250. To cause the flow of the water in the syphon, it
is essential that one branch of the tube should be longer
than the other ; and the motion is toward the longer
branch. The water flows out of the longer branch in
consequence of its weight ; but as a vacuum would thus
be produced in the upper part of the tube, the water from
the vessel rises in it by atmospheric pressure, as in the
suction-pump. If the longer leg were immersed in the
water instead of the shorter one, and then filled by ex-
hausting the air by the mouth, the liquid would immedi-
ately flow back into the vessel. The length of the
branch immersed in the vessel is always to be measured
from the surface of the water, D E. That the atmos-



QirBSTiON ^9. Of what does the syphon consist? How is the tube to be filled at
first ? 250. Must one branch of the tube always be longer than the other ? In what di*
rection does the water flow 1 How is It shown that the pressure of the atmosphere ii
necessary to cause the water to flow through the syphon? Why may not large sr •
prions be used with adrantage for practical purposes?



PNEUMATICS.
161:



pheric pressure is concerned in the operation of the
syphon is shown from the fact that it entirely fails to act
in a vacuum ; and also from the further fact that in the
open air, water refuses to pass a syphon-tube, the shorter
eg of which exceeds 34 feet.
