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Date Posted: Monday, November 24 2008, 11:19:56
Author: Bob Pass
Subject: Re: mercury lens floation
In reply to: Bob Pass 's message, "mercury lens floation" on Saturday, August 23 2008, 14:00:24

When I first saw the contribution From Michael Spencer I thought we had an explanation, regretfully more consideration reveals the flaws. The key is the assertion that the lens will float on “mercury which is not there, that is, displaced”. If this were true why would you want to use the 55 litres in his example, why not make the trough smaller, and then use even less mercury thus economising on what must be a costly product.

I am still with Archimedes when it comes to floatation, that is that a floating body displaces it own weight of the fluid in which it floats. Were this not the case you would have to come up with another explanation to account for the force acting upwards, which resists the force acting downwards, due to the mass, of the floating object. In Michael’s example the 2500kg lens would not float with the addition of 750 kg of mercury, but would I am afraid remain firmly on the bottom of the trough.

Perhaps Sherlock Holmes was well advised not to say “it's the amount of mercury that's not there that is significant, not the amount that is”.

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