CATARACTS, AND INUNDATIONS. 81 



waters and to supply them afresh. And in such cases the flux and 

 reflux of the upper head of uater must necessarily depend upon the 

 state of that below; and the causes which alternately augment and 

 diminish the latter must produce a similar effect upon the former. 

 And it is possible that these causes may be, as we shall presently 

 find was long ago ingeniously conjectured by the younger Pliny, 

 regular currents of air produced by the penetrating influence of the 

 sun, a communication with the sea itself: or a periodical return 

 of subterranean heat or other agency below the interior reservoir 

 that may drive additional waters into it, or expand those of which 

 it consists. 



In the present day, however, the common principle upon which 

 this phenomenon is accounted for is that of the hydraulic machine, 

 called the Cup of Tanlalus : an instrument consisting of a vessel fur- 

 nished with a siphon or tube with two legs, the one shorter than the 

 other, and which may be attached to it in different ways. But this 

 will not account for fountains with irregular ebbs and flows; 

 and hence, Mr. Gough, while he attributes the regularly recurrent 

 springs to the explanation of a siphon, has proposed another theory 

 to account tor those of a different kind, and which, in truth, is not 

 far removed from one of the modes conjectured by the younger 

 Pliny. Mr. Gough's theory, together with his explanation of the 

 common theory of the siphon, we shall give in a subsequent part 

 of the present section, allotted to an account of the alternating well 

 at Giggleswick, in Yorkshire. 



In Switzerland springs and lakes of this kind are peculiarly com- 

 mon ; and Mr. Addison in his Travels endeavours to account for 

 those which he met by a different process, but a process however 

 which it must be obvious can only apply to a few. We saw, says 

 he, in his description of Geneva and the lake, in several parts of 

 the Alps that bordered upon us, vast pits of snow ; as several 

 mountains, that lie at greater distance, are wholly covered with it. 

 I fancied the confusion of mountains and holkw s, I here observed, 

 furnished me with a more probable reason than any I have met 

 with, for these periodical fountains in Switzerland which flow only 

 at particular hours of the day. For as the tops of these mountains 

 cast their shadows upon one another, they hinder the sun's shining 

 on several parts at such times, so that there are several heaps of 

 snow which have the sun lying upon them for two or three hour* 



