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salting. This is very clearly shown in diagram No. 4. This is based on 

 the result of tests on over 100 pipes, each plotted point being the value 

 from a group of three to eight 1 in. pipes, each 6 ft. to 7 ft. long. No two 

 pipes were less than 6 ft. apart or more than 12 chains, and they were 

 placed in groups of three to eight over the tract of country described, and 

 therefore in all classes of soil. Salting was effected by pouring into each 

 4 gallons of saturated-brine solution, and then filling the pipe with salt. 

 Tests were repeated within an hour or so of salting. Initial resistances up 



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/ni/'ial reiisfance /n oh/ru of indific/ua/ pipes _ z^. C. bridge 



Fig. 4. 



to 250 ohms are in loam, sand, or clay, and over 700 are in heavy shingle ; 

 others are in a mixture of earth and shingle. As the degree of penetration 

 of the salt into the surrounding soil is a matter of considerable uncertainty, 

 the percentage decreases in resistance are much more uniform than might 

 be expected. The curve drawn through the plotted points suggests the 

 probable average reduction. Thus it appears that the reduction of re- 

 sistance by salting is largely dependent on the magnitude of the initial 

 resistance, and the salting is highly efficacious in reducing high resistances. 



A further interesting feature of salting is the improvement of the 

 " earth " with time. Apparently the salt continues to work into the 

 surrounding soil for a considerable time after the initial salting. In time 

 the salt must be dissolved by successive rains, and finally the " earth " 

 must deteriorate. Curves are given demonstrating this in diagram No. 5. 

 Apparently the brine continues to percolate into the surrounding soil, 

 partly by soakage and partly by capillarity. Eventually the solution is 

 weakened by the addition of rain and subsoil water until the resistance 

 again increases. 



From a cursory inspection of these values it is evident that — at least, in 

 many cases- — single-pipe earths, however well salted, will not give the value 

 of resistance desired. The only loophole remaining is by way of placing 

 a number of pipes in a group. The question immediately arises as to how 

 close such pipes may be placed and still remain effective. A consideration 

 of curves of the decrease in resistance with increase of distance indicates 



