INSTRUMENTAL SURVEY OF BOREHOLES 47 



d. It should be immune from injuries due to water or 

 mud pressures, chemical actions in the hole or strata, 

 etc. 



e. It should be uninfluenced by local attractions such as 

 are set up by magnetic strata, metalUc linings and rod 

 magnetism. 



/. It should be simple and free from many technicalities 

 and therefore less liable to derangement and needing less 

 supervision. 



g. It should be easily understood and, if possible, capable 

 of being read direct with few adjustments. 



h. It should be capable of registering at great depths, i.e., 

 it should be of small diameter. This claim is a failing of 

 most instruments. 



i. Its data should always be subject to check up and down 

 the hole and also by different means. 



The several methods invented to investigate the course 

 of boreholes may be broadly classified under the following 

 general heads, though certain instruments may be included 

 under two or more of these : 



1. Fluid methods utilizing the shape of the fluid outline 

 in a cylindrical retainer. Such a fluid may be hydrofluoric 

 acid, cement, gelatine, mercury, copper sulphate, wax or 

 paraffin. 



2. Plummet and magnetic needle methods in which the 

 dip and deflection are read on special arcs in the instrument 

 or by core measurers aboveground. 



3. Electrical methods, wherein plummets are actuated or 

 pricking cones are set in motion, also electrolytic deposition 

 devices, wax-warming arcs, and other registration con- 

 trivances. 



4. Pendulum methods either simple or compound. 



5. Photographic methods wherein the position of plum- 

 mets and compasses is recorded, or where kinemato- 

 graphic records of successive positions of these, or direct 

 photographic views of the unlined sides of the hole, are 

 provided. Multiple photographic devices and multiple 

 views of shaped notches, etc., are included here. 



