C AMERICAN you
state that the floating of solid iron on melted iron is on the same
principle as the floating of ice in water. I do not quite understand
how it can be. Please explain. A. Solid iron, at an elevated
temperature, floats upon molten iron for the same reason that ice
floats upon molten ice-water--because it is specifically lighter. You
will find the subject discussed at length in Tyndall's "Heat as a Mode
of Motion."

(17) J. W. will find full directions for canning corn, etc., on p. 394
(4), vol. 39, SCIENTIFIC AMERICAN.

(18) "Amateur" writes: I wish to make some small bells that have a
clear ring. What metal or metals can I use that I can melt easily? A.
Use an alloy of tin and antimony. See SCIENTIFIC AMERICAN SUPPLEMENT
NO. 17.

(19) H.--A nutritious mixed diet is unquestionably the best, care
being taken to avoid an excess of meat.

(20) W. F. writes: I have made an engine, and would like to find out
what size of boiler it will require. The cylinder has 21/4 inch bore and
3 inches stroke. A. It depends upon pressure and speed to be
maintained; probably a vertical tubular boiler, 15 inches diameter,
and 32 to 36 inches high, would suit you.

(21) R. G. (Salt Lake).--Please send full name.

(22) J. M. G. asks: If two persons each pull one hundred pounds on
opposite ends of a rope, what will be the strain on the rope? A. The
strain on the rope will be 100 lbs.

(23) W. M. M. asks: In laying off a mill stone in furrows, what
draught is given? What amount of the space of a stone is given to
furrows and what to grinding surface? A. There is considerable
difference in the practice of various millers, and we would be glad to
receive communications from those experienced in the art of dressing
millstones.

MINERALS, ETC.--Specimens have been received from the following
correspondents, and examined, with the results stated:

S. (New Orleans.)--The powder consists of a mixture of zinc oxide and
finely powdered resin. A quantitative analysis would be necessary to
determine the proportions.

Any numbers of the SCIENTIFIC AMERICAN SUPPLEMENT referred to in these
columns may be had at this office. Price 10 cents each.

* * * * *

COMMUNICATIONS RECEIVED.

Life Preserving Stone. By J. D. W.
On Ventilation. By D. W.
What is Mental Action? By N. K.
Panama Railroad or Canal. By G. R. P.
A Problem. By K.
On the Gary Motor. By G. F. M.
Magnetic Motor. By G