"This generosity of Torricelli," says Playfair, "was, perhaps, rarer than his genius: there are more who might have discovered the suspension of mercury in the barometer than who would have been willing to part with the honor of the discovery to a master or a friend." Torricelli's discovery was made in 1643, less than two years after the death of his master.

Galileo had observed that water will not rise in an exhausted tube, such as a pump, to a height greater than thirty-three feet, but he was never able to offer a satisfactory explanation of the principle.

Torricelli was able to demonstrate that the height at which the water stood depended upon nothing but its weight as compared with the weight of air.

If this be true, it is evident that any fluid will be supported at a definite height, according to its relative weight as compared with air.

Thus mercury, which is about thirteen times more dense than water, should only rise to one-thirteenth the height of a column of water--that is, about thirty inches.