The sundial is dated 1743 and so belongs to the original building. It has the motto UMBRA SUMUS, "we are a shadow." The words come from an ode of Horace (4.7). You can read the entire poem in Latin and English online. The stanza in question poses a lovely contrast between the moon's renewal and our own mortality:
Damna tamen celeres reparant caelestia lunae; Nos ubi decidimus, Quo pater Aeneas, quo dives Tullus et Ancus, Pulvis et umbra sumus.
The moons, however, quickly repair their heavenly losses; when we have fallen to where Father Aeneas fell, and wealthy Tullus and Ancus, we are dust and shadow.(Aeneas is the Trojan hero who founded the Roman people; Tullus Hostilius and Ancus Marcius were the third and fourth kings of Rome.)
Photo by Paula Bailey at Flickr: