On Tuesday, 14 August 2012 21:27:52 UTC+1, Martin Brown wrote:
On 14/08/2012 19:02, djornsk wrote:

...

Mercury metal was used until fairly recently for zenith transit

telescopes and has made a resurgence for spinning mirror scopes eg:

It is used in the Souter Lighthouse (http://www.nationaltrust.org.uk/sout...-and-the-leas/) today. The whole lens assembly floats in a ring of mercury, making it so frictionless that you can rotate the two-tonne assembly with a finger.

-- Jason