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I don't understand how Watt measured the volume of a mass of water vapor compared to that same mass in liquid phase. In this text he describes the process by himself, but I can't understand it. Could someone please rephrase it? I know it was an imprecise way of measuring what he wanted to measure, but I don't even get how what he describes accomplishes that.

Description of the process

Thank you for your time in advance.

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  • $\begingroup$ Watt had a known volume of water vapor, which was the volume of the flask. What he measured was the mass of that water vapor. $\endgroup$ Apr 6, 2022 at 18:12

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Watt started off with a flask containing water liquid and air. A tube ran down through a seal at the top of the flask to almost the bottom of the flask.

Watt heated the flask so that the water liquid started to evaporate and converted into water vapour (steam) which expelled the air from the flask. The tube was there to make sure the denser air was displaced by the less dense water vapour and expelled though the tube.

When there was no more water liquid in the flask and it’s contents, water vapour, we’re cooled rapidly leaving a residue of water in the flask. Watt then made three weighings.

The mass of the flask and condensed water vapour $a$, the mass of the dry flask $b$ and the mass of the flask full of water liquid $c$ were found.

Watt then reasoned that $a-b$ of water vapour had the same volume as $c-b$ of water liquid.

437.5 grains = 1 ounce

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