. Complete arithmetic . ends by the cir-cumference, is called a diameter. A straight line drawnfrom the center to any point in the circumference is calleda radius. A radius is equal to half a diameter. 186. Measuring Arcs. The cirTcumference of every circle is di-vided into 360 equal parts calleddegrees. Corresponding to every arc, like ABin the figure, is an angle at the center,A OB. Therefore to every degree on thecircumference {arc degree) there is adegree at the center {angle degree), andan arc has as many degrees as its angle. 187. Right Angle and Perpendicular. The angle at thecenter of
. Complete arithmetic . ends by the cir-cumference, is called a diameter. A straight line drawnfrom the center to any point in the circumference is calleda radius. A radius is equal to half a diameter. 186. Measuring Arcs. The cirTcumference of every circle is di-vided into 360 equal parts calleddegrees. Corresponding to every arc, like ABin the figure, is an angle at the center,A OB. Therefore to every degree on thecircumference {arc degree) there is adegree at the center {angle degree), andan arc has as many degrees as its angle. 187. Right Angle and Perpendicular. The angle at thecenter of a circle corresponding to one fourth of a circum-ference is called a right angle, and the two lines that forma right angle are said to be jyerjpendicular to each other. Thus angle -4 Oi? is a right angle, and OB is perpendicular to OA,A right angle, therefore, contains \ of 360 degrees, or 90 degrees. An angle less than a right angle is called an acute angle, and onegreater than a right angle is called an obtuse ANGLE AND ARC MEASURES 181 188. Angle Measure and Arc Measure. The following arethe tables of circular measure: Angle Measure 60 seconds () = 1 minute ()60 minutes = 1 degree (°)360 degrees = 4 right angles Arc Measure 60 seconds () = 1 minute ()60 minutes = 1 degree (°)360 degrees = 1 circumference That is, angle measures and arc measures are practically the degree of the circumference of the earth at the equator contains60 geographical miles, or statute miles. 189. Miscellaneous Measures. The following tables are used: JSfumhers 12 units = 1 dozen (doz.)12 dozen = 1 gross (gr.)12 gross = 1 great gross20 units = 1 score Paper 24 sheets = 1 quire20 quires = 1 ream 2 reams = 1 bundleo bundles =: 1 bale The great gross and score are becoming obsolete. For conveniencein counting, 500 sheets are now commonly called a ream. EXERCISE 94 Express as seconds: 1. 17. 3. 48. 5. 3°. 7. 3° 5. 9. 8° 5 8. 2. 32 4. bQ>\ 6. 2r 8. 4° 8. 10. 9° 62. Express a
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