Antonine era, it will be seen that, although there is nothing more than probable evidence in favour of the above supposition, yet it has more in its favour than any of the other conjectures mentioned. The present building was erected by Innocent XII. at the end of the seventeenth century in order to prevent the fall of the columns, which had become dangerously disjointed. The entablature has been restored in many parts, and a kind of attica erected over it, which gives the ruin the appearance of being in better preservation than it really is. [Sidenote: Gnomon Obelisk.] North of the Piazza Capranica, in the open space called the Piazza di Monte Citorio, is a large obelisk of red syenite. This is the Gnomon Obelisk, of which Pliny gives an interesting account in his ‘Natural History.’ It was brought by Augustus from Egypt, with that which is now in the Piazza del Popolo, and was erected on the Campus Martins under the directions of the mathematician Facundus Novus to serve as a sun-dial, by which not only the hour of the day, but also the day of the month, might be shown. For this purpose the pavement of the piazza in which it stood was marked out with a complicated system of lines in bronze; and, to prevent any disturbances caused by the settlement of the foundations, they were laid as deep below the ground as the height of the obelisk itself. Pliny remarks that when he wrote, the gnomon had ceased for thirty years to mark the time rightly, and he ascribes this inaccuracy to some displacement of the obelisk due to natural causes, such as earthquakes or inundations.[96] It is more probable that the inaccuracy of the Julian calendar gradually produced the change. Ammianus Marcellinus, the Notitia, and the anonymous writer of the Einsiedlen MS., all mention this obelisk as still standing on the place where Augustus placed it. It was then--after the ninth century--lost for a time, but discovered again in 1403 with a part of the figures of the dial. Marliani, in the first half of the sixteenth century, mentions a part of the obelisk as lying neglected in a cellar near S. Lorenzo in Lucina, and it was not erected upon the present site until 1792. To the east and north of the Monte Citorio lay the great buildings of the Antonine era, of which we still have some remains in the base of the Pillar of Antoninus Pius, now in the Giardino della Pigna of the Vatican, the magnificent Pillar of M. Aurelius in the Piazza Colonna, and the remains of the arch of the latter emperor, now in the Palace of the Conservators of the Capitol. [Sidenote: Pillar of Antoninus Pius.]