136 SECTION PHYSICS. 



ness (leggerezza positive?) ; he measured the greatest expansion of air, 

 freed from the surrounding pressure ; he determined the weight of air 

 compared to that of water ; he suggested the famous experiment with 

 the silver ball, to test the compressibility of water ; and another experi- 

 ment on the propagation of sound in vacuo ; he studied the con- 

 traction of various liquids in cooling ; and in the researches on the 

 velocity of propagation of light, he was the first to construct the 

 Heliostat (Eliostata) ; he published, whilst lecturer in Pisa, the 

 "Euclides restitutus," in which he reduces to 230 propositions the 

 elements of ancient geometry ; and afterwards the treatise on the force 

 of the blow, which together with that on the natural motions depend- 

 ing on will, serve as an introduction to his great work " Del moto 

 degli Animali ;" he studied the optic nerve, and the organs of respira- 

 tion in fishes, and was the first to occupy himself with the anatomy of 

 the torpedo ; he observed the comet of 1664, declaring that it was not 

 an accidental meteor, or vapour, but a solid body moving, not round 

 the earth, but round the sun, in a line resembling a parabola, and thus 

 laid the foundations of the theory of the comets ; he published the 

 theory of the " Stelle Medicee," in which he clearly proved that the 

 orbit of the satellites is not in the same plane as that of Jupiter, and 

 he compared these satellites to the moon, and thus alluded to the 

 principle of universal attraction ; he was the first to make known that 

 Venus can be seen two days running, as a morning and as an evening 

 star ; he wrote an account of the eruption of Mount Etna which took 

 place in 1669 ; he studied the constitution of liquids, and was the first to 

 point out the phenomenon of the contraction of the liquid vein; 

 and he illustrated by new experiments Galileo's idea of the fall of 

 weights in vacuo. 



But we must now turn, for a short time, to the principal works of the 

 Academy, although very much must necessarily remain unsaid of the 

 individual academicians, and especially of Redi and Stenone and 

 Cassini. 



And coming now to the experiments, the first series refers to the 

 natural pressure of the air. In a discussion on Torricelli's experi- 

 ment, and on the reason why solid bodies do not lend themselves to 

 it, and among liquids, mercury should adapt itself the best, there is to 

 befound noted down,with regard to liquids, a most important hypothesis 



