

ENGINEERING FOR LAND DRAINAGE. 



17, place an upright staff not less than 10 feet long. 

 Two or three hours before noon mark the extremity A 

 of its shadow. Describe an arc of a circle with S the 

 foot of the staff for centre and SA the distance to the 

 extremity of the shadow for radius. About as many 



Fig. 17. — Obtaining Meridian by Equal Shadows of the Sun. 



hours after noon as it had been before noon when the 

 first mark was made, watch for the moment when the 

 end of the shadow touches the arc at another point B, 

 Bisect the arc AB 2± N, Draw SN and it will be 

 the true north and south line. Set up the compass at 

 vS, sight on N or 5i\^ produced and read the needle. 

 The reading will be the declination of the needle at 

 that place. It is more important, however, to record 

 on the notes the declination used than it is to go into 

 the niceties of obtaining and using an absolutely correct 

 declination angle for line work of the character herein 

 described. 



If the compass has a variation plate, set off the dec- 

 lination assumed or determined and record all bearings 

 as read. If there is no such provision for mechanically 



