SCIENTISTS OF XVTH AND XVlTH CENTURIES. 101 



body moving slowly or swiftly is equally affected by gravity 

 irrespective of its weight thus eradicating the old Aris- 

 totelian error that a light body falls less rapidly than a heavy 

 one; showed also that a body projected horizontally must 

 have a horizontal motion and also an accelerated motion 

 downward determining, that is, the trajectory described by 

 a body not falling vertically ; he determined (before Newton 

 came, as we see) the connection between the spaces of descent 

 and the times and illustrated facts experimentally by the 

 use of inclined planes ; he demonstrated that the earth's 

 ATTRACTION ACTS EQUALLY on all bodies, proving it by 

 the use of hollow spheres in which various substances of 

 different weight suspended by strings of equal length oscil- 

 lated in equal times ; and from the Pisa Tower proved that 

 the velocity of falling bodies is independent of their weight ; 

 so that he disclosed the law of falling bodies and that of the 

 composition of forces two great generalisations of terrestrial 

 physics, being thereby the founder of terrestrial dynamics. 

 But collaterally with these great discoveries he had established 

 the THREE LAWS OF MOTION : I. A body remains at rest 

 or perseveres in uniform motion in a straight line unless 

 disturbed by another force; II. The change of motion is 

 proportional to the motive force impressed and in the 

 direction of the line in which the force is impressed ; III. To 

 every action is ever opposed an equal reaction ; or, the 

 mutual actions of two bodies upon each other are ever 

 equal and opposite. To Galileo, therefore, we owe the 

 discovery of the law of the uniformity and perpetuity of 

 motion indeed, he ESTABLISHED the doctrine of the PER- 

 MANENT GOVERNMENT OF THE WORLD BY LAW a doctrine 

 only strengthened by the discoveries of Kepler and Newton. 

 Further, Galileo, concurrently with Cavalieri (1598 1647), 

 found the CALCULUS OF THE INDIVISIBLE, and concurrently 

 with Stevinus, placed HYDROSTATICS AND HYDRODYNAMICS 

 on exact foundations, by his discovery of the laws (known 

 by the Alexandrian school) respecting the mechanical 

 properties of water i.e. the laws of the EQUILIBRIUM OF 

 FLUIDS. He was the first to discover that water raised by 

 the suction pump cannot ascend higher than 34 feet, but it 



