58 PHYSICS. 
pressure is greater than that of the atmosphere; when it is less they are 
themselves kept closed by the pressure of the external air. 3 
Upon the pressure of the atmosphere or compressed air depends thes ac- 
tion of very many important and useful arrangements, apparatus, and 
machines, some of which will here be considered. 
The pressure of the air amounts to about 14 pounds to the square inch ; 
if then a vessel be constructed with an opening of not more than one 
square inch, and the pressure of the fluid therein contained does not exceed 
14 pounds, then when filled it may be covered with a sheet of paper and 
inverted without the escape of the fluids on the withdrawal of the paper, 
this escape being prevented by the atmospheric pressure. Upon this prin- 
ciple depends the straight syphon, fig..54. This is a tubular vessel, con- 
tracted above and still more below, and open at both ends. When dipped 
into a liquid so as to be completely filled, and the thumb placed upon the 
upper extremity, the tube may be elevated without the escape of the liquid, 
which only occurs on the withdrawal of the thumb. The Syphon, fig. 55, 
is a bent tube, bsb’, whose legs are of unequal length. If now the shorter 
limb be immersed in a liquid, and the entire tube filled by suction or other 
means, the liquid will continue to flow from the extremity of the long limb, 
until the opening at 0 is laid bare—provided, however, that the extremity of 
the long limb always occupies a position lower than that of the shorter one. 
For the purpose of more conveniently filling the syphon and obviating any 
danger of getting the fluid into the mouth, a sucking tube, as at fig. 56, is 
frequently attached. Closing the opening at b’, and sucking at ¢, the whole 
limb, sb’, will become filled; the escape of fluid will commence on re- 
moving the obstruction at b’, and continue until the fluid has run off to the 
level bn. This is sometimes called a Poison Syphon. 
In this place belong the various forms of apparatus depending on the 
syphon, and called Cup of Tantalus. They are used principally for pur- 
poses of amusement, or to excite astonishment when a vessel filled with 
water empties itself spontaneously. Fig. 59% represents a mefallic vessel 
divided by a floor somewhere near the middle into two parts. Through an 
opening of this floor passes a glass tube open at both ends, over which a 
larger tube is placed, fig. 59”, hermetically closed above, and with only a 
small opening near the floor to admit the water. On pouring water into 
the vessel it passes through the small aperture into the large tube, standing 
in this as high as in the vessel itself. On risimg as high as the top of the 
smaller tube the water runs over into the lower division, for which purpose 
the latter must have a vent-hole above to admit the escape of air. The 
water will then run off until its level reaches the aperture in the large tube, 
the lower part of the vessel thus becoming full as the upper is emptying. 
The experiment becomes most astonishing when the mechanism is con- 
cealed by some figure. Figs. 57 and 58 represent vessels which, properly 
filled, retain the water when standing erect, allowing it to flow out, how- 
ever, when inclined. The vessel in fig. 57, as in fig. 59, is divided into two 
parts ; through the bottom of the upper one there passes the long leg of a 
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