46 GENERAL SCIENCE 



His apparatus, which was placed in an inverted position, consisted 

 of a tube with a very smooth interior in which a closely fitted piston 

 was placed. Weights were applied to this piston to see how much 

 pull was necessary to draw it down. 



Before his death he advised his pupil (Evangelista Torricelli) to 

 continue these experiments. 



Torricelli's decisive experiment ascertained the length of a column 

 of mercury sustained by the same cause, whatever it might be, 

 which supported the column of water. 



As the weight of mercury is 13.6 times greater than that of water 

 he reasoned that the height of the mercury column should be only 

 about TS as high as the water column. 



Torricelli proved his idea on the subject by taking a glass tube 

 about 3 feet in length, closed at one end, and filled with mercury. 

 Putting his finger on the open end, he inverted this tube in a small 

 bowl, containing mercury, and when he removed his finger, he 

 found that the mercury sank in the tube until its level was about 

 29 inches above the level of the mercury in the bowl. % 



Otto von Guericke invented the water barometer by erecting a long tube 

 reaching from a cistern in the cellar up through the roof of his house in 

 Magdeburg, Germany. A wooden image was placed within the tube, floating 

 upon the water. On fine days this novel weather-prophet would rise above 

 the roof-top and peep out upon the queer gables of the ancient city, while 

 in foul weather he would retire to the protection of the garret. The accuracy 

 of these movements attracted the attention of the neighbors. Finally, be- 

 coming suspicious of Otto's piety, they accused him of being in league with 

 the devil. So the philosopher removed his " wooden man " and the staid old 

 city was once more at peace. 



Measuring the Pressure of the Atmosphere. Take a long hol- 

 low glass tube and put one end in a dish of mercury. Attach an 

 air pump to the upper end of the tube and remove the air. The 

 mercury will rise in the tube to about 30 inches at sea level, but no 

 higher. Now a cubic inch of mercury weighs about .49 of a pound. 

 The air is pressing on the surface of the mercury hard enough to 

 make the mercury rise to a height of about 30 inches. Then the air 

 must be pressing about 14.7 pounds per square inch. This great 

 pressure of J4.7 pounds per square inch amounts to a tremendous 

 total when we consider the number of square inches on our bodies. 



