»». HI.] DISSERTATION SECOND. 115 



equal progress. The laws which determine the weight of 

 bodies immersed in fluids, and also the position of bodies 

 floating on them, had been discovered by Archimedes, and 

 were farther illustrated by Galileo. It had also been dis- 

 covered by Stevinus, that the pressure of fluids is in pro- 

 portion to their depth, and thus the two leading principles 

 of hydrostaticks were established. Hydraulicks, or the 

 motion of fluids, was a matter of more difficulty, and here 

 the first step is to be ascribed to Torricelli, who, though 

 younger than Galileo, was for some time his contemporary. 

 He proved that water issues from a bole in the side or bot- 

 tom of a vessel, with the velocity which a body would ac- 

 quire, by falling from the level of the surface to the level 

 of the orifice. This proposition, now so well known as the 

 basis of the whole doctrine of Hydraulicks, was first pub- 

 lished by Torricelli at the end of his book, De Motu Gra- 

 vium et Projectorum ; but it is not the greatest discovery 

 which science owes to the friend and disciple of Galileo. 

 The latter had failed in assigning the reason why water 

 cannot be raised in pumps higher than thirty-three feet, 

 but he had remarked, that if a pump is more than thirty- 

 three feet in length, a vacuum will be left in it. Torricelli, 

 reflecting on this, conceived, that if a heavier fluid than 

 water were used, - a vacuum might be produced, in a way 

 far shorter, and more compendious. He tried mercury, 

 therefore, and made use of a glass tube about three feet 

 long, open at one end, and close at the other, where it ter- 

 minated in a globe. He filled this tube, shut it with his 

 finger, and inverted it in a basin of mercury. The result 

 is well known ; — he found that a column of mercury was 

 suspended in the tube, an effect which he immediately as- 

 cribed to the pressure of the atmosphere. So disinterest- 

 ed was this philosopher, however, that he is said to have 

 lamented that, Galileo, when inquiring into the cause wh\ 



