(27) H. C. P. writes: In the Scientific American of September 18, Mr. B. Y. D., query 26, asks whether a sun dial, made for latitude 48 deg. 15', can be utilized in latitude 38 deg. 50' for showing correct time. To make his dial available in the lower latitudes, he has only to lift the south side, so as to give the face a slope to the north, equal to the difference of the latitude, in this case 9 deg. 25'. For then the plane of the gnomon being in the plane of the meridian, the edge of the gnomon casting the shadow will be parallel with the earth's axis; and the face of the dial will be parallel with the horizon of the latitude for which the dial was made, and the graduation will show the time required; that is, on the supposition that it was correctly made, and for a horizontal dial. (28) O. M. M. asks for a cheap process of plating steel case knives with tin. A. Clean the metal thoroughly by boiling in strong potash water, rinsing, pickling in dilute sulphuric acid, and scouring with a stiff brush and fine sand. Pass through strong aqueous salammoniac solution, then plunge in hot oil (palm or tallow). When thoroughly heated remove and dip in a pot of fused tin (grain tin) covered with tallow. When tinned, drain in oil pot and rub with a bunch of hemp. Clean and polish in hot sawdust. (29) V. R. P. writes: I have an aquarium which contains 4-2/3 gallons of water. How many fish must I have in it--average length of fish 11/2 to 2 inches to insure the health of the fish? At present, I refill the aquarium semi-weekly. Please tell me a process by which I can lengthen the time. A. Put in three fish, 11/2 inches in length, to one gallon of water, one small bunch of fresh water plants to one gallon of water. Tadpoles (after they have cast their branchia or gills), newts, and rock fish can be used to the extent of six to the gallon. The aquatic plants will supply the fish with sufficient oxygen, so that the water will seldom require changing. (30) A. S. writes: I am about to construct an aqueduct 1,200 feet in length, the water level differing 40 feet. By placing a forcing pump in the valley I could then raise the water to a height of 40 feet, and having erected a tank at that height and connected it by means of pipes with another tank 1,200 feet distant, but on the same level, the water according to a law of nature would travel over the distance of 1,200 feet. But finding it very difficult to erect tank 40 feet high, I would prefer to construct the whole on the incline. Will the forcing pump having just power enough to raise the water 40 feet perpendicularly into the tank have sufficient power to force it into a tank of the same elevation through 1,200 feet of pipe running on the incline, or must I have more power, and how much more? A. The forcing pump must have enough more power to overcome its own additional friction and the friction of water in the long inclined pipe. Allow 20 per cent more power at least.