LENGTH OF A SHAFT OR ADIT. 



135 



In any of the above calculations, should the measurement 

 be by yards and feet, the number of square yards in the 

 land divided by 4,840 will give the number of acres. (See 

 Measures, APPENDIX.) 



To find the distance between the points where one is in- 

 accessible from the other for instance, on the other side of 

 a river. 



Required the distance between B and A. 



FIG. 64. 



Pace off from B, at right-angles to the direction B A, a 

 distance B E ; then continue pacing off a distance E c, so 

 that E c may be some even fraction of B E (say one-fourth 

 or one-eighth). Proceed, at right angles to c B, along c D 

 until a point D is reached, where DBA seem in one and the 

 same straight line. 



Then : 



Required length A B = 



CD X EB 

 EC 



Very frequently the prospector may wish to form some 

 idea of the length of an adit necessary to meet a perpen- 

 dicular shaft sunk from a certain known spot, or the length 

 of a vertical shaft necessary to be sunk to meet an adit 

 driven in from a certain point. To solve such problems 

 (as well as many others in connection with surveying) a 

 very limited knowledge of the properties of a right-angled 



