An apparatus for the measurement of quasi-static gravity gradients comprising a flexible string and output means for producing an output which is a function of said gravity gradients, wherein the string is fixed at both ends, and wherein the apparatus comprises sensing means for detecting the transverse displacements of said string from its undisturbed reference position due to a gravity gradient around said string. The output means is responsive to the detected displacement to produce said output which is a function of the gravity gradients. The apparatus further comprises a means mounted at a position corresponding to the mid-point between the fixed ends of the string and constructed to prevent movements of said string which correspond to its odd modes (starting from C-mode) whilst not affecting at least the movement of said string which corresponds to its second fundamental mode (S-mode).