south longitudinal plane through the instrument in the well as seen in fig3 the gyroscope may be rotated about axis 24 so that its signal output 39a is maximized when spin reference axis sra passes through the north south longitudinal plane and is zero when that axis is normal to that plane one usable gyroscope is model gi g6 a product of northrop corporation the accelerometer 21 which is simultaneously rotated with the gyroscope has an output as represented for example at 45 under instrument tilted conditions corresponding to tilt of axis 24 in north south longitudinal plane i e the accelerometer output is maximized when the gyroscope output indicates south alignment and again maximized when the gyroscope output indicates north alignment fig2 shows tilt of axis 24 from vertical 46 and in the north south plane for example further the accelerometer maximum output is a function of the degree of such tilt i e is higher when the tilt angle increases and vice versa therefore the combined outputs of the gyroscope and accelerometer enable ascertainment of the azimuthal direction of bore hole tilt at any depth measured lengthwise of the bore hole and the degree of that tilt fig5 diagrammatically illustrates the functioning of the accelerometer in terms of rotation of a mass 40 about axis 24 tilted at angle u03c6 from vertical 46 as the mass rotates through points 44 at.