On Mon, May 10, 2010 at 6:38 PM, Robin Hamilton <
[log in to unmask]> wrote:
> Neatly spotted, Alison!
>
> He's right that the OED isn't quite correct here, but his suggestion that
> it's all gravity is possibly equally misguided.
>
I had to take fluid mechanics in various forms in Engineering school, and
this stuff is still rather brain-bending, but I according to my memories, he
is right. Atmospheric pressure is essentially equal on the surface of the
reservoir fluid and on the exposed surface of the end of the siphon. There
is infinitesimally more pressure on the latter because it is infinitesimally
closer to the Earth's center of mass, but that effect is overwhelmed by the
pressure exerted by gravity, which is hydrostatic pressure within the fluid
itself, and not imparted by the surrounding fluid (the atmosphere).
For purposes of a dictionary entry it makes sense to just leave it at
"gravity", but at the same time, I like your suggestion of
"by means of hydrostatic pressure"
Considering that hydrostatic pressure is nothing but pressure developed by
the nature of a fluid in a force field (i.e. gravity). Anyone who takes
that entry as a clue to revise their fluid dynamics will quickly remember
that the relevant hydrostatic pressure comes from the siphoned fluid itself,
and not from the atmosphere, so all would be well.
--
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