182 ANNUAL OF SCIENTIFIC DISCOVERY. 



direction and the velocity of the flow are intimately connected with the centrifu- 

 gal force. "Without a rotation, and the centrifugal force which rotation produces, 

 the earth's figure of equilibrium would be a sphere. In this event, the Mis- 

 sissippi would flow northward. Its southern direction, under existing circum- 

 stances, may therefore be fairly attributed to the centrifugal force. If the earth 

 did not rotate, and the sphere were the figure of equilibrium, the Nile would 

 flow in direction as it now does, but much more rapidly. Under existing cir- 

 cumstances, the same centrifugal force which accelerates the flow of the Missis- 

 sippi retards the flow of the Nife. 



If the inquiry be made whether the Mississippi runs up Mil or down, I 

 reply that this is simply a question of definition. If down means towards the 

 earth's centre of figure, then the Mississippi runs up. If down means towards 

 that part of the earth's surface where the attraction is greatest, then also the 

 Mississippi runs up. We cannot 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 down, 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 equilibrium," and by up " above 

 the surface of equilibrium," then our definition will be as broad as nature's 

 laws, and will lead to no paradoxes, all of which nature abhors more than 

 a vacuum : then all the rivers will be found flowing downwards. 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, therefore, 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 universal standard of level. 



It may be concluded from what has been said, that the new hydrostatic 

 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 language 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. 



ON THE ORIGIN AND TRANSFORMATION OF MOTIVE POWER. 



The following paper, by Prof. ~Wm. Thompson, recently read before the 

 Koyal Institution of Great Britain, is one of the most valuable contributions 

 to physical science made during the past year : The speaker commenced by 

 referring to the term work done, as applied to the action of a force pressing 

 against a body which yields, and to the term mechanical effect produced, which 

 may be either applied to a resisting force overcome, or to matter set in motion. 

 Often the mechanical effect of work done consists in a combination of those 

 two classes of effects. It was pointed out that a careful study of nature 



