 In his 1638 Discorsi , Galileo 's character Salviati , widely regarded as Galileo 's spokesman , held that all unequal weights would fall with the same finite speed in a vacuum . But this had previously been proposed by Lucretius and Simon Stevin . Cristiano <unk> 's Salviati also held it could be experimentally demonstrated by the comparison of pendulum motions in air with bobs of lead and of cork which had different weight but which were otherwise similar . 
