Robert's age and 99, is 77 To which George adding his age 35 ---- The sum will be 112 ---- 12 1 ---- Then by taking away the first figure, 1, } and adding it to the last figure, 2, } 13 the sum is } Which added to Robert's age 22 ---- Gives George's age, which is 35 _The Boundless Prospect._ Take a square box, about six inches long and twelve high, or of any other proportionate dimensions. Cover the inside with four flat pieces of looking-glass placed perpendicular to the bottom of the box. Place at the bottom any objects you please, as a piece of fortification, a castle, tents, soldiers, &c. On the top, place a frame of glass shaped like the bottom of a pyramid, as in Fig. 8, and so formed as to fit on the box like a cover. The four sides of this cover are to be composed of ground glass, or covered inside with gauze, so that the light may enter, and yet the inside be invisible, except at the top, which must be covered with transparent glass: when you look through this glass, the inside will present a pleasing prospect of a boundless extent; and, if managed with care, will afford a deal of amusement. [Illustration: Fig. 8.] _To set Fire to a combustible Body by Reflection._ Place two concave mirrors at about twelve feet distance from each other, and let the axis of each be in the same line. In the focus of one of them place a live coal, and in the focus of the other some gunpowder. With a pair of strong bellows keep blowing the coal, and notwithstanding the distance between them, the powder will presently take fire. The mirror may be either made of glass, metal, or pasteboard gilt. _To find the Number of Changes that may be rung on Twelve Bells._ Multiply the numbers from 1 to 12 continually into each other, as follow: and the last product will give the number required. 1 2 -- 2 3 -- 6 4 -- 24 5 ---- 120 6 ---- 720 7 ----- 5,040 8 ------ 40,320 9 ------- 362,880 10 --------- 3,628,800 11 ---------- 39,916,800 12 ----------- 479,001,600 _To find how many square Yards it would require to write all the Changes of the Twenty-four Letters of the Alphabet, written so small, that each Letter should not occupy more than the hundredth part of a square Inch._