OF ARTS AND SCIENCES. 233 



the attraction is greatest, then also the Mississippi runs up. We can- 

 not say, with Mr. Mann, that down means towards the earth's centre 

 of gravity, because the earth has no single centre of gravity. His 

 definition of up and dow7i, therefore, is without any meaning, and is 

 not, as he says, based upon the only philosophical idea we can have 

 of these terms. The only standard level of altitude is the surface of 

 equilibrium. If we understand by down ' below the surface of equi- 

 librium,' and by up ' above the surface of equilibrium,' then our defi- 

 nitions will be as broad as nature's laws, and #ill lead to no para- 

 doxes, all of which nature abhors more than a vacuum : then all the 

 rivers will be found flowing downward. On a small scale, and in 

 local mechanics, an inclined plane is one which is inclined to the local 

 plumb-line. But on a large scale, such as will take in the whole length 

 of a great river, every plane surface is inclined to every plumb-line 

 but one, and the surface which is not inclined, and on which, there- 

 fore, a body has no tendency to slide, is a surface which is everywhere 

 perpendicular to the plumb-lines which intersect it ; that is, it is the 

 earth's surface of equilibrium. This is the only true, broad, and uni- 

 versal standard of level. 



" It may be concluded from what has been said, that the new hy- 

 drostatic paradox is of man's invention, and that nature is in no way 

 responsible for it. Science abounds in such paradoxes ; and men of 

 science are too prone to array the merest truisms in paradoxical lan- 

 guage which catches the popular ear, though at the sacrifice of making 

 science itself vulgar. Moreover, if the explanation which I have given 

 of the paradox under consideration is beyond the knowledge or above 

 the comprehension of a child, then the question which involves it is 

 unfit to be addressed to him." 



Professors W. B. Rogers and D. Treadwell offered some re- 

 marks upon the subject, and expressed their concurrence in 

 the view taken of it by Mr. Levering. 



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 FROM OCTOBER, 1854, TO APRIL, 1856. 



Academy of Natural Sciences of PhiladelpMa. 



Proceedings, Vol. VII., Vol. VIII. No. 1. 8vo. Philadelphia. 

 Journal. New Series. Vol. III. Part II. 4to. Philadelphia. 

 VOL. III. 30 



