ON INSTRUMENTS FROM ITALY. 123 



apparently larger appearance of stars on the horizon ; and found the 

 ingenious manner of making sight more distinct without employing 

 lenses, by making use of a little hole, and thus clearly demonstrating the 

 utility of diaphragms in optical instruments. Then, forestalling the 

 modern theories of heat, he clearly pointed out how different the 

 warming of bodies is according to their nature and to the state of their 

 surfaces ; and in his publication on the magnet, he was the first to 

 explain the arrangement which iron filings make on paper spread over 

 the poles of a magnet according to lines, which were afterwards named 

 " lines of strength." Castelli, who had made Torricelli's acquaintance 

 in Rome, and who was convinced of his genius, introduced him to 

 Galileo, who was at once eager to know him ; but it did not fall to 

 Torricelli's lot to spend in the society of that great man more than the 

 last three months of his laborious existence. 



Torricelli extended the mechanical discoveries of Galileo, ingeniously 

 applied the method of indivisibles, conceived by Galileo, to the squaring 

 of the circloide, which he was the first to demonstrate, and to the 

 measurement of hyperbolic solids. Being appointed mathematician 

 to the Grand Duke, he established himself in Tuscany, occupying 

 himself, among many other things, some of which we have mentioned 

 above, in the perfecting of telescopes. We also owe him a microscope 

 simpler than Galileo's, being made of only one lens, or rather of a little 

 glass ball, which he fashioned by the flame of a lamp. But the in- 

 vention which contributed the most to his celebrity was, as every one 

 knows, that of the barometer. 



Galileo, by condensing air, had demonstrated its weight, and in his 

 Dialogue on the resistance of solids, he says of water, that in suction- 

 pumps, it does not rise higher than about eighteen " braccia," leaving 

 the space above empty. Torricelli, pondering over this fact, was led to 

 think of what would happen if in the place of water, mercury, which is 

 so much heavier, were used ; for he argued that by its means there 

 would be much greater ease in obtaining a vacuum, in a much shorter 

 space, than that necessary for water. And he then made a long tube 

 of glass of the length of about two braccia which terminated at one end 

 in a ball, likewise of glass, and remained open at the other ; through 

 this aperture he proposed to fill the tube and the ball completely with 

 mercury ; and then holding it with his finger and turning it upside- 



