sh. Fifty-two
degrees of cold is thus easily obtained, and the machines used for the
purpose can produce several tons of ice each day in the hottest
countries. Much artificial ice is now made by compressing atmospheric
air, and by this method a freezing temperature is obtained on vessels
employed in carrying fresh meats from distant countries.

INK BOTTLE.--1. Mineralogists apply the term "pyrites" to a large group
or family of minerals, compounds of metals with sulphur, or with
arsenic, or with both. The name was originally given to the sulphuret of
iron, known as iron pyrites, in consequence of its striking fire with
steel (from the Greek _pyr_, fire), and it was used for kindling powder
in the pans of muskets before gun-flints were introduced. Iron pyrites
is commonly of a bright brass-yellow color, and is found crystallized in
cubes, dodecahedrons and many other forms. It is a very widely diffused
and plentiful mineral, and seems to belong almost equally to all
geological formations. 2. Eagle cents issued in 1858 are of no value to
collectors, because they lack rarity. 3. Your exchange is too trivial.

J. B. D., of Chicago, kindly informs us that he has been able to get a
slight shock from a telegraph battery in the following manner: "On every
learner's instrument there are two binding-posts, and to one of them is
joined a wire from the battery; a small file is fastened to the other;
the key is closed, and then the other wire of the battery is taken in
your wet fingers, and, with the other hand, also wet, upon the file,
the wire is run along the surface of the file, and a shock results."

WALTER R.--What is known as the registry system is intended to secure to
valuable mail-matter in its transition through the mails the utmost
security within the province of the Post Office Department. The fee on
any registered matter, domestic or foreign, is fixed at ten cents on
each parcel or letter, to be affixed in stamps, in addition to the
postage. The money-order system is intended to promote public
convenience, and to secure safety in the transfer through the mails of
small sums of money. The rates may be ascertained by inquiring at a
local office.

AN ADMIRER OF G. D.--1. Two French scientists, Captain Renard and
M. Tissaudier, have invented a balloon whose motive power is
electricity. The dynamo machine used by them is an intensely
concentrated bichromate battery of one and a half horse-power. It is
very light, weighi