John, sometimes called Eugenius Theodidactus, was a great pretender to skill in the Rosicrucian philosophy and the celestial signs, in the reign of King Charles I., and wrote a number of chemical and astrological works. This empiric was much consulted by the Duke of Buckingham, who believed in judicial astrology. He employed Heydon to calculate the king's and his own nativity, and was assured that his stars had promised him great things. The duke also employed Heydon in some treasonable and seditious practices, for which he was sent to the Tower. The astrologer lost much of his reputation by telling Richard Cromwell and Thurloe, who went to him disguised like cavaliers, that Oliver would infallibly be hanged by a certain time. This period, however, he outlived several years.