e line described; and this, together with his
statement that no such line can be discovered, appears to indicate
that the writer relied upon his knowledge of the laws of magnetism to
state what would be the result, without testing it experimentally. It
is probable that this reversal of polarity is susceptible of
explanation by the known laws of magnetic currents, but if it has
hitherto escaped observation, its discovery is certainly deserving of
notice, and may lead to valuable results. Of the fact, any one may
easily convince himself by the simple experiments above described.

G. H. FELTON, M.D.

Haverhill, Mass., February 28, 1879.

* * * * *

PNEUMATIC CLOCKS.

_To the Editor of the Scientific American:_

In the description of the pneumatic clock, copied from _La Nature_,
and published in your journal of date 1st of March, the invention is
credited to me. Such is not the case. By an arrangement between Mr.
Wenzel, Mr. Brandon of Paris, and myself, patents have been obtained
in France, England, etc., for the clock, and issued in my name; but
the honor of the invention belongs exclusively to Hermann J. Wenzel,
of San Francisco.

Yours faithfully,

E. J. MUYBRIDGE.

San Francisco, Cal., February 27, 1879.

* * * * *

THE ICE CAVE OF DECORAH, IOWA.

_To the Editor of the Scientific American:_

Some years ago I visited the "Ice Cave" of Decorah, Winneshiek county,
Iowa, and having since been unable to receive any explanation of the
wonderful phenomenon exhibited by it, I write, hoping that you or some
correspondent may explain the paradox.

The thriving town of Decorah lies in a romantic valley of the Upper
Iowa River, and the cave is almost within its corporate limits.
Following the left bank of the stream, one soon reaches the vicinity,
and with a hard scramble through a loose shale, up the side of a
precipitous hill, forming the immediate bank of the river, the
entrance is gained--an opening 5 feet wide and 8 feet high. These
dimensions generally describe the cave's section. From the entrance
the course is a steep decline--seldom less than 40 deg.. At times the
ceiling is so low that progress on hands and knees is necessary. About
125 feet from the entrance the "Ice Chamber" is reached. At this spot
the cave widens into a well proportioned room, 8 by 12 feet. The floor
is solid ice of unknown thickness, and on the right hand wall of th