FLUID METHODS OF SURVEYING BOREHOLES 97 



shatter the flask. However, for small depths and pressures 

 bottles are still used, and in order to make the filling of the 

 flask as convenient as possible choose a flask with a wide 

 neck. The flasks used by chemists with ground-in glass 

 stoppers suit very well; however, common preserving bottles 

 with screw joints can be used. The external diameter 

 of the flask should be about 3 to 10 mm. smaller than the 

 tube diameter. In order to center the flask therein it is 

 wound about with band tape to the suitable thickness. 

 It is advisable not to put the flask directly on to the lower 

 joint but to interpose between a small cushion or wad. 



The hydrofluoric acid is to be got in the trade in various 

 strengths, mostly at 40 per cent acid. This is diluted 

 by water down to about 20 per cent, which will give good 

 results. Otherwise it is advisable just before the test to 

 fill a flask with the fluid and to determine how much time 

 is required to get a clearly visible mark at the fluid surface. 

 With a 20 per cent acid 15 to 20 min. are usually 

 required. 



There is yet to mention the strong etching action of the 

 acid, for carrying which it appears advisable to have strong 

 leather gloves. Especially should care be exercised in 

 taking the vessel out of the borehole, as it may at any time 

 occur that it is broken or has been eaten through by the 

 acid, which then flows out of the opened joints over the 

 hands of the person. The vessel is taken out of the tube 

 at the end of the measurement, emptied and the interior 

 and exterior rinsed in clear water; its further handling will 

 not be dangerous. 



The advantages of fluid methods of borehole surveying 

 are: 



1. The apparatus is easily constructed, read and manipu- 

 lated. 



2. It is cheap and the parts obtainable anywhere. 



3. It can be employed in boreholes of small diameter. 

 The principal disadvantages of the method are : 



1. It does not provide continuous registration in and 

 out of the hole. 



