1 68 



HISTORY OF SCIENCE. 



blished formula in accordance with facts. His usual sagacity appears 

 to have been at fault in the explanation of the phenomenon which 

 he suggests in his Dialogues, where he compares the column of water 

 in the barrel of the pump to a rod of metal suspended from its upper 

 end, which may be lengthened until it breaks with its own weight. 



The true explanation of the rise of the water in the common pump 

 was first given by a friend and pupil of Galileo, the distinguished 

 TORRICELLI (1608 1647). Torricelli made some additional dis- 

 coveries in the mechanics of moving bodies, and he investigated some 

 general theorems relating to the centre of gravity and the equilibrium 

 of bodies, He showed also that water issues from a hole in the side 

 or bottom of a vessel with the same velocity that it would acquire in 

 falling from the level of the orifice. The famous experiment which 

 is called by Torricelli's name was devised by him in order to verify 

 his explanation o! the rise of the water in an exhausted tube to -the 

 height of 33 feet and no more. The column of water, said Torricelli, 

 ascends to such a height that its weight 

 exactly balances the pressure of the atmo- 

 . v phere. In other words, the atmosphere, 

 laving weight, presses on the surface of the 

 water in the well ; and when this pressure 

 is removed from the liquid within the pump- 

 barrel, the liquid is forced up by the atmo- 

 spheric pressure outside, until the weight of 

 the column of water produces an equal and 

 counter-pressure. In order to show that the 

 height to which a liquid would rise in an 

 exhausted tube depended on the weight of 

 the liquid, Torricelli devised his famous 

 experiments, which may be easily under- 

 stood by the aid of Fig. 70. He took a 

 strong glass tube about 3 feet long, and 

 closed at one end. This tube he com- 

 pletely filled with quicksilver, which is a 

 liquid thirteen times heavier than water. 

 Having closed the mouth of the tube by 

 his thumb, pressed against it as shown on 

 the right-hand side of the figure, he inverted 

 it, and plunged its mouth beneath the sur- 

 face of some more quicksilver contained in 

 a basin b, before withdrawing his thumb. 

 When the thumb was then removed from 

 the surface of the tube, the quicksilver 

 FIG. 70. immediately descended, until it stood at a 



certain height within the tube, as at a, 

 where it was about 30 inches above the level of the surface of the 



