PURPOSE:To measure land subsidence at a prescribed depth by detecting a magnetic field formed by a magnet inserted into the side wall of a boring hole at a required depth by a magnetic measuring probe descended into the boring hole with time intervals. CONSTITUTION:The measuring probe 6 is descended into the boring hole 2 and each position of the magnet 5 is searched by a measuring device 12. A base board 1 is fixed as an immovable point and each depth of the magnet 5 is read out. The depth of each magnet 5 is measured with time intervals and respective quantity of displacement from an initial measured value is observed as the quantity of land subsidence. The output polarity of resistance elements 8a, 8b are inverted each other to offset both outputs each other and the detected value is read out by a galvanometer through a balancing circuit 13. Therefore, the relation between the output from the balancing circuit 13 and a vertical distance from the magnet 5 indicates zero because outputs from the resistance elements 8a, 8b are offset when the intermediate point O of the elements 8a, 8b reaches most close to the magnet 5. The scale at that time indicates the precise depth of the magnet.
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