20 will trace their relative path on record plate 13 such a trace is indicated at 203 fig4 fig5 is a side view of the instrument 10 showing more detail with a displaced position indicated in phantom or broken lines lower mounting plate 11 shown in section may be a ribbed disc like casting of appropriate known design made of aluminum alloy or other suitable material upper plate 12 may be similar tripod legs 31 33 are fastened rigidly at their upper ends to plate 12 to the surface of lower mounting plate 11 is attached record plate 13 by suitable removable means such as screws 80 the space between the plates 11 12 is enclosed by a flexible extensible cover 50 which may be conveniently cut from a length of ventilating hose of the sort made of waterproof rubberized cloth and having circumferential pleats like a bellows it is conveniently secured around the circumference of each mounting plate 11 12 by flexible straps or hose clamps 51 52 when for example the upper plate 12 is displaced to the left respective to lower plate 11 as indicated by arrow 200 fig5 the arm means 30 is moved to a new position as indicated in broken lines at 30 39 and stylus plunger 20 is moved from its initial position at a point 201 to a new position at 202 a stylus trace is made in this process the cross section of the instrument 10 as seen from the side changes from rectangular to rhombic the protective cover 50 bends and stretches to accommodate the.