“We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight." Air has mass.
Another example of the many basic principles about the physical world which we accept, but required observation and discovery to become so.
On this day 382 years ago Evangelista Torricelli described an experiment in a letter to a friend. Fill a meter-long glass tube with mercury, close it, flip it into a basin of mercury. The column falls to 76 cm. Always 76 cm. The gap above the mercury was the first vacuum ever made, something Aristotle had called impossible. Torricelli explained what holds the column up:
"We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight."
The mercury is not pulled from above. It is pushed from below by the weight of the atmosphere on the basin. One page, two revolutions: vacuums exist, and air is an ocean with us at the bottom. Four years later Pascal had a barometer carried up a mountain and watched the column drop.