NATURAL SCIENCE IN SEVENTEENTH CENTURY 257 



He resided with Galileo and acted as his amanuensis from 1641 

 until Galileo's death. In experimenting with mercury he found 

 that this did not rise to 33 feet, but instead to hardly as many 

 inches. He next proved, by comparing the specific gravity of 

 water and mercury, that the same "pressure" was at work in 

 both cases, and boldly affirmed that this pressure was that of the 

 atmosphere. The tube of mercury used in his experiments was 

 what we now call a barometer (baros, weight), but it was for a 

 long time called "the Torricellian Tube," as the empty space 

 above the mercury is still called the "Torricellian vacuum." 

 This invention or discovery of Torricelli's was one of the most 

 fertile ever made, for at one blow it demolished the ancient super- 

 stition that "nature abhors a vacuum," explained very simply 

 two ancient puzzles (why water rises in a pump, and why it rises 

 only 33 feet), determined accurately the weight of the atmosphere, 

 proved it possible to make a vacuum, and gave to mankind an 

 entirely new and invaluable instrument, the barometer. Torri- 

 celli's results and explanations were received at first with incredu- 

 lity, but were soon confirmed, notably by Pascal (1623-1662) 

 in a treatise, New Experiments on the Vacuum. In one of these 

 Pascal used wine instead of water or mercury in the Torricellian 

 tube, with satisfactory results, and in another, reasoning that if 

 Torricelli were right, liquids in the tube should stand lower on a 

 mountain than in a valley, persuaded his brother-in-law, Perier, 

 to ascend the Puy de Dome (near Clermont, France) in September, 

 1648, on which mountain the column was found to be much 

 shorter. This and other brilliant work by Pascal have given him 

 a high rank among natural philosophers. 



Since it was now easy to obtain a vacuum by the Torricellian 

 experiment, fresh attempts were made to produce vacua otherwise. 

 Von Guericke, burgomaster of Magdeburg in Hannover, after 

 many failures, finally succeeded in pumping the air out of a hollow 

 metallic globe. It was in this experiment that the air-pump was 

 introduced. Guericke found that his globe had to be very strong 

 to resist crushing by the atmospheric pressure, and in the popular 

 demonstration now known as that of the Magdeburg hemi- 



