Phil Stracchino submitted a more detailed look at the problem:
Since the upper end of both tubes is within the bulb and below the
liquid surface, the same partial vacuum that draws liquid up the
lower tube from the dish to the bulb will also act to draw liquid
through the siphon towards the bulb.
This force is counteracted in each tube by the weight of the hydraulic
head of water in that tube.
Although the siphon tube is higher than the straight lower tube,
that portion of the siphon which is above the level of water in the bulb
is balanced and in equilibrium, and does not contribute to the hydraulic
head of the siphon.
The hydraulic head in the siphon tube is based upon the height of
the water column between the water level in the bulb and the higher
of the water level in the dish or the end of the siphon tube,
not from the total height of the siphon.
It is immaterial how deep within the liquid either end of either tube is,
because so long as both tips of either tube are below the surface,
hydraulic head in that tube acts between the surfaces of the two bodies
of water, not between the ends of the tube.
Any increase in the weight of water in either tube caused by extending
the tube deeper into either body of liquid is exactly counterbalanced
by the increased opposing hydrostatic pressure at the opening resulting
from the greater depth of water above it.
There is therefore nothing to be gained by putting the end of the
siphon deeper into the dish; it changes nothing (or more to the point,
the changes it creates cancel out).
This is why you can't siphon water from a lower level to a higher,
only from a higher to a lower.
Now, consider first the state in which the end of the siphon is below
the liquid level in the dish.
In that case, the siphon and the lower tube, although taking different paths,
are actually equivalent closed routes between exactly the same two
water reservoirs.
The same pressure differential applies to both, and the system is
in equilibrium.
No water will flow because there is no net force favoring one path
over the other.
Secondly consider the case in which the lower end of the siphon
is raised out of the dish, as shown in the diagram.
The siphon, in this case, now ends not at the pool of water in the
dish, but some distance above it.
The siphon, since it now ends above the water surface and thus
above the effective end of the lower tube, now has less hydraulic
head than the lower tube; thus, the weight of water in the lower
tube exceeds the weight of unbalanced water in the siphon.
Water will flow down the lower tube out of the bulb, causing water
to be drawn up the siphon to replace, it until the siphon tube is empty.
At that point, air enters the bulb through the siphon,
collapsing the partial vacuum, and all remaining water in the bulb
drains into the dish via the straight tube.
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