CHRISTIAN HUYGENS 75 



as the essential point in the movement of the simple pendu- 

 lum the predetermined path of the bob, along which path 

 the bob moves under the influence of gravity, according 

 to the laws already given by Galileo for motion on an inclined 

 plane; the latter had also himself investigated falling on a 

 circular path. 



Further consideration of the motion of the compound 

 pendulum, that is, an oscillating body which cannot even 

 approximately be regarded as a single point (as for example a 

 swinging rod), led Huygens deep into a knowledge of the 

 essential features of rotational motion generally. This 

 opened the way to a general mastery of every kind of motion 

 of solid bodies, including also the heavenly bodies: the prob- 

 lem of linear motion under the influence of given forces 

 having been solved in all essential respects by Galileo. If 

 we attempt to follow the main lines of Huygens' thinking in 

 this important matter, we realise that even advances in know- 

 ledge relating to very complicated matters are based on 

 simple trains of thought, though the complete working out 

 may be very elaborate. 



When a rod hung up at one end swings as a pendulum, each 

 of its single parts swinging about the given axis forms a 

 simple pendulum, and the periods of oscillation of all these 

 single pendulums can be calculated from their lengths. 

 The shorter of these will swing faster, and the longer ones 

 correspondingly slower. But all these single pendulums are 

 rigidly connected together to form a rod, and hence are ob- 

 liged to move at the same rate, and the question is, what this 

 rate will be. Huygens arrived at a correct answer by making 

 use, as his great countryman Stevin had previously done, of a 

 generally known and therefore well assured existing experi- 

 ence. It related to the centre of gravity. In the present case 

 this tells us that when the pendulum rod moves under the 

 influence of gravity, nothing will ever happen to raise its 

 centre of gravity to a greater height than that from which it 



