Weighing the World.

Taranaki Daily News, Volume XLVII, Issue 7755, 6 March 1905, Page 3

Weighing the World.

I "In practice, all the methods of weighjing the earth," writes Mr W.A. iShenstone in the Cornhill Magazine;, j-'-'resotve themselves into experiments in which we measure the attraction between two bodies having known | masses placed at a known distance from each other on the earth's surface, and then compare this with the attraction of the earth on some known mass of matter, also on its' surface. The following illustration, taiken from a lecture by Professor J. H. 'Poyntmg, will moke the idea clearer " ' Suppose yon hang a- weight of 501b from ' a spring balance a few feet above the earth. Then the pull of the earth, whose centre >is about 4000 miles or 20,000,000 ft away, is 50Jb Now suppose you bring a second weight, this time let us ray a weight of 3501b, to a position 1 foot from the first one, and between the hitter and the earth, so that its pull ■is added to that of the earth. Then, if your balance is sufficiently sensitive, you will- find the smaller mass no longer weighs 50Tb, but a little more—in fact, about - l-250th of a grain more—that is to say, the pull of the 3501b weight at the distance of Ift is equal to the l-250th of a grain, or 1-1,750,000 of lib, or the pull of the earth at : a distance of the earth would be 400 billion times 90,000,000 times that of a 3501b sphere. But, as already at equal distances these pulls are proportional to the masses concerned, ami thus, by doing a little, more arithmetic, wc should find that the earth weighs about 12,500,000,000,000.000, 000,000,0001b.''