On a window-sill in a southerly position, fix in a solid, permanent manner, a small cupful of quicksilver; cover it with a lid made of varnished metal, and pierced in its center by a small round hole about a quarter of an inch in diameter. This lid must fit well, but not too tightly, so as to permit its being lowered in close proximity to the surface of the quicksilver. When the window is open the solitary ray reflected on the mercury will be projected on the ceiling of the room. At the exact noonday the center of the mirror and the center of the reflected image are in the meridian plane. It remains only to be traced. At the moment of its passage one marks in B, for example, a spot corresponding to the center of the reflected image; one knocks a small nail there, and with a string connect this point with another outside the window, so that the string passes through the center of the diaphragm, M. The line, B M, is the meridian plane. From A, suspend a lead-line which meets the string, B M. All you have to do now is to join on the ceiling the points, A B, and continue them to D. A black thread may be stretched to serve as the line, and this is the meridian required. To get the mean time one has only to note the exact passage, and deduct the corrections given in various astronomical papers. To Measure the Height of a Mountain. One can, without instruments, take the height of a building or a mountain, provided you are able to measure their base. A yardstick and two ordinary sticks are enough. Suppose the height of the tower, E F, is to be taken. [Illustration] Some distance off plant a stick, a yard high, A B; one yard from this we plant another and longer one, C D. Measure exactly the distance, B F, and applying the eye at A, we aim at the summit of the tower, E; mark on the stick, C D, the point where the visual ray meets the stick, _i.e._, point G. Then, by measuring the distance, D G, and subtracting one yard you get G I, and may be expressed in the following statement: A H : A I :: E H : G I In the given example let us suppose that A H = 150 yards, A I will, of course, be equal to one yard; G I =, say four fifths of a yard; the problem will be: 150 yards : 1 yard :: _x_ : four fifths of a yard. Work the sum out, and the value of _x_ is 120 yards. Having taken our lease, A H, at one yard from the ground, we must add one yard to 120, making 121 yards, which is the height of the tower wanted. To Take Up Four Knives with One. Here is one more trick of equilibrium, which appears to be interesting enough to find a place among these experiments. We need not give any long explanations, for our figure fully illustrates the way in which it has to be executed. [Illustration]