ace, a portion
at a time, with good glue; immediately roll the glued surface in emery
spread out on a board. 2. How can I fasten small pieces of looking
glass on iron? A. Use equal parts of pitch and gutta percha together.

(3) W. C. asks: 1. What is the power of the simple electric light
described in SUPPLEMENT NO. 149? A. When supplied with a strong
current it is equal to 5 or 6 5-foot gas burners. It is designed for
temporary use only. 2. What is the cost of manufacturing the
dynamo-electric machine in SUPPLEMENT NO. 161? A. The one shown in the
article referred to cost about $35.

(4) L. D. asks: 1. Which is the better conductor, silver or copper? A.
Silver. 2. And the comparative resistance offered to the electric
current by water and the above? A. Taking pure silver as 100,000,000,
the conductivity of distilled water would be 0.01.

(5) H. J. F. writes: In SUPPLEMENT 162 a simple electric light is
described. I wish to light a room 20x20x10 feet. 1. How large is the
bell glass? A. 21/2 inches. 2. Can I use battery carbon? A. Use a
carbon pencil made for electric lamps. 3. How can I make tray water
tight after putting wire through? A. With gutta percha. 4. I have one
large cell Bunsen and one Smee. How many more and of what kind shall I
get? A. One of the batteries described in SUPPLEMENTS 157, 158, 159,
will do, probably 8 or 10 Bunsen elements would be the best.

(6) W. B. F. writes: I tried to make an electric pen, like the one
described in your SCIENTIFIC AMERICAN, of February 22d, 1879, using a
Smee's battery, a circuit breaker, and an induction coil, but it did
not work. Is there anything wrong, or is a condenser different from an
induction coil? A. A condenser consists of a number of sheets of tin
foil separated from each other by larger sheets of paper. One half of
the tin foil sheets are connected with one terminal of the primary
coil, the other half with the other terminal; the tin foil sheets
connected with one terminal alternate with those of the other
terminal. The condenser is essential to the working of the coil. For
complete directions for making induction coils, see SCIENTIFIC
AMERICAN SUPPLEMENT NO. 160.

(7) J. De F. asks: 1. Knowing the resistance of a wire of given
conductivity, length, and diameter, will the resistance of any other
wire be in proportion inversely? A. Yes. 2. Is there heat enough
developed in the secondary coil of an induction coil to prevent the
use of paraffine as an ins