Description of the Hemisphere, the Twelve Columns, and their Bowls
Opposite these gates the crowning part of the whole was the hemisphere, which rose to the very summit of the church. This was encircled by twelve columns (according to the number of the apostles of our Saviour), having their capitals embellished with silver bowls of great size, which the emperor himself presented as a splendid offering to his God.
Source: Life of Constantine (New Advent)