36 ELEMENTARY GENERAL SCIENCE CHAP. 



rate is regulated by means of the pendulum, the properties of 

 which can be best understood by an experiment. 



EXPT. 45. Attach a weight to the end of a cord. Fix the 

 cord in such a way that the pendulum can oscillate freely. Set 

 it oscillating, and notice how long it takes for the pendulum to 

 complete a given number, say twelve, swings. Keeping the 

 cord exactly the same length, attach a heavier w r eight and 

 repeat the experiment. The time of swing remains unaltered. 

 Keeping any one weight, observe the time taken to complete 

 twelve swings when the length of the cord is varied. It will 

 be seen that the time of swing varies with the length of 

 the cord. Notice also that it does not matter if the pendulum 

 makes a wide oscillation or a very small one, the time taken 

 being the same. 



If it were possible for the student to perform the experiment, 

 it would be found that the time taken for the pendulum to 

 swing backwards and forwards varies as it is taken from the 

 equator to the poles, on account of the fact that the earth is not 

 exactly spherical in shape. Or, putting the same fact in another 

 way, in order that a pendulum may swing backwards and for- 

 wards in the same interval of time, it is necessary to alter the 

 length of the cord in our experiment as we travel from the 

 equator towards either pole. A pendulum of such a length that 

 the distance from the point of suspension to the centre of the 

 bob is 39'139 inches, swinging at Greenwich, completes one 

 swing in a second of time. In a clock we have a mechanical 

 contrivance for maintaining the swinging of a pendulum. We 

 must content ourselves with referring the reader to books on 

 astronomy and horology for an account of the construction of a 

 clock. In watches the place of the pendulum is taken by a care- 

 fully suspended balance wheel. 



Definition of Motion. The word motion' is meant to convey 

 the idea of change of place. The simplest forms of motion nre 

 changes in the positions of bodies with regard to one another. 

 When a boy runs down the street he is in motion ; as regards the 

 houses and lamp-posts he moves. To fully describe the boy's 

 motion it would be necessary to know the tllm-limi in which he 

 is moving or the line along which ho runs, and the -ratu or 

 /v/nr/7;/ with which he travels. 



Velocity. Velocity is, then, the rate at which a body moves, 



