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INTRODUCTION TO MECHANICS. 



greater the number of degrees, and two angles are said to be equal when 

 they contain an equal number of degrees : the two angles (ADC and 

 C D B, Jig. 4) are together just equal to half a circle ; they contain, there- 

 fore, 90 degrees each : 90 being a quarter of 360. An angle of 90 degrees 

 is called a right angle, and when one line is perpendicular to another, it 

 forms a right angle on either side. Angles containing more than 90 

 degrees are called obtuse angles (Jig. 5) ; and those containing less than 

 90 degrees are called acute angles (Jig. 6). Thus the angles of a square 



Fig. 6. 



Fig. 5. 



table are right-angles, those of an octagon table obtuse angles, and 

 those of sharp pointed instruments, acute angles. 



When a billiard player strikes the ball perpendicularly against the 



Fig. 7. 



cushion, it returns in the same direction 

 but when he sends it obliquely to the cushion 

 it rebounds obliquely on the opposite side; 

 the ball in this latter case describes an 

 angle, the point of which is at the cushion. 

 The more obliquely the ball be struck against 

 the cushion, the more obliquely it will re- 

 bound on the opposite side, so that a billiard 

 player can calculate with great accuracy in what 

 direction it will return. Fig. 7 represents a 

 billiard table : if a line AB be drawn perpen- 

 dicular to the cushion from the point where 

 the ball A strikes, it will divide the angle 

 which the ball describes into two parts, or two 

 angles ; the one will show the obliquity of the 

 direction of the ball in its passage towards the 

 cushion, the other its obliquity in its passage 

 back from the cushion. The first is called the 



