er,
6,900 lb. 2. What is the contents (in gallons) of a tank 15 feet deep,
10 feet in diameter, top and bottom diameters being equal? Please give
me a formula. A. Area of 10 feet diameter = 78.54 x 15 feet deep = 1,178
cubic feet, and, allowing 71/2 gallons per cubic foot = 1,178 x 7.5 =
8,835 gallons.

(26) C. L. W. writes: I have constructed a small induction coil to be
used for giving shocks. It is 3 inches long. The primary coil is wound
with 3 layers of No. 18 cotton covered wire, and the secondary consists
of about 12 layers of No. 38 silk covered. 1. How many cells and what
kind of battery shall I use to get the best results? A. For temporary
use one cell of Grenet battery would answer, but for continued use some
form of sulphate of copper battery is to be preferred. 2. Is it
necessary that the spring and screw in the interrupter should be coated
with platinum? A. Yes; otherwise they would soon burn out.

(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 se