THE EARTH AS A PLANET 191 



one turn of this wheel. From the number of oscillations per 

 minute of the pendulum (by your, watch), determine the 

 time required for one turn of this wheel. 



By counting the number of teeth of the pinion of this 

 same wheel and of the second wheel into which the pinion 

 works, calculate the time for one turn of this second wheel. 

 In case a hand were attached to the axle of this second wheel, 

 what time interval would be counted off by its every com- 

 plete turn? What purpose has a portion at least of the 

 wheelwork of a clock? 



7. Note the result as the escapement at the upper end of 

 the pendulum is pulled forward by the teacher sufficiently 

 to free the first wheel in the train, and the effect of replacing 

 the escapement on the rate of the wheel motion. State (a) 

 the cause of the motion in the wheelwork; (6) the use of 

 the pendulum with reference to such rate of motion. Re- 

 call what was true after a time of the motion of the pen- 

 dulum in paragraph 1 . What is true of the motion of a clock 

 pendulum? Explain the difference. Name another use of 

 the wheelwork of a clock. What is the sole purpose of the 

 pendulum in the clock ? 



8. Calculate the number of seconds in a day. How is 

 the natural unit of time we call the day fixed for us ? From 

 what time till what time is the true solar day measured? 

 Since in a day there are 86,400 seconds, what will be true of 

 the relative lengths of these equal parts if the day's length 

 varies during the year (as it does) ? 



9. With 86,400 swings of a second's pendulum counting 

 off a day-period, what will be true of noon by the clock, 

 and noon by true solar time (meridian passage of sun) ? 



