GALILEO AND HIS JUDGES. 93 



will be uniform and perpetual upon the plane, if 

 the plane be extended to infinity." 



This of course involves the principle of the first 

 of the three laws of motion, the Newtonian laws, 

 as they are frequently called, because the man whose 

 name they bear was the one who used them clearly 

 and consistently as the basis of a great astronomical 

 theory. The law, as now usually stated, is fuller 

 and more explicit than that given by Galileo, and 

 may be enunciated thus : " Every body perseveres 

 in its state of rest, or of uniform motion in a straight 

 line, unless it is compelled to change that state by 

 forces impressed on it." 



It is, however, greatly to the scientific credit of 

 Galileo that before the close of his life be should 

 have emancipated himself from the erroneous idea 

 that circular motion alone is naturally uniform, and 

 should have stated in the language just quoted the 

 true mechanical doctrine, unknown to his prede- 

 cessors, unknown even to Kepler, a doctrine which 

 involved nothing less than a revolution in the con- 

 ception of the laws of motion. Nor was this his 

 only contribution to the science of mechanics ; he 

 it was who first understood the law that regulates 

 the velocity of falling bodies ; he perceived that 

 they were acted upon by an uniformly accelerating 

 force, that of terrestrial gravity, and that the velocity 

 at any given point is proportional to the time of 

 descent. 



The principle of virtual velocities is said by some 



