ulating material? A. With proper battery
power, no. 3. How high in the list of non-conductors does paraffine
stand? A. It is one of the best. 4. Will a cotton insulator soaked in
paraffine answer as well as silk? A. No, because it renders the
covering of the wire too thick. 5. Can you recommend any insulating
material for making induction coils which will dry rapidly? A.
Alcoholic shellac varnish. Rosin to which a little beeswax has been
added is an excellent insulator; it must be applied in a melted state.
6. What is the composition of the black material covering the
Leclanche porous cell? A. Gutta percha. 7. Is the magneto-electric
machine described in the SCIENTIFIC AMERICAN SUPPLEMENT patented? A.
To which do you refer? Most, if not all of them are patented.

(8) B. V. F. writes: With reference to item 8, on page 139, of
SCIENTIFIC AMERICAN, March 1, 1879, I think there is some mistake
about the coal you think required to heat 1,000 cubic feet space. I
burn some 8 tons coal to heat, in the whole year, such part of my
house as must exceed 25x20x18=9,000 cubic feet. We keep up a moderate
heat at night. Ventilate more than most families do; take part only of
the cool air, and only in part of the coldest weather, from the
cellar, which at such times is opened into the main entries. House
wood, back plastered, and stands alone. If 100 lbs. coal would heat
1,000 feet one day, I ought to burn 900 lbs. a day, or nearly 14 tons
in December and 14 more in January. A. We are glad to receive these
data, which correspond quite closely with some obtained by recent
accurate experiments. The estimate given in the SCIENTIFIC AMERICAN
also agrees well with experiments on the use of hot air heaters for
very small buildings or rooms. Of course, the larger the space to be
heated, the more economically it can generally be done.

(9) W. M. S. asks: Will the coil described in SUPPLEMENT NO. 160 do
for the electric pen described in a recent number of the SCIENTIFIC
AMERICAN? If not how must it be changed? A. It is too large; make it
one half the size given.

[Illustration]

(10) B. G.--In reply to your inquiry as to Mr. Stroh's telephone
experiment, we give the following, which we clip from the _English
Mechanic_: A singular experimental effect, of special interest just
now from its possible bearing on the theory of the source of sound in
the Bell telephone, has just been observed by Mr. Stroh, the well
known mechanician. If a telephon