144. HISTORY OF 



If this acquired velocity be quite spent, and the plain througb 

 which the river passes is entirely level ; it will, notwithstanding, 

 still continue to run from the perpendicular pressure of the wa- 

 ter, which is always in exact proportion to the depth. Tliis 

 perpendicular pressure is nothing more than the weight of the 

 upper waters pressing the lower out of their places ; and conse- 

 quently driving them forward as they cannot recede against the 

 stream. As this pressure is greatest in the deepest parts of the 

 river, so we generally find the middle of the stream most rapid •, 

 both because it has the greatest motion thus communicated by the 

 pressure and the fewest obstructions from the banks on either side.* 



sometimes cover spaces of several hundred leagues in length and breadth. 

 Such are tlie famous lakes of Xarayes and Paria, inscribed on maps of 

 America, and expunged from them by turns; it is probable that Africa con- 

 tains a great many of this description. If there exist now in the numerous 

 cavities of the earth subterraneous lakes of this kind, and if these communi. 

 cate with other lakes which are visible, it is easy to imagine that the waters 

 of these last may sometimes entirely disappear, by sinking down into the 

 basin of the subterraneous lakes in proportion as thoy dry up. This lower 

 basin again filling itself anew, the waters issue from it to till the superinr 

 basin. If, in a supposable series of subterraneous cavities, the last link of 

 tiie chain happen to be a mass of subterraneous water, situated at an elevated 

 level in the bosom of a mountain, the periodical return of the waters in the 

 visible basin may be accompanied by a motion similar to that of the spouting 

 fountains. It is by means of such hydraulic machinery that nature keeps up 

 the wonders of the lake of Cirknitz in Illyria, and in many others of the 

 same description. 



The variations and motions of lakes, which do not depend upon an augmen. 

 tation of quantity, present very complicated questions : That any lakes com- 

 municate under ground « ith the sea, and owe their regular tides to such 

 communication is much to be doubted. The equilibrium of the atmosphere, 

 deranged by electricity, or by any other cause, may occasion water to rise up, 

 by altering the pressure which retains it at its level. There is a bay in lake 

 Huron where electric clouds continually remain, and no traveller has ever 

 passed it without hearing thunder. In Portugal there is a priol near Beja. 

 in Alentejo, which, by its loud rtnise, ind'cates the approach of a etorm. 

 Other lakes appear agitated by the disengagement of subterraneous gases, 

 or by winds which blow in some cavern with which the lake communicates. 

 Near Boleslaw, in Bohemia, a lake of unfathomable depth sometimes in win- 

 ter emits blasts of wind sufficiently strong to raise up in the air pieces of ice 

 several quintals in weight. Two considerable lakes, Loch Loinond in Scotland, 

 and the Welter in Sweden, often experience during the serenest weather vio- 

 lent agitations. In the Murche of Brandebaurg, the pool of Krestin often 

 commences in fine weather to boil up in whirlpools so as to engulph the little 

 boats of the fishermen. Perhaps the decomposition of calcareous stones has an 

 influence upon some of these phenomena. 

 * Many great rivers in factttow with an almost imperceptible declivity. ITie 



