Hal Clement's science-fiction stories are models of scientific ingenuity, excellent examples of the kind of 'pure' science-fiction that delights critics and readers. Mission of Gravity is his best.The planet Mesklin was in effect an enormous discus and the variations in gravity on its surface were colossal. At the centre the force was 600 times that on Earth and it was impossible for human beings to live at either of the two poles. In order to recover the scientific instruments from an Earth rocket which had gone astray on Mesklin, Charles Lackland, from his space ship, strikes a bargain with one Barlennan, a Mesklinite merchant skipper, who is ready to collaborate in return for scientific knowledge. He and his crew of tiny insect-like creatures go to work.But Barlennan has little hope of becoming Mesklin's Prometheus: the gap between human and Mesklinite culture is too vast.