24 Wilde, Evolution of the Mental Faculties. 



by its acceptance. The ground for this opinion was that 

 the principal lunar and planetary motions were already 

 well explained by Kepler's third law, the law of falling 

 bodies discovered by Galilei, and the hypothesis of central 

 forces acting at some function of the distance as assumed 

 by Hooke and others. Hence the small progress that the 

 law of gravitation, as the inverse square of the distance, 

 made at Cambridge and on the Continent during the 

 lifetime of its discoverer. 



Newton's first law of motion, from its common accept- 

 ance, appears to be so self-evident that a denial of its 

 universality would, at first sight, indicate a spirit of 

 perverse contradiction rather than a serious desire to 

 elucidate the truth of nature. Nevertheless, as will be seen, 

 the common idea of inertia^ like the simian notion of the 

 immobility of the earth, and the Cartesian quantity of 

 motion in a body, has its origin in an erroneous interpreta- 

 tion of phenomena as presented to the senses. 



It is laid down by Newton in his first law of motion 

 that '■'' Every body perseveres in its state of rest, or of uniform 

 motion in a right line, unless it is compelled to change that 

 state by forces impressed thereon" " Projectiles persevere in 

 their motions, so far as they are not retarded by the 

 resistance of the air or impelled downwards by the action 

 of gravity. A top, whose parts by their cohesion are per- 

 petually drawn aside from rectilinear motions, does not 

 cease its rotation, otherwise than as it is retarded by the 

 air." 



Commenting on this law, Maclaurin states,* "It is 

 part of the same law, that a body never changes the 

 direction of its motion, of itself, but by some external 

 influence only, and it is as natural a consequence of the 



* Account of Newtoii' s FJiitosophical Discoveries, p. 113. 



