180 ANNUAL OF SCIENTIFIC DISCOVERY. 



importance. I am willing, and Dr. Smith no doubt is willing, that Mr. Mann 

 should have the numbers as he states them. Suppose then that the length 

 of the Mississippi River, measured on a meridian, is only fourteen hundred 

 miles, and that the mouth is only about two and a hah" miles more distant 

 from the earth's centre than the source. The question arises whether the 

 flow of the river from the north to south is caused by the centrifugal force, or 

 whether the criticism of Mr. Mann upon this mechanical solution of the pro- 

 blem is sound. The critic asks : " Why then does not the mighty force which 

 sends the Mississippi up hill four miles send the Nile back to the Mountains 

 of the Moon?" And again he asks : "Why does not the centrifugal motion 

 of the earth drive the waters of the Atlantic and Pacific Oceans towards the 

 equator, at the rate of ninety-six miles a day?" 



Let us attend next to Mr. Mann's own explanation of the flow of the 

 Mississippi. After enlarging upon the protuberant matter at the earth's 

 equator, he continues: "Now water, like eveiy other material thing, is 

 attracted towards the centre of gravity. The centre of gravity is that point 

 about which all the parts are in equilibria. Or, in popular language, water, like 

 everything else, being attracted by matter, is most attracted, other things 

 being equal, by the greatest quantity. The only philosophical idea we can 

 have of up or down is from or towards the point of greatest attraction, that is, 

 from or towards the centre of gravity." Elsewhere, this writer speaks of the 

 earth "being an oblate spheroid, having the greatest quantity of matter, and 

 therefore the greatest attraction, under the equator." Finally he says: "The 

 whole truth is, that the waters of the Mississippi are constantly tending to 

 the common centre of attraction ; but, being prevented from approaching 

 that centre in a direct line, they approach it indirectly, by moving forwards 

 along the bed of the channel. They are constantly approaching the centre 

 of gravity, that is, they are constantly descending.' 11 



One error into which Mr. Mann has fallen is that of supposing that the 

 attraction which the earth exerts at any particular point of its surface is a 

 local phenomenon, and not the resultant of the aggregate attractions of every 

 particle of matter in the earth. This error leads him to a conclusion contra- 

 dicted by the experiments and observations of the last two centuries ; namely, 

 that where there is the most matter, there is also the most attraction, and 

 that consequently the attraction is stronger at the equator than it is at the 

 poles. We might ask Mr. Mann why this mighty force of attraction does not 

 send the Nile back to the Mountains of the Moon. My own answer is, that 

 this excess of attraction at the equator does not exist, and therefore neither 

 carries the Mississippi towards its mouth, nor tends to carry the Nile back 

 from its mouth. To many the assumption will seem a plausible one, that 

 the excess of matter at the equator should be accompanied with a redundancy 

 of attraction there. They forget that the whole earth attracts everywhere. 

 And calculation proves that the attraction of the whole earth upon a body at 

 the surface is greater the nearer this body is to the poles; and for this 

 obvious reason, the excess of equatorial matter operates to the prejudice of 

 equatorial gravity, by keeping the rest of the earth at an unusually large dis- 

 tance. Moreover, it is of no importance to the flow of the Mississippi whether 



