CIRCLE 



1387 



CIRCULATION OF THE BLOOD 



circle) with the area of the square in which 

 it is. The eye tells you it is about % of it. 

 Is it more or less than % of it? It is a little 

 more than % of the square. So we see that 

 a+6+c+rf, or the whole circle, is a little more 

 than 4X%, or 3 and a fraction, times as great 

 as the square of the radius. With the eye we 

 cannot tell just what this fraction is, but it 

 has been proved that the area of a circle is 

 3.14159+ times as large as the square built on 

 the radius of the circle. It is put into a short 

 form as follows: 



Area of a circle = 3.1416Xradius2 

 or 



Area of circle = irxr 2 

 or 



Area of circle = irr2 



This can be seen very clearly by making your 

 drawing on squared paper thus: 



QJ 

 O. 



c 

 en 



Radius 



Here you may find by actual count of the 

 little squares how many times as large the 

 circle is as the square built on the radius. 



Problems. 1. What is the circumference of a 

 cylindrical iron pipe whose diameter is 10 

 inches? 



Solution. Circumferencen27rr. 



Circumference in inches = 2X3. 1416x5 = 31. 416. 



2. How many tulip bulbs set 6 inches apart 

 will be required for the outside row of a flower 

 bed 6 feet in radius? 



Solution. Circumference = 2irr. 

 Circumference in feet = 2xirx6. 

 Number of bulbs = 2x3. 1416X6X2 = 75. 3984. 

 Practically 75 bulbs. 



3. What is the circumference of a stack of 

 hay at the bottom if the radius is 9 feet? 



4. What is the circumference of a circular 

 pond 40 feet in diameter? 



5. John and Harry set up a circular tent in 



their yard at the foot of a tree. They erected 

 the center pole at a point six feet from the 

 tree. How many square feet of ground did 

 the tent cover? How many yards around was 

 the space enclosed by the tent? See illustra- 

 tion. 



Solution. The distance from the pole to the 

 tree is the radius of a circle. 



Area of circle =irx" 2 . 



Number sq. ft. covered = 3. 1416x62=113. 0976. 



The distance around the space enclosed by 

 the tent is the circumference of the circle 

 whose radius is 6 feet. 



Circumference =2 XT X?"- 2 



Number of yards around = 2 X 3.1416 Xff = 

 12.5664. 



6. Find the cost of clearing a circular skating 

 pond 300 feet in diameter, when the snow is 

 6 inches deep on the ice and it costs $.10 a 

 cubic yard to clear it away. 



7. A cart-wheel, 3 feet in diameter, turns 

 around at the rate of 40 miles an hour; how 

 many revolutions does it make in 1 hour? A.H. 



CIRCULATION OF THE BLOOD. The 



arteries, veins and capillaries form a complete 

 system of pipe lines in the body, through 

 which the blood is driven by the central pump 

 the heart. While all the vessels of this very 

 complete pipe line form one complete system, 

 this is in reality three systems, each having a 



