NYMPHÆA (amongst the ancients), doubtful what structures they were; some take them to have been grottoes, deriving their name from the statues of the nymphs with which they were adorned; but that they were considerable works appears from their being executed by the emperors, (Ammian, Victor, Capitolinus) or by the city prefects. In an inscription, the term is written nymphium. None of all these nymphæa has lasted down to our time. Some years since, indeed, a square building of marble was discovered between Naples and Vesuvius, with only one entrance, and some steps that went down to it. On the right hand as you enter, towards the head, there is a fountain of the purest water; along which, by way of guard, as it were, is laid a naked Arctusa of the whitest marble; the bottom or ground is of variegated marble, and encompassed with a canal fed by the water from the fountain: the walls are set round with shells and pebbles of various colours; by the setting of which, as by so many strokes in a picture, are expressed the 12 months of the year, and the four political virtues; also the rape of Proserpine; Pan playing on his reed, and soothing his flock; besides the representations of nymphs swimming, sailing, and wantoning on fishes, &c.
It seems pretty evident that the nymphæa were public baths; for at the same time that they were furnished with pleasing grottoes, they were also supplied with cooling streams, by which they were rendered exceedingly delightful, and drew great numbers of people to frequent them. Silence seems to have been a particular requisite there, as appears by this inscription, Nymphis loci, hibe, lava, tace. That building between Naples and Vesuvius, mentioned above, was certainly one of these nymphæa.