20 Domestic Science 



therefore 0'07 cm. must be added to 18' 1 cm. to 

 obtain the reading at C correct to 0*01 cm. 



The vernier described above is of the type known 

 as " forward reading ", i.e. the numbers of the divisions 

 of the vernier and those of the scale both increase in 

 value from left to right, and the reading of the vernier 

 is taken in the same direction as that of the scale. 

 A vernier 1*1 cm. long, with ten equal divisions as 

 before, is said to be " backward-reading ". Tn this 

 case, the zero of the vernier scale is at the right-hand 

 end, and the numbers of its divisions increase in value 

 from right to left, so that when the zero is placed in 

 the required position, the reading on the vernier is 

 obtained by looking along the scale in the opposite 

 direction to the usual. The latter type possesses the 

 advantage of a more open scale, the slightly larger 

 divisions upon it being more readily distinguished, 

 and hence causing less eye-strain. This may be still 

 further obviated by reading the vernier with the help 

 of a lens of low power. 



Many other forms of vernier are used in con- 

 nection with various scientific instruments, but the 

 consideration of their methods of construction and 

 their uses is beyond the scope of our work. Practice 

 in reading a simple vernier may be obtained by exami- 

 nation of those which may be found attached to the 

 scales of an ordinary standard barometer. 



QUESTIONS 



1. The radius of a penny is 0'6" and that of a 10-centime piece 

 is 1'5 cm. By how many mm. is the circumference of a penny 

 longer than that of a 10-centime piece ? 



2. A 60-inch tape-measure is actually 60| inches long. What 

 would be the error in the length of a cricket pitch, which should 



