60 INTRODUCTION TO GENERAL SCIENCE 



body has made one revolution. It is easy to count the revo- 

 lutions, but we sometimes desire to measure rotary motion in 

 fractions of a revolution. 



A complete revolution is called 360. One quarter of a 

 revolution, therefore, is 90. Each degree is divided into 

 60 equal parts called minutes, meaning small. The minute 

 was found not to be small enough, and a second division was 

 made which consists of the 60 equal parts of the minute, 

 which are called seconds. 



Not only is rotary motion measured by this means, but the 

 same system is used to indicate direction. Thus a road may 

 cross another road at what is called an angle of 30. This 

 means that a wagon, in going from one road to the other, 

 must turn 30 in order to take the desired direction. We 

 reckon direction, in regard to the north and south line, by 

 this same system. See Sections 49 and 61. 



Altitude may be given in feet, or other units of length, or it 

 may be stated in degrees, minutes, and seconds. We can say 

 that a building is 62 high, from a certain position, which 

 means that it is necessary to raise our eyes 62 above the 

 horizontal plane in order to see the top of the building. 



References : 



Any text in Geometry. See Angular Measurement. 

 1. Forest Service Bulletin 36 : 98-106. Instruments for Measur- 

 ing Heights. 



Experiment 22. To Make and Use a Clinometer. 



a. On a piece of cigar-box wood 6" X 4" draw a semicircle 

 with the center at the middle of the long side. Mark every 

 5 with the protractor and number from the right angle mark, 

 each way, 0-90. 



Drive a brad at the center of the semicircle and one at each 



