Note to Romeo and Juliet, 2.1.24. "To raise a spirit in his mistress' circle"
In Shakespeare's time it was widely believed that a philosopher such as Dr. Faustus [the star of Christopher Marlowe's play] could "raise" a spirit from the underworld by drawing a circle and conjuring, by saying a charm. A magician could "lay" [cause to descend] the spirit with another charm.
Mercutio pretends to be talking about spirits in this sense, but he uses double-entendres to make one sex joke after another.
Frontispiece to a 1620 printing of Doctor Faustus
showing Faustus conjuring Mephistophilis