WEIGHT. 



35 



How do we Ob- 66. In constructing a System of Weights 

 i!l-dof weight's ^^^ Measures, it is necessary, in the first place, 

 and Measures? ^q g^ uijou somc dimension which shall forever 

 serve as a standard from which all other weights and 

 measures may be derived, and by which they may be com- 

 pared and verified. If an artificial standard were taken, 

 it is evident that it might be falsified, or even entirely lost 

 or destroyed, thus creating great confusion. It is, there- 

 fore, necessary to fix upon some unchanging and invariable 

 space or size in nature, which will always serve as a stand- 

 ard, and which the art of man can not affect. In th^ 

 English System of Weights and Measures, such an un- 

 varying dimension, or standard, is found in the length of 

 a pendulum. 



Describe the ^7. A peudulum is a heavy body, suspended 



Pendulum. from a fixed point by a wire or cord, in such a 

 manner that it may swing freely backward and forward. 

 The alternate movements of a pendulum in opposite di- 

 rections are called its vibrations, or oscillations, and 

 the part of a circle over which it moves is called its akc. 



In Fig. 5, A B represents a pendulum ; D 



C, the arc in which it vibrates. 



Now, it has been found that 

 a pendulum, of any weight, 

 which in the latitude of Lon- 

 don will vibrate, or swing over 

 the same arc, or from the 



highest point on one side, to the highest point 



on the other side, in one second of time, will 



always, under the same circumstances, have 



the same length. The length of this pendulum 



(the part A B, Fig. 5) is divided into 391,393 



equal parts. Of these parts, 1 0, 000 are called 



an inch, twelve of which make one foot, 



thirty-six of them one yard. Thus we ob- 

 tain standards of linear measure. 



„ . ^ To obtain a Standard of 'Weight, a cubic inch (accuratelii ob- 



How do wc ob- \ i. 1 J /. 



tain a Standard tained from the pendulum) of distilled water, of the temperature 

 of Weight? ^f g20 Fahrenheit's thermometer, is taken and weighed. 

 This weight is divided into 252,458 equal parts; and of these, 1,000 will be 

 a grain. The grain multiplied, gives ounces, pounds, etc. 



How does the 

 Pendulum fur- 

 nish a Stand- 

 ard of Meas- 

 ures of Length ? 



Fig. 5. 

 A 



