(12548) C. A. E. H. says: I have seen it stated that the mouths of some rivers are higher than their sources. I can see that this may be true of rivers such as the Mississippi, which flow toward the equator through a number ot degrees of latitude. If the water is actually farther from the center of the earth at the moutli, will you kindly explain the force or forces acting upon it to raise It'.' A. All rivers which flow south In tile northern hemisphere, or north in the southern hemisphere, are farther from the center of the earth at their mouths than at their sources. The earth's diameter Is 26 miles longer through the equator than it Is from pole to pole. The theory is that the earth was once soft. and in rotation, and bulged at the equator by centrifugal force. The earth has cooled and become solid, but the centrifugal force remains. The yielding water takes the position which the rotation of the earth requires it to take. The very definition ot a level surface Is one which Is parallel to tile surface of still water. The Mississippi River flowing south does not take this position. It is below that position at its mouth. It really flows down to the Gulf of Mexico, although in so doing it goes farther from the earth's center. The name of the force raising it farther from the center of the earth is “Centrifugal Force." (12549) ,A. J.. 1. says: Kindly settle the following argument for Ile: A friend ot mine contends that after water has assumed the solid state as Ice, provided the pressure is constant, its temperature will remain at 30 deg. F. so long as the temperature of the surrounding medium remains below 32 deg. F., and that if the surrounding medium was to go down to 8 deg. F'. , the temperature of the ice would remain constant. 1 contend that the temperature of the ice varies with the temperature of the surrounding medium, as also does its volume. A.- Water, in the solid state. Is no different from any other solid. All solids rise and fall in temperature as the place in which they are Is heated or cooled. Everybody knows that ice cream, which is largely frozen water, is cooled much below the freezing point and packed to make it keep. Ice also expands and contracts by heating and cooling, just as iron or any other solid does. People are misled by the fact that ice cannot be heated much above 32 deg. Fahr. It melts. but so does every other crystalline solid when it is heated to its melting point. All such substances cannot be heated above the melting point any more than ice can. The peculiarity ot ice Is that it mefts at a very low temperature. Most crystals melt at a much higher temperature than does ice. Ice when below 32 deg. becomes dry. It is easy to see the difference between ice in cold air and ice in warm air. The writer has cooled ice to the temperature of liquid air, more than 300 deg. Fahr. below zero. It crumbled into very small pieces .by Its coptraction at that temperature. (12550) A. F. H. asks: Will ' you please explain through Notes ' and Queries, tile following terms as applied to steel rails : Horizontal Inertia. Vertical Inertia, Moment of Inertia, Section Modulus, Radius of Gyration? A. Consult one of the books on mechanics. such as Merrlman's “Mechanics of Materials,” ($5 It ordered from our Book Department), for a full explanation. Briefly, “moment of inertia” is a term signifying the sum of the masses ot the particles of a body,' each multiplied by the square ot its distance from the center of inertia, which may be a point or an axis. In the rail section herewith, the sum of the masses of the particles multiplied by their distances squared, would be the polar moment of inertia. it all the distances ran together at the crossing of the axes; or the moment' of inertia about the horizontal axis, if the distances all ran vertically to the horizontal line AA; or the moment about .the vertical axis, It the distances ran perpendicular to that axis. The section modulus is the moment of inertia divided by the distance of - the most distant particle from the pole or the axis of inertia. Its value, like the values of the moment of inertia, may be found in the pock-etbooks, for all standard rail sections. The radius of gyration is the distance from the pole or the axis (as the case may be) of inertia to .that point at which, if a 11 the mass of the body were concentrated, the moment of inertia would be the same as tor the original shape. 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This article was originally published with the title "Notes and Queries" in Scientific American 105, 16, 358 (October 1911)