WHY DO SFEIJVGS AND WELLS OVERFLOW? 405 



" This was believed to have been found in a locality not far from 

 Dijon ; but there as well, in spite of appearances, the rain-waters re- 

 ceived on the portion of land overlooking the spring could amply suf- 

 fice for its supply." 



After referring to the former ignorance of people concerning the 

 quantity of rain, of dew, and of snow, falling in different regions, 

 Arago continues : " For example, people did not believe that the basin 

 of the Seine .... received annually by rain a quantity of water equal 

 to the tribute which the Seine bears to the sea in the same space of 

 time. Perrault and Mariotte first studied the question experimentally^ 

 and they found, as is usual in such cases, that the vague conceptions 

 of their predecessors were precisely the opposite of the truth. . . . 

 The volume of water which passes yearly under the bridges of Paris 

 is hardly the third of that which falls in rain into the basin of the 

 Seine. Two thirds of that rain either return into the atmosphere by 

 evaporation, or sustain vegetation and the life of animals, or drain 

 into the sea by subterranean passages." 



Without insisting further on the fact that the rain-waters, dews, 

 and snows falling on higher grounds must be sufficient to account for 

 all flowing springs and wells (except, possibly, such cases as the gey- 

 sers), let us see how Mr. Green's subterranean water-deposits are to be 

 driven to the surface of the earth by his " newly discovered force." 

 Why, by making the eartb's centrifugal force act in the direction of 

 the tangent to the earth's surface, and then getting the resultant of 

 this force and of gravity ! Further, since the question of the relative 

 intensities of these two forces " does not enter into the problem," you 

 may assume that they are equal, and thus you will find that "the 

 direction of the resultant itself is, say, 45° from the direction of the 

 force of gravity. . . . Moreover, since the resultant has been shown " 

 (by saying that the diagonal either of a square or of a parallelogram 

 is longer than either of its sides) " to be greater under all circumstances 

 than gravity, certainly the vast aggregations must also be greater than 

 the aggregated gravity, and will be able to overcome it under the con- 

 ditions stated. . . . The intensity of the centrifugal force will increase 

 with the distance from the center of the earth, while gravity will de- 

 crease ; the resultant ioill also increase. Thus we find the strongest 

 and most abundant overflows at the tops of mountains or on high 

 plateaus." 



As a specimen of mechanical exposition this is almost unique,* but 

 it is too ludicrous to mislead. In point of fact, as every schoolboy ought 

 to know, the centrifugal force due to the earth's rotation, on a particle 

 at any place on the earth, does not act in the direction of the tangent 



* Mr. Green is not quite the first writer who, in undertaking to overthrow a well- 

 established mechanical explanation of natural phenomena, has assumed that the earth's 

 centrifugal force acts in the direction of the tangent to its surface. (See discussions on 

 " The Tides," in vols. xi. and xii. of this magazine.) 



