Measurement of Area and Volume 33 



This is the same ratio which we obtained in Experiment 

 3, and called TT. 



From these results, the following rule for finding 

 the area of a circle is obtained : 



Measure the length of the radius, square this, 

 i.e. multiply the number of units of length in the radius 

 by itself, and multiply the result by TT. 



Example. The radius of a circle is 6*40 centi- 

 metres. Its area is consequently 



(6-40 x 6-40 x 3-14) sq. cm. - 129 sq. cm. (nearly). 



Another method of finding the areas of irregular or 

 curvilinear figures will be described in a later chapter 

 dealing with the use of the balance. 



Note. Instead of the squared paper divided into 

 quarters and hundredths of a square inch as described 

 above, paper similarly divided into square centimetres 

 and square millimetres may be used. The areas may 

 thus be obtained in square centimetres instead of in 

 square inches. The use of such paper has, however, 

 the great drawback that the counting of very small 

 squares is a somewhat trying task, more especially 

 to anyone suffering from defective vision. 



19. The measurement of volume, i.e. the quantity 

 of space occupied by a body, is carried out on prin- 

 ciples similar to those governing the measurement of 

 area. Thus the unit of volume is a cube, the length 

 of each edge of which is unity. (A cube is a solid 

 figure with six faces, each of which is a square.) The 

 British units in common use are the cubic inch, cubic 

 foot and cubic yard, while in the Metric System, the 

 cubic centimetre and cubic metre are usually employed. 

 In measuring a brick, its volume is generally stated in 

 cubic inches or cubic centimetres ; the volume of air 



H. D. S. 3 



