The existence of currents of electricity passing through the earth's crust and on its surface along the lines of least resistance has long been an established fact. Experiments conducted at Harvard, U.S.A., by Professor Trowbridge have proved beyond a doubt that, by means of such delicate apparatus as the telephone and microphone, it is possible for the observer to state in which direction, from a given point, the best line of conductivity runs. Under certain conditions the return current is so materially facilitated when brought along the line of a watercourse or a moist patch of the earth's crust, that the words heard through a telephone are distinctly more audible than they are at a similar distance when there is no moist return circuit. Deflections of the compass, due to the passing of earth-currents along the natural lines of conductivity in the soil or the rocks, are so frequently noticed as to be a source of calculation to the scientific surveyor and astronomer. It can thus be shown not only that definite lines of least electrical resistance exist in the earth, but also that natural currents of greater or less strength are almost constantly passing along these lines.
Some of the curious and puzzling empirical rules gained from the life-long experience of miners in regard to the varying richness and poorness of mineral lodes, according to the directions in which they strike—whether north, south, east or west—may very probably be explained, and to some extent justified, by the fuller light which science may throw upon the conditions determining the action of earth-currents in producing results similar to those of electro deposition. If, in a given region of a mineral-bearing country, the geological formation is such as to lend itself to the easy conduction of currents in one direction rather than in another, the phenomenon referred to may perhaps be partially explained. But, on the other hand, the origin of the generating force which sets the currents in motion must first be studied before the true conditions determining their direction can be understood. In other words, much that is now obscure, including the true origin of the earth's magnetism, must be to some extent cleared up before the reasons for the seemingly erratic strike of earth-currents and of richness in mineral lodes can be fully explained.
Practice, however, may here get some distance ahead of science, and may indeed lend some assistance to the latter by providing empirical data upon which it may proceed. When once it is clearly seen that by delicate electrical instruments, such as the telephone, the microphone and the coherer as used in wireless telegraphy, the line of least resistance on any given area of the earth's surface or any given piece of its crust may be determined, the bearing of that fact in showing the best lines of moisture and therefore the likeliest lines for mineral lodes will soon be recognised in a very practical manner.
No class of men is keener or more enterprising in its applications of the latest practical science to the getting of money than mining speculators. Nor have they at all missed the significance of moist bands occurring in any underground workings as a very favourable augury for the close approach of highly mineralised lodes. If, then, moisture be favourable, first to the presence of mineral-bearing country and secondly to the conductivity of electrical lines, it is obvious that there is a hopeful field for the exercise of ingenuity in bringing the one into a practical relation to the other.
The occult scientific reasons for the connection may not be understood; but it is sufficient for practical purposes to know that, in a certain line from the surface outcropping of a mineral lode, there has been given a demonstration of less electrical resistance along that line than is experienced in any other direction; also to know that such a line of least resistance is proved to have been, in almost innumerable instances, coincident with the best line of mineral-bearing country. The case is similar to that of the rotation of crops in its relation to scientific microbiology. The art of mining may get ahead of the science of physiography in respect of earth-currents and lines of least resistance, as showing where mineral lodes may be expected. Yet there is no doubt whatever that science will not in the one case lag so far behind as it has done in the other.
The first notable service rendered by systems of the kind indicated will no doubt be in connection with the rediscovery of very valuable lodes which have been followed up for certain distances and then lost. In an instance of this description much fruitless exploration drives, winzes and "jump-ups" may have been carried out in the surrounding country rock near the place where the lode last "cut out"; but, in the absence of anything to guide the mine manager and surveyor as to the direction which the search should take, nothing but loss has been involved in the quest. Several properties in the same neighbourhood have, perhaps, been abandoned or suspended in operation owing to very similar causes.
The whole group may perhaps have then been bought by an exploration company whose modus operandi will be as follows: The terminal of the electrical exploration plant is fixed at the end of the lode where it gave out, or else immersed in the water of the shaft which is in connection with the lode system; and another similar terminal is fixed by turns in each shaft of the contiguous group. The electrical resistances offered to the return currents, or to the wireless vibrations, are then carefully measured; and the direction of the lost lode is taken to be that which shows the least resistance in proportion to the distance traversed. The work of carrying out such an investigation must of necessity be somewhat elaborate, because it may be necessary to connect in turn each shaft, as a centre, with every one of the others as subsidiaries. But the guidance afforded even of a negative character, resulting in the avoidance of useless cutting and blasting through heavy country, will prove invaluable.
Many matters will require attention, in following out such a line of practical investigation, which are to some extent foreign to the usual work of the mining engineer. For example, the conditions which determine the "short-circuiting" of an earth-current require to be carefully noted, because it would be fallacious to reason that because the line of least resistance lay in a certain direction, therefore an almost continuous lode would be found. Moreover, the electrical method must only be relied upon as a guide when carefully checked by other considerations. Other kinds of moist formations, both metalliferous and non-metalliferous, may influence the lines of least electrical resistance, besides those containing the particular metal which is being sought for.
The water difficulty has enforced the abandonment of very many valuable mines in which the positions of the lodes are still well known. Sunken riches lying beneath the sea in old Spanish galleons have excited the cupidity and the ingenuity of speculators and engineers; but the total amount of wealth thus hidden away from view is a mere insignificant fraction of the value of the rich metalliferous lodes which lie below the water level in flooded mines.
The point in depth at which the accumulation of the water renders further following of the lode impracticable may vary in different countries. In China, throughout whole provinces, there is hardly a mine to be found in which the efforts of the miners have not been absolutely paralyzed directly the water-level was reached. But in Western lands, as well as in South Africa and Australia, the immense capacity of the pumps employed for keeping down the water has enabled comparatively wet ground to be worked to a very considerable depth.