402

AMERICAN RAILROAD

J OURN AL.

and lastly that the work would require the hands of boiling point, amounts under the latter heat to a
force higher than that under the former by about 37

more men than could. be fed in the actual Situation
of the country. Here our operations in this quarter
ended. The metal was probably within our reach,
but if we had found it, the expense of carrying tt to
Montreal must have exceeded its marketable value.
It was never for the transportation of copper that
our company was formed, but always with a View
to the silveriwhich it was hoped the ores, whether
of copper or lead, might in suflicient quantity con-
tain.’

Eighteen years after the failure of this attempt,
(1789) l\/Iclienzie passed through Lake Superior on
his first voyage of discovery into the north west, and
in his description of Lake. Superior says———* On the
same side (the south) at the river Tennagon is found
a quantity ofvirgin copper. The Americans, soon
after they got possession of that country, sent an
agent thither, and I should not be surprised to hear
of their employing people to worlc the mine. In-
deed it might be well worthy the attention of the
British subjects to work the mines on the north coast,
though they are not supposed to be so rich as those
on the south.’ll

The attention of the United States Government

appears first to have been turned toward the subject
during the administration of President Adams,
when the sudden augmentation of the navy render-
ed the employment of domestic copper in the equip-
ment of ships, an object of political as well as pe-
cuniary moment; and a mission was authorised to
proceed to Lake Superior. Of the success of this
mission, as it has not been communicated to the pub-
lic, nothing can with certainty be stated, but from
the inquiries which have been instituted during the
recent expedition, it is rendered probable that the
actual state of our Indian relations at that period,
arrested the advance of the commissioners into the
regions where the most valuable beds of copper
were supposed to lie, and that the specimens trans-
mitted to government were procured through the in-
strumentality of some friendly Indians employed for
that purpose.”

Ilnprovernent in Diving Bells.

The principle of the diving—bell is illustrated fa-
miliarly by holding the mouth of a glass tumbler
evenly on the surface of water, and then immersing
it without allowing the air included within it to es-
cape. , A paper placed on the inside of the tumbler
will be found after the immersion perfectly dry.

Air, owing to the repulsive property of its mole-
cules, has always a tendency to enlarge its volume.
This tendency is called its tension or elasticity; and
differs from the elasticity of solids and fluids, inas-
much as, continuing to act: in every degree of den-
sity, it appears to have no limit. The Law of lVIa-
riotte shows that the tension of air is in the inverse
ratio of its volume: it is therefore measured by the
force that, increasing the density, compresses the
volume. At the level of the sea the compression is
such as to give a tension or expansive force of about
14-76 lbs. avordupois to the square inch : this is tali-
en as the standard pressure of what is called one at-
mosphere. if atmospheric air under the piston ofa
cylinder be taken at a pressure of 15 lbs. on the sq.
inch, the density by pushing the piston half-way in-
to the cylinder is doubled; and the pressure of the
air is therefore raised by that means to 30 lbs. on
the square inch. Again: the higher the tempera-
ture of air the greater is its expansive force; and
this increase of force, rising in equal proportions
along every degree between the freezing and the

per cent.

a column of water 33 feet high.

density and tension are double that under ordinar
circumstances, or about 30 lbs. to the square incl

mersion coupled with the increase of this pressui
occasioned by the lights sometimes used in the bel

temperature within.

The pump must be able to throw in at every strolr

the volume consumed by the men during the period during the period of immersion.

Now if a chest of air be lowered with the mouth
downward to a depth of 33' feet in water the air
within the chest will sustain besides its original one-
atmosphere pressure an additional pressure equal to
But a water-col
umn 33 feet high is, as illustrated by the common
sucking-pump, equal to one-atmosphere; and con-
sequently the pressure on the air within the chest
being equal at 33 feet deep to two (Ll77t.()Sfl>LC7'£’S, the

Bul the pressure is still further increased by an in-
crease of temperature as seen above; and therefore
the diving bell may be either dangerous or fatal un-
der the pressure resulting from certain depths ot'im—

insufliciency or irregularity in the supply of air, or
any other circumstance leading to an increase 0!

The supply ofair must be adapted to the depth of
the water anti the consumption of the worl:men.—— the bell from a forcing pump through a leather hose;

sure of the head above it. This may of course be
done by lessening or decreasing the number of
strokes within a given time. Wlien the water rises
inconveniently within the bell the men signal for an
increased supply; and no other inconvenience than
that of tension results from a super—abundant sup-
ply, seeing that the excess will pass off under the
edges of the bell, and break in bubbles on the sur-
face. Sl’I16£1tOtl’S bell has undergone such improve-
ments from time to time that it is now quite safe and
highly useful in hydraulic works. The bell used
by Alexander Nimo at VVellsley Bridge on the
Shannon in Ireland, and that usect on the other
works of that river were very fine examples of the
diving bell. The bell at the government dock yard,
Plymouth, England, is cast iron, weighing some
four tons. It is 6 feet by 4 at the mouth, and 5 feet
high. Mr. Frazer has published some ingenious
improvements inthe diving belt in the Transactions
of the Institute of Civil Engineers of Ireland for
18-16.

A series of convex lenses on the top admit suffi-
cicnt light into the bell to enable the men to work
i11 very considerable depths. The air is passed into

y
l.

n‘.

17

c relays of men working the pump in regular strokes
Apla ofleather

of the stroke, and a further supply equal to the pro- on the inside is secured over the air hOlL the spaces
duction of a volume to fill the bell under the pres- between the screws admitting the passage of the air

ig. 1.

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ll McKenzie’s Voyages, p. 29.

  

     

    

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