OBSERVATIONS OF CHANGE OF POSITION 27 



at the rate of one metre per second, or to have, if the displace- 

 ment is uniform, a speed of one metre per second. It is im- 

 material which body is considered as fixed in position. 



The numerical value of the speed of a body so moving is the 

 same as the number of units of length which are added in the 

 unit of time to its distance from a selected origin. 



23. Change of Speed, or Acceleration. Motion is not 

 always constant. It may be variable. If it be constant, 

 the speed of a material particle, measured at two different 

 times, is found to be unchanged. If it be variable, the speed 

 is said to accelerate. The acceleration may be positive or 

 negative, that is, there may be an increase or a decrease of 

 speed. The change in speed occurring in a given interval 

 gives the conception of rate of acceleration, just as speed 

 is derived from combining the magnitude of the distance 

 traversed with the time occupied. It is also obvious that 

 the rate of acceleration may similarly be either constant 

 or variable. 



It will be seen that the numerical value of rate of rectilinear 

 and uniform acceleration is measured by the number of units of 

 speed which are gained or lost in a unit of time. 



24. Measurement of Time. We are directly conscious of an 

 order or sequence of events. The experience of all generations 

 has led men to regard certain events as recurring with suffi- 

 cient regularity to be constituted into fixed points, from which 

 the more variable events may be dated, and by which they 

 may be compared with one another. These regular events are 

 the alternation of day and night, the changes of the moon, and 

 the apparent maximum height of the sun. From these crude 

 reckonings our more exact conceptions have grown ; but we 

 still have to depend upon the rotation of the earth for our 

 measure of time. 



A chronometer or clock is an instrument constructed so 

 that the index moves over one division of the second-dial in 

 s"gcorr ^ * ne mean solar day. The number of seconds, elapsed 

 since the beginning of the day, is calculated by means of the 

 minute and hour fingers. A chronometer is compared with 

 the revolution of the earth in the following manner : 



