft ground in the country he finds is not 

 above a hundred and thirty-three yards above the level of 

 the fea ; and what is fuch an inconfiderable rife of land, 

 to perennial condenfation of vapours, fit to afford fo con- 

 fiderable a fpring ? or the high lands of the whole country, 

 to the maintaining all its fountains and rivulets ? 



Other riaturalifts, therefore, have had recourfe to the fea, 

 and derive the original of fprings immediately thence ; but 

 how the fea-water fhould be raifed up to the furface of our 

 earth, and even to the tops of the mountains, is a difficulty, 

 in the folution of which they cannot agree. Some fancy a 

 kind of hollow fubcerranean rocks to receive the watery va- 

 pours raifed from channels communicating with the fea, by 

 means of an internal fire, and to aft tiie part of alembics, in 

 freeing them frwm their faline particles, as well as condenfing 

 and converting them into water. This kind of fubterranean 

 laboratory, ferving for the dilHUation of fea-water, was the 

 invention of Defcartes. (Princip. part iv. J 64.) Others, 

 as M. de la Hire (Mem. de I'Acad. an. 1703), &c. fet 

 afide the neceflity of alembics, and think it enough 

 that there be large fubterranean refervoirs of water at 

 the height of the fea, whence the warmth of the bottom 

 of the earth, or even the central fire (if there be fuch a 

 thing), may raife vapours ; which, pervading not only the 

 intervals and fillurcs of the ftrata, but the bodies of the 

 ftrata themfelves, at length arrive near the furface ; where, 

 being condenfed by the cold, they glide along on the firll bed 

 of clay they meet with, until an aperture in the ground lets 

 them out. M. de la Hire addi!, that the falts of ftones and 

 minerals may contribute to the detaining and fixing the va- 

 jjours, and converting them into water. But we have a ftill 

 more natural and eaiy way of exhibiting the rife of the fea- 

 water up into mountains, &c. by putting a little heap of 

 fand, arfies, a loaf of bread, or the like, into a balon of 

 water ; in which cafe the fand. Sic. will rcprefent the dry 

 land, or an idand ; and the bafon of water the fea about it. 

 Here the water in the bafon will rife to or near the top of 

 the heap, in the fame manner, and from the fame principle, 

 as the waters of the fea, lakes. &c. rife in the hilU. The 



principle 



