WONDERFUL THINGS ABOUT WEIGHTS.

King Country Chronicle, Volume VI, Issue 432, 20 January 1912, Page 2

WONDERFUL THINGS ABOUT WEIGHTS.

TWO THOUSAND MILES UNDER

THE EARTH

We take the word o", scientists that' a body weighs more at the poles than at the equator, and that it is because it is nearer the centre of the earth. If laymen were to reason from that 'simple statement, however,, we should assume that a body would get heavier if we took it down into a deep mine ; for it is certainly getting closer to the centre of the earth. Such, however, is not the case. A body taken under the surface of the earth loses in weight in proportion to distance, until when the centre of the earth is reached it weighs nothing. The same thing will be observed if the body is taken up into the air or on a high mountain ; it, loses its weight. So it is rem that a body always has its greatest weight on the surface of the earth. Tho' law governing this weight (which, of course, is nothing more or less than a measure of gravity attraction) is railed the law of universal gravitation, which states thai every particle of matter in the universe attracts every other particle with a force proportional to the product of the masses divided by the square of the distance between the centres of the masses. From this we deduce several interesting things. Suppose a body is taken two thousand miles under the earth, which is halfway to the centre. If it weighed ten pounds on the surface, it will weigh less than five. Now let us compute how far above the surface we shall have to take it foi it to weigh only that. Working by the formula, we get only seventeen hundred miles ; therefore, we sse that a body loses weight faster as we leave the earth. If this law be read again, the reason a body weighs nothing at the centre of the earth can be readily seen. It states that every particle attracts every other. Suppose, then, we have our body at the centre of the earth i; we should have as many particles on ono side attracting us in the opposite direction, no mattei which way we turned. Of course, then the sum of the attractions would be zero and our body would not move, and, as weight in pounds is only an expression for the attraction of gravity, our body would weigh nothing. Also, it can be seen why a body taken under the surface gets lighter instead of heavier, as it would seem proper for it to do, as it is getting nearer the centre, as before, wc find that a great particles are now attracting the body towards the surface ; but the majority of course are attracting towards tho centre, and the body would go in that direction anc? would, consequently, weigh something, but not so much as at the surface. An easy rule applies to all such cases. It can be proved that a body in a spherical shell, placed at any point in it, is not attracted by the shell at all ; that is, tha sum of the shell's attraction is zero. Of course, a body under the surface of the earth one mile would be inside of a shell of earth one mile thick, which, according to the law, has no attraction. Therefore, a body taken down would suffer the same diminu-

tion that it would if it. was taken to earths of smaller and smaller diameters, and, conssquently, of

smaller and smaller masses. It seems almost impossible that a ball held in one hand is attracting one held in the other hand, ( and that they arc trying to get together. Such however, is the case and can be proved experimentally, and the attraction and tho motion can be measured. Two bras:? halls of known weight were attached to the ends of a rod,, which in turn was suspended by a long thin wire from the ceiling. The positions of the balls were accurately determined with a microscope, and then two big lead balls were placed alongside! the brass ones. The mutual attraction caused the brass balls to move towards the lead ones. .By a n experiment similar to this Cavendish also weighed tho earth. —"Popular Science Sittings."