ABC OF THE STEEL SQUARE 41 



Various writers have discussed the proper- 

 ties of the right-angled triangle, but we all 

 know that a square erected on a hypothenuse 

 of a right-angled triangle is equal to the sum 

 of the squares erected on the base and perpen- 

 dicular. This is a well-known mathematical 

 truth, and it may be applied in the tests we 

 are making. Those carpenters who have had 

 occasion to lay out the foundations of houses 

 are well acquainted with the old rule frequently 

 known as "the 6, 8 and 10," which depends up- 

 on the relationship of the squares o-f the perpen- 

 dicular and the base to the square of the hypoth- 

 enuse. Thus the square of 6 is 36, the square of 

 8 is 64. The sum of 36 and 64 is 100. And the 

 square of 10 is 100. Now let us make applica- 

 tion of this rule to test the steel square. 



For the sake of accuracy we want to take* 

 figures which are as large as possible, so as to 

 reduce the possible error in measurement to the 

 smallest possible dimensions. Let us take for 

 dimensions, 9, 12 and 15 inches. That these 

 will serve is easily demonstrated. The square 

 of 9 is 81. The square of 12 is 144. The sum 

 of these squares is 225, and the square of 15 

 is 225. Therefore, if the tool that we are test- 

 ing shows a dimension of exactly 15 inches meas- 

 ured from 9 on the outside of the tongue to 12 



