This is represented below. It is balanced on a pivot at F. The light hollow brass ball, A, represents the sun, and pith balls, B and D, the earth and moon, rotating about the pivot E. The metallic points projecting from B and D (in opposite directions, of course) cause these to rotate round each other; hut the leverage of the point D being, From its position, greater than the Leverage of B, it sets the long arm of the orrery in rotation upon the pivot F. Galvanometer, To make a simple.-Take an ordinary pocket -compass and wind 100 feet of No. 18 insulated copper wire around it.

Electrical Orrery

Kaleidoscope, to make a. Take two strips of glass, 8 or 10 in. long, 1 to 1-1/2, in. broad at one end and about 1/2. as broad ;at. the other. Blacken one side of each with black varnish. put two smooth straight edges together, and form a hinge by gluing a stripof cloth over the two edges Make the angle between the strips of glass an aliquot part of 180 as 20°, 30°, or 45°. Cover the open side of the triangular prism with black velvet. Place in a tin or pasteboard tube so that the angle of the smaller end of prism is nearly in the centre. Cover top of tube with clear glass, and cover this with paper, except a small hole in centre. In bottom of tube, form a cell by placing two pieces of glass 1/8 in. apart (the lower one of ground glass). In this cell place fragments of broken colored glass, beads, etc. They must be capable of free movement in the cell when the tube is turned in the hand.