It was erected originally at Heliopolis to Psammeticus I., of the
twenty-fourth dynasty, more than six centuries B.C. It is 72 feet
high. Its first site in Rome was in the Campus Martius, where is now
the Piazza dell'Impresa, where it was found and taken to its present
site. The Roman pedestal with inscription is in the Church of S.
Lorenzo in Lucina. The obelisk was repaired, and its present pedestal
formed of fragments of the Antonine Column, which stood near by. The
obelisk was brought to Rome by Augustus at the same time as the one in
the Piazza del Popolo, and was put up, according to Pliny (xxxvi. 15),
as a sun-dial:--
"The one that has been erected in the Campus Martius has been applied
to a singular purpose by the late Emperor Augustus--that of marking
the shadows projected by the sun, and so measuring the length of the
days and nights. With this object, a stone pavement was laid, the
extreme length of which corresponded exactly with the length of the
shadow thrown by the obelisk at the sixth hour on the day of the
winter solstice. After this period the shadow would go on day by day
gradually decreasing, and then again would as gradually increase,
correspondingly with certain lines of brass that were inserted in the
stone--a device well deserving to be known, and due to the ingenuity
of Facundus Novus, the mathematician. Upon the apex of the obelisk he
placed a gilded ball, in order that the shadow of the summit might be
condensed and agglomerated, and so prevent the shadow of the apex
itself from running to a fine point of enormous extent, the plan being
first suggested to him, it is said, by the shadow that is projected by
the human head. For nearly the last thirty years, however, the
observations derived from this dial have been found not to
agree,--whether it is that the sun itself has changed its course, in
consequence of some derangement of the heavenly system; or whether
that the whole earth has been in some degree displaced from its
centre--a thing that, I have heard say, has been remarked in other
places as well; or whether that some earthquake, confined to this city
only, has wrenched the dial from its original position; or whether it
is that, in consequence of the inundations of the Tiber, the
foundations of the mass have subsided, in spite of the general
assertion that they are sunk as deep into the earth as the obelisk
erected upon them is high."
_Regaining the Corso, the first turning on the right, Via Pietra,
leads into the_ PIAZZA DI PIETRA, in which are the ruins of