Letter from Thomas Gold to Dr Cecil Green, Science Services Division, Texas Instruments, Dallas, Texas He is sorry that Dr Green cannot come to the dedication [of the Space Sciences Building at Cornell University] on 19 October. Says that things are going well. Says that 'Arecibo [Ionospheric Observatory] is making great strides: we are, for example, on the point of proving that the rotation of Venus is locked to the slight tidal perturbation it receives from the earth. Such a lock can only occur if Venus had a liquid core. Radar from the earth therefore can establish the existence of a liquid core on Venus!' 26 September 1967