The only classical notice of the employment of these engines for moving the invisible world (not, however, for good, but for evil) is to be found in the Annals of Tacitus (ii. 69), who thus enumerates them amongst the means, real or imaginary, whereby Livia's agent, Piso, occasioned, or aggravated, the final illness of the too popular Germanicus. "The severity of the attack (a fever) was heightened by the suspicion of poison on the part of Piso: and in fact there were discovered, hidden in the…