•222 The Electrical Method of Soil Moixtiirc Deterinlnation 



use might be made of the method in arid sandy regions for determining 

 the approximate depth at which the water table lies, e.g. supposing the 

 depth of this were 50' — starting with electrodes 6" deep say and a short 

 distance apart on increasing the distance the resistance would gradually 

 increase to a point but on reaching a distance apart of about 200' the 

 beaker experiment would lead us to expect a fairly sharp fall due to the 

 water at 50' depth. A few observations would suffice to determine at 

 what fraction of the distance apart when the resistance falls the water 

 table would be found. It is perhaps worthy of note in this connection 

 that the resistance of earth returns in telegraphy is very low; though of 

 course, as large plates are usually buried for this purpose, we should not 

 erpect anything like the resistances encountered in the soil experiments. 



Resistance-moisture Curves. 



Experiments were made in the laboratory to determine the character 

 of the resistance-moisture curve. Artificial mixtures, natural sands and 

 soils were employed and results obtained in accordance with those of 

 Whitney (^c), except that at low moisture contents definite discon- 

 tinuities were observed. 



The method of carrying out the experiments was alike in all cases. 

 500 grams of the soil or other substance used was placed in a wooden 

 box whose internal dimensions were 2|" x 6" x 2^" and the electrodes, 

 small carbon cones, were placed at each end. An extra amount of the 

 same sample was used as a reserve from which samples removed for 

 moisture determinations could be replaced, maintaining the original 

 weight of dry soil. The hygroscopic moisture was found not to affect 

 the resistance within the capacity of the instruments used; that is to 

 say the air dry substance had in all cases a resistance under the con- 

 ditions of experiment exceeding one megohm. As wetting a sample in 

 such a way as to render it only slightl)- moist is difficult the following 

 plan was adopted: the sample was wetted slightly with distilled water 

 and then thoroughly mixed, after which it was warmed to about 30° C. 

 and allowed to cool again once or twice. This seemed to distribute the 

 moisture evenly through the mass and was preferred to the alternative 

 of gradually drying out a saturated mass as its character is changed by 

 this treatment. The damp sample was then placed in the box and packed 

 down to a constant leveP — a thermometer placed in the middle of the 

 mass enabled the temperature to be read and the resistance was taken 



' Great clifTicultv was found in this in the case of the boulder einy; the observations 

 were less satisfactory on this account. 



