Psellus founded a system of demonology, which had for its basis the natural history and habitation of demons. His first class consisted of fiery devils, that wandered in the regions near the moon, but were prevented from entering that luminary. They displayed their power in blazing stars, in counterfeit suns, moons, and meteoric lights, and prevented foul weather. These demons, we are informed, occasionally resided in the furnaces of Hecla, Etna, or Vesuvius. His second class was made up of aërial devils, that inhabited the atmosphere, caused tempests, thunder, and lightning, rended asunder trees, burned down steeples and houses, struck men and beasts, showered stones, wool, and frogs from the skies; counterfeited in the clouds the battles of armies, raised whirlwinds, fires, and corrupted the air so as to spread disease. The third class was terrestrial devils, such as lares, genii, fauns, satyrs, wood-nymphs, foliots, robin-goodfellows, or trulli. The fourth class was aqueous devils, as the various descriptions of water nymphs. The fifth class consisted of subterranean devils, known by the name of Getuli or Cobals. They preserved treasure in the earth, and prevented it being suddenly revealed; they were also the cause of horrible earthquakes. Psellus's sixth class of devils was named lucifugi. They delighted in darkness, entered into the bowels of men, and tormented those whom they possessed with frenzy and the falling sickness. An opinion prevailed that devils possessed corporeal frames, capable of sensation; that they could feel and be felt; that they could injure and be hurt; that they were nourished with peculiar food; that they did not hurt cattle from malevolence, but through a desire to obtain natural temperate heat and moisture from the animals they killed; that they disliked the sun's rays; and that they attained a great age. Of all the kinds of demons we have heard of, the most loathsome are the vampires. Horst speaks of a vampire as a "dead body which continues to live in the grave, which it leaves, however, by night for the purpose of sucking the blood of the living, whereby it is nourished and preserved in good condition, instead of becoming decomposed like other dead bodies." Fischer, who believed there were vampires, informs us that the bite of a vampire left no mark upon the person, but that the bite speedily proved fatal, unless counteracted by the injured person eating some of the earth from the vampire's grave, and smearing himself with his blood. These precautions had only a temporary effect, if at all successful; for the bitten victim, sooner or later, became a vampire himself--died and was buried, but continued to follow the examples of old vampires in nourishing themselves, infecting others, and propagating vampirism.