Lightning Calculation

One-third memory, one-third practico and one-third trick- t.hat is the secret of most of the rapid calculators who figure before the public. There are very few calculations of which there is not a short way to the solution, but perhaps none of them is so easy and at the same time so surprising to the ordinary mind as the instan taneons extraction of the cube root. This is a feat which has gained great applause for its performers from the days of Hutchins, "the lightning calculator," till now. The extreme rapidity with which it is worked and the difficulty of the solution by the ordinary methods render it one of the most taking of feats. Before explaining the method of perforruing this extraction of the cube root ! it may be well for the benefit of those , readers who have forgotten some of their ; early school knowledge to explain what a cube root is. Multiply a number by itself and the product by the original nuinber, and the result is a cube. Tiras: 9 x 9 = 81 (the square of 9); 81 x 9 =. 729 (the cube of 9). Then 9 is the cube root cf 729. Now for the method. First you obtain from one of the andience an exact cube of not more than six places of figures, thongh with moderate practice the latter condition need not be insisted on. Say the cube given is 140,608, of which the root is 52. You know the cubes of the units by heart, thus: The cube of 1 is 1 The cube of 2 is 8 The cube of 3 is 27 The cube of 4 is t4 The cube of , 5 is. 125 The cube of 6 is 210 The cube of 7 is 343 The cube of 8 is 512 The cube of 9'is 728 Now, as the thousands in the cube given exceed 125 and are less than 216, the tens in the reply must be 5. For the second figure, or units, a curious trick comes in. The cubes of 1, 4, 5, 6 and 9 end in the same figures; the cube of 2 is 8; the cube of 3 ends in 7, and reversely the cube of 8 ends in 2 and the cube of 7 in 3. So when the qnestioner says 140,000 (here you say to yourself 50) 608, you say out loud on the instant, 52. Take another, 39,304. The thousands exceed 27; therefore the root is thirty something. The last figure is 4; fore the root is 84.