APPENDIX 



557 



The unit of length in the English system for the United 

 States is the yard, defined as above. The relation between 

 the centimeter scale and the inch is shown below. 



100 MILLIMETER8=r 10 CENTIMETERS = 1 DECIMETER = 3. 937 INCHES. 



Square inch 



INCHES AND TENTHS 



CENTIMETER AND INCH SCALES 



Measures of Surface. In the metric system the unit 

 of area used in the laboratory is the square centimeter 

 (cm. 2 ). It is the area of a square the edge of which is 

 one centimeter. The square meter (m. 2 ) is often employed 

 as a larger unit of area. In the Eng- 

 lish system both the square inch and the 

 square foot are in common use. Small 

 areas are measured in square inches, while 

 the area of a floor and that of a house lot 

 are given in square feet ; larger land areas 

 are in acres, 640 of which are contained in 

 a square mile. 



The square inch contains 2.54 X 2.54 

 = 6.4516 square centimeters. The relative sizes of the two 

 are shown in the accompanying figure. 



k The area of regular geometric figures is obtained by computation 

 from their linear dimensions. Thus the area of a rectangle or of a 

 parallelogram is equal to the product of its base and its altitude 

 (A = b X h) ; the area of a triangle is half the product of its base 

 and its altitude (A = \b X h) ; the area of a circle is the product of 

 3.1416 (very nearly 2 ^) and the square of the radius (A = Trr 2 ) ; 

 the surface of a sphere is four times the area of a circle through its 

 center (A = 4 Trr 2 ). 



SQUAKE CENTIMETER 

 AND SQUARE INCH 



