PREPARATION OF THE DESIGN 79 



too small a triangle for accuracy, and it is better 

 to take 6, 8 and 10, or even 9, 12 and 15. On A B 

 mark off the equivalent of 3 ft., and stretch out the 

 tape to complete the required triangle, remember- 

 ing to have the side representing 5 opposite where 

 the right angle is required. Stretch all tight and 

 the point C will be obtained which can be extended 

 as far as necessary, from A. This last method is 

 said to be the way by which the bases of the pyra- 

 mids were squared. 



In stretching triangles two people are needed to 

 prevent the tape riding up the stake while the 

 apex is being found. The angle where the ring is 

 can be secured by using two pins instead of one 

 thrust through, and on many occasions besides 

 the two pins is a useful device. 



The foregoing operations in surveying have all 

 been said to be performed by a tape. The sur- 

 veyor more often uses a chain composed of 100 

 links, the length of which may be seen marked 

 at the back of the measuring tape, and it is 66 ft. 

 long. At every ten links is a brass tab showing 

 the number of links from the ends, as 10, 20, 30, 

 40, and the centre is distinguished by a circular 

 disc. Thus the chain can be easily read from 

 either end. The link measure assists in the calcu- 

 lation of areas as 100,000 square links is an acre. 



It is a heavy instrument, and has that advan- 

 tage over a tape in windy weather, but in measuring 

 through shrubberies and herbaceous beds it 



