(10) P. Y. P. writes: 1. To find the number of acres in a farm of valley and hillside land, is it by measuring the general contour of the land, allowing its actual surface, or by measuring and allowing only the imaginary face of the plane of it? A. The latter is the correct method. 2. Can more grain, say rye, be raised on a farm of valley and hillside land, as described above, than on a farm having a flat surface, the area of which is equal to the plane of the former, all other things supposed to be equal? A. No. (11) Inventor asks: 1. Can you tell me of a book on sound boards? A. We do not know of a book especially devoted to the subject. 2. Also the best kind of wood to make them out of? A. Spruce. (12) F. C. A. writes: I wish to construct a bar electro-magnet to go in a cylinder 1 inch in diameter and 1 inch long. 1. What size ought the core to be? What number of wire shall I use, and what number of Léclanché cells shall I use (not to exceed twelve) to obtain the greatest possible attractive power, distance 1/10 of an inch? A. Make the core 3/8 inch, wind it with No. 24 silk covered wire. Use 6 or 8 cells. 2. In the same space, could a horseshoe magnet be used, with a gain of power over the bar magnet? A. A cylindrical magnet, which is substantially the same as a horseshoe, might be substituted with advantage for the bar magnet. (13) W. C. H. writes: In turning a tapering shaft in an engine lathe, will the tool if raised above the centers of the lathe turn the taper true from end to end, _i. e._, neither concave nor convex, the taper to be made by sliding the tail center the required distance? A. The taper will be concave. (14) H. E. H. asks how to make lime light. A. The lime light is made by directing the jet of an oxyhydrogen blowpipe against a cylinder of lime. The blowpipe is contrived to take the proper proportion of oxygen and hydrogen gas, and the lime is placed in the reducing focus of the jet. (15) L. F. asks: 1. How many Daniell's or Smee's cells would it require to produce the same effect as 50 Bunsen cells? A. About 100. 2. Is the diaphragm equally necessary in Bunsen's, Smee's and Daniell's cells, or can it be omitted in any one of them easier than in the others, and why so? A. The diaphragm or porous cell is required in Daniell's and Bunsen's batteries, but is not used in Smee's. The porous cell is used only in two fluid batteries; its object is to allow the current to pass, but to prevent the mixture of the two liquids. 3. Is the thickness of the zinc of any importance? A. Only that the thicker zinc lasts longer. 4. Which is the cheapest way to produce electric sparks and to charge a Leyden jar, and what will be the expense? A. By means of a frictional electrical machine. The machines cost from $10 upward.