Vol. 10
July Issue
Year 2009

Off the Beaten Track

in Vol. 10 - July Issue - Year 2009
An Ocean of Air

Evangelista Torricelli (1608-1647) inventing the mercury barometer, 1644. Torricelli demonstrated that liquid will rise in a tube unless the weight of the column of liquid is equal to the pressure of the air pressing on an equal section of the reservoir of liquid.

It was an oddly-mixed group of men which slowly climbed along the narrow path up the Puy de Dome on the pleasant morning of 19 September 1648. Two clerics and three elegantly-dressed gentlemen who would have looked more in their place at a town council meeting, rather than on a mountain trail in central France. A casual observer might have mistaken a small wooden box which they carried for a sort of food container, perhaps for a picnic under the clear blue sky. But this couldn’t be their purpose, for the group bore no resemblance to holiday makers out for a day of sunshine and merriment. Their gait was too determined, their dress was too formal. Who were they and what were they up to?


Seven years earlier, a young mathematician by the name of Evangelista Torricelli, from Faenza in northern Italy, had become interested in the subject of the vacuum. In the course of his studies, he had been informed of a mysterious problem which was afflicting mining industries in Europe. More precisely, the mines were constantly prone to flooding and the piston pumps which were being used to drain these mines could not draw the water higher than about 10 meters above the surface of the flood level. Torricelli became convinced that the fact that the water would not rise higher was somehow related to the weight of the air pushing down on the water at the bottom of the mineshafts. To try to solve this riddle, Torricelli would have had to build equipment sufficiently big to recreate the working conditions of the water pumps, which would have meant building equipment much too big and unwieldy. He overcame this difficulty by using mercury instead of water. Since mercury is fourteen times denser than water, he could build his experimental equipment down to scale fourteen times smaller.

Sometime in 1644, one of Torricelli’s assistants filled a two-meter long tube with mercury and upended it in a dish full of the same metal, with the open end beneath the surface of the mercury in the dish. The mercury ran out of the tube into the dish as soon as the assistant removed his finger from the open end of the tube. However, against all expectations, the tube did not empty completely, but there remained something less than a meter of mercury in the tube. Torricelli concluded that the weight of the air pushing down on the mercury in the dish must have been equal to the weight of the mercury remaining in the tube. If this were not so, the tube would have emptied completely.

When describing this experiment in a letter to a colleague, Torricelli wrote that, if his theory was correct, “we live submerged at the bottom of an ocean of air.” He further reasoned that the pressure of the air in this “ocean” varied with the altitude at which the pressure was measured.

Torricelli did not have the courage to publicly announce his theory or to carry out further tests in Italy, as his ideas would have been branded as heretical. However his letters were copied and circulated among other scientists and experimenters in Europe, until they came to the attention of a Frenchman named Blaise Pascal, who verified Torricelli’s ideas by repeating the experiment with full scale equipment and water. Now the theory needed to be tested at a certain altitude, but unfortunately there were no mountains where Pascal lived, so he wrote to his brother-in-law, François Périer, who lived in an area surrounded by mountains. About one year after Torricelli’s death, Périer set out to verify the Italian’s theory. He left one tube of mercury upended in a dish full of mercury at the base of the Puy de Dome and, accompanied by some local dignitaries, took an identical set of equipment to an altitude of over 1,300 meters, where the experiment was repeated several times in different spots at the top of the mountain. As Torricelli had predicted, the level of the mercury in the tube was lower than the level of the mercury in the tube left at the foot of the mountain.

The success of this experiment would have enormous impact on future technical and scientific developments in many fields of application, for it marked the invention of the barometer.

By Giovanni Gregorat, Contributing Editor MFN & Sales Manager, Pometon Abrasives

Author: Giovanni Gregorat