SCIENCE OF COMMON THINGS. 20 



Pendulum. Common clock. 



166 Will a mass of iron, weighing one hundred pounds, let fall from 

 an elevation, reach the ground any quicker than a mass weighing only one 

 pound, ktfall at the same time and from the same place f 



No ; the lighter mass will fall with the same velocity, 

 and reach the ground as soon as the larger one. 



Before the time of Galileo it was taught and believed, that if two 

 bodies of different weights were let fall from any height at the same mo- 

 ment, the heavier body would reach the ground as much sooner as ita 

 weight was greater than the smaller. Galileo, on the contrary, main- 

 tained that they would both strike the ground at the same time, and, as 

 his doctrine was generally disbelieved, he challenged his opponents to a 

 practical trial. The experiment was made from the top of the celebrated 

 leaning tower of Pisa, in the presence of a great concourse of people, and 

 resulted in the complete triumph of Galileo. 



167 What is the rule by which the height from which a body falls may 

 be found, the time consumed in falling being known? 



Multiply the square of the number of seconds of time 

 consumed in falling, ~by the distance which a body will 

 fall in one second. 



168 If a stone is five seconds in falling from the top of a precipice, hoio 

 high is the precipice f 



The square of five seconds is 25 ; this multiplied by 

 16, the number of feet a lody will fall in one second, 

 gives 400, the height of the precipice. 



169 What is a pendulum ? 



A pendulum is a heavy body, as a piece of metal, 

 suspended by a wire or cord, so as to swing backwards 

 and forwards. 



170 When is a pendulum said to vibrate f 



When it swings backwards and forwards ; and that 

 part of the circle through which it vibrates, is called 

 its arc. 



17*1 What is a common clock f 



Merely a pendulum, with wheel- work attached to it, 

 to record the number of vibrations, and with a weight 

 or spring, having force sufficient to counteract the re- 

 tarding effects of friction and the resistance of the air. 



17S How long must a pendulum be to beat seconds 1 



About 39 inches. 



178 Why does a common clock go faster in winter than in summer t 

 2* 



