1643 – barometer

Galileo knew that pumps had a problem pushing water to an elevation above thirty feet and had theorized that a vacuum might be involved. At the time, the idea of a vacuum was controversial and there was no observational evidence that it could exist. He assigned one of his students, Torricelli to work on this problem. In 1641, an associate of Torricelli named Gasparo Berti set up an experiment to test for the existence of a vacuum using a tube of water. His experiment seemed to confirm that a vacuum was formed at the top of the tube, holding the water up and preventing it from flowing completely out of the tube.

Evangelista Torrecelli suggested that maybe it was the weight of the air outside of the tube pressing the water up into the tube and arranged an experiment using mercury instead of water because mercury is heavier and a smaller tube could be used.

Descartes and Pascal seem to have advanced these ideas toward actually measuring the pressure of the atomsphere.

Robert Hooke later created a circular barometer which made it much more effective as a useable tool.

PREREQUISITE:
pumps and siphons
mercury
the untested theory of a vacuum

SUBSEQUENT:
Concepts of gas pressure
Refinements in pumps, including eventually the mercury pump which led to the invention of the first vacuum tube and modern electronics

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