Bobbing cork
How do you make a cork float in the centre of a still glass using just water?
Normally a cork will always be drawn to the side of a glass due to the upwards meniscus of the water - the water is attracted to the glass and so "sticks" to the side and forms a curve. As the highest point, the cork invariably gravitates towards the sides.
This effect isn't inevitable - anyone with a old fashioned mercury thermometer can observe the opposite effect, a downwards meniscus, as the mercurys is more attracted to itself rather than the glass.
So how do you win a hypothetical bet and make water have a downwards meniscus? Simply fill the glass to the brim. The water still tries to "stick" to the rim - saviour of bar staff the world over - creating a downwards meniscus and making the cork float in the middle.
Relevance to this cache? You'll have to bob down and up to get to it and, as the river hopefully isn't full to the brim, you'll need to look on the edge of the bank to find it. If it is, it's supplied with a handy cork, so you'll find it in the middle of the river... somewhere between here and London!
As with all my riverside caches this is very likely to be inaccessible when river levels are high.
Below: mercury downwards mensicus on the left, and an upwards meniscus on good old H₂O on the right.
