1. Introduction
Pascal in his ‘sea of air’ hypothesis [1] imagined air to be like a liquid. As one ventured deeper and deeper into the
fl
uid the pressure would increase. If air acted like a liquid then this behavior could be explained by imagining the gas at any depth holding up the weight of
fl
uid above. The mercury column in a barometer was then doing the same thing, and opposed by only a vacuum at the top of the evacuated tube. These ideas challenged long held Aristotelian notions, but the evidence gathered by carrying a barometer higher and higher up the Puy-de- Dome in France was clear; air pressure varied with depth and the inevitable inference was that air had measurable weight just like liquid water.