Hydraulics. 49 



height, will not flow unless exposed to the pres- 

 sure of the atmosphere, will be plain from filling 

 a cask or other vessel full of this fluid. If the 

 bung is perfectly tight, and there is no aperture 

 above for the air to press upon it and force it out, 

 it is in vain that we shall attempt to draw it off 

 by opening a passage for it below. Hence the 

 use of vent-holes, and vent-pegs in casks: by 

 raising the vent-peg air is admitted, which forces 

 the liquor to flow out at the cock or faucet, where- 

 as if the vent-peg were kept tight no liquor 

 whatever could be obtained. The Valencia is a 

 common instrument made of tin, the lower part 

 of which is in the figure of an inverted cone, (see 

 PI. V. fig. 22.) with an orifice at the bottom , 

 and one at the top b. It is used for taking sam- 

 ples of liquors out of the bung-holes of casks. In 

 order to use it, the operator puts it into the 

 bung-hole with both orifices open, and the liquor 

 rises through the orifice at bottom to the top of 

 the instrument ; he then puts his thumb on the 

 hole or aperture at top, so as to exclude the air' 

 completely, and the liquor will not run out at the 

 bottom till the air is admitted by the thumb be- 

 ing removed, which is done in order to let it flow 

 into the cup or vessel which is to receive it. 



Thus it is plain that fluids, circumstanced as 

 above, are put in motion, or caused to flow, by 

 the pressure of the atmosphere ; and it will be 

 shown, that whenever that pressure is removed, 

 they will rise above their natural level, and flow 



VOL. i. D 



