As the water heats up, it creates steam that slowly moves the hot water through the tube into the ground coffee. The sealed container is filled with water and heated, and the second is filled with ground coffee. The Napier coffeepot has two vessels, one sealed except for a tube running from near its base to the bottom of the other that is open to the atmosphere. Brewing coffee simply by boiling ground water can burn it and impart bitter flavors, so a machine that could brew a better cup was considered of some importance at the time. Here in the 21st century we take our fancy coffee machines for granted, but a century and a half ago, making coffee was a much simpler and less precise process. What really is a vacuum coffee maker and what makes it so special? The answer lies in the temperature at which the coffee is prepared. Media Vacuum coffee makers: impressive, but slow
