e, T, with the circuit of its coil
left open, be held to the ear, and a powerful magnet, M, be moved
gently up and down along the length of the magnet, as shown by the
arrow, and at a distance of an inch or two from it, a faint breathing
sound will be heard, the recurring pulses of sound keeping time with
the up and down motion of the magnet. The sound may be aptly compared
to the steady breathing of a child, and there is a striking
resemblance between it and the microphonic sounds of gases diffusing
through a porous septum as heard by Mr. Chandler Roberts. We
understand that Professor Hughes is investigating the cause of this
curious sound by help of the microphone.

(11) "Enterprise" asks: What part of its volume will iron expand in
passing from a temperature of 60 deg. to melting temperature? A. The
cubical expansion of iron for each degree (C.) between 0 deg. and 100 deg. is
0.00003546 of its volume, its volume being 1. This ratio however,
increases somewhat at higher temperatures, since the mean coefficient
of expansion for each degree between 0 deg. C. and 300 deg. C. is 0.00004405.
The question you ask has probably never been settled. You may form an
approximation by the use of the above ratios, knowing the melting
point of the iron.

(12) P. L. O. asks for a good chemistry for a beginner to study
without a teacher. A. Fownes' "Chemistry;" Gorup-Besanez, "Inorganic,
Organic and Physiological Chemistry."

(13) L. E. M. asks: What is the best method of keeping fine guns from
rusting, and what oil should be used? A. For the outside, clear gum
copal 1 part, oil of rosemary 1 part, absolute alcohol 3 parts. Clean
and heat the metal and apply a flowing coat of the liquid by means of
a camel's hair brush. Do not handle until the coat becomes dry and
hard. For the inside of the barrel a trace of refined sperm oil is as
good as anything, but an excess should be avoided.

(14) A. H. B. asks how much weight, falling 10 feet, will be required
to produce one horse power for five hours? A. One horse power for 5
hours = 33,000 x 300 = 9,900,000 foot pounds--so that the weight
required is 9,900,000 / 10 = 990,000 lbs.

(15) A. D. R. asks: 1. In renewing a Leclanche battery, do the zincs
have to be amalgamated? A. They are usually amalgamated. 2. Will two
cells large size Leclanche battery give any light, using the simple
lamp described in SUPPLEMENT NO. 162? A. No.

(16) H. L. J. writes: In a recent issue of the SCIENTIFI