124 SECTION PHYSICS. 



down, submerge the orifice of the tube below the level of more mercury 

 in a large vessel ; and that being done, take away his finger and open 

 the tube, thinking that the quicksilver would detach itself from the ball, 

 and having glided down, and remained suspended, according to the 

 various calculations, at about the height of one and a quarter braccia, 

 would, in all probability, leave a vacuum in the ball above and in part 

 of the tube. He communicated this thought to his great friend, 

 Viviani, who, most anxious to see the result, agreed to the experiment, 

 which he himself carried out, and was, hence, the first, about a year after 

 Galileo's death, to see Torricelli's ingenious idea confirmed by the fact. 

 He hastened to his friend, who, most joyful at the news of this evidence, 

 was all the more persuaded that the weight of the air was really that 

 which was in equilibrium with the column of water or mercury. Indeed, 

 being asked by Viviani what would have happened if the experiment 

 had been made in closed space, Torricelli, after having reflected for a 

 short time, answered : The same thing ; since the air is already com- 

 pressed in it. This most important discovery was communicated by 

 the author himself to Ricci in Rome, and by Ricci to Signer de Verdus 

 who, in his turn, made Padre Mersenne acquainted with it, from whom 

 Pascal learnt it and made it famous, as every one knows, in his cele- 

 brated Puy-de-D6me experiment. It is very probable, however, that, 

 v/ithout detracting in the least from the great merit due to Pascal, the 

 experiment of the sinking of the mercury in the barometrical tube in 

 proportion to the increase of the height to which it is carried, was made 

 for the first time in Italy. Carlo Beriguardi, in his " Circolo Pisano," 

 published in 1643, while endeavouring to clothe in an Aristotelian dress, 

 both this and other experiments, says : That the tube of quicksilver 

 leaves more space empty when placed at the top of a tower or of a 

 mountain, than at the foot. And in a letter to Ricci, Torricelli himself 

 observes : That it would be possible, by means of his instrument, to 

 get to know when the air was lighter or heavier ; and that it might be 

 the case that this air, which is most heavy upon the surface of the 

 earth, becomes more and more light and pure as we rise higher and 

 higher to the tops of the loftiest mountains. There is, accordingly, 

 reason to believe that he himself, or others following up his indications, 

 really made the experiment and saw the mercury sink as the height 



