20-4 UtfCHRS Of THE LAKE OP OEWfeVjk. 



water, one mach more extenfive than the other, and conrteflecf 



by a narrower portion or gut. He then tfates that if the at* 



mofpheric preflure be greater upon the larger furface than on 



the fmaller, the firil will be depreffed and the latter will rife, 



and that tru difference of elevation in each furface occafioned 



by the palling of any given quantity of water will be greater 



the fmaller the furface. 



No atmofphcric This is very true; but it can in no cafe happen, that the 



change caa make difference between the level of one water and the other can 

 a greater alter- 

 ation in the hke am <>unt to a greater quantity than that ot a water barometer, 



than the corref- by a jjk e change, namely, about fourteen lines for every line 



fill of a water °* var »ation in the common barometer. That is to fay, if the 



barometer, barometer were to rife and fall again through half an inch, in 



tib 'than reatfy ^ e murt t,me °* a ^ e ' c ' ie * which I believe fcarcely if ever hap- 



ukes place. pens, the feiche itfelf could not rife above (even inches. The 



whole range of variation in the barometer could only caufe a 



rife of three feet and a half iniiead of five which fometimes 



happens. 



Another theory * would venture to conjecture that this phenomenon is one 



ctfered; that the among the numerous ofcillatory procelfes which take place 



•"toiretneron w ' ,en two variable natural powers are oppofed to each other 



the rigidity of in the production or modification of any event. Moft fmall 



iity ? Ji dtfehar re" ' a ^ es are ^ ormec * Dv l * ie enlargement of a river, by which the 

 Jake is fupplied at one end and evacuated at the other. The 

 quantity of water in the lake itfelf will, in thefe circumftances, 

 be always more than would be fufficient to fill its capacity, 

 taken from the level of the loweft point of difcharge. How 

 much more it may be than this quantity will depend upon the 

 ftreams which enter and pafs out. An increafe in the quantity 

 of (apply will keep the level higher, and fo likewife will any 

 increafe in the obftacles to its flowing off; and on the contrary, 

 if the fupply be diminiftied, or if the facility of difemboging 

 be encreafed, the level will be deprefled. Thefe eflfecls will 

 take place moft ftrikingly at firft at that end of the lake where 

 the efficient caufe operates. When any change has once taken 

 place, fuch as that of the depreflion, it will continue for a 

 thort time after the caufe has ceafed to act; fo that the depref- 

 fion would itfelf be followed by a rife, even if the circum- 

 ftances which caufecT it were not alfo fubjeel to a like variation. 



This effedr. is Changes of this kind, on a fmall fcale, are obfervable in mill- 



fctfl in brook-, dams, and even in the frnooth places in brooks or rivalets, as 

 and anil-ponds. * 



may 



