The Mexican calendar-stone should be photographed on a large scale, and studied yet more carefully than it has been, for only a part of the divided circles which surround it have been explained. It should be photographed, because, to my certain knowledge, Mayer’s drawing gives the year, above the figure of the sun which indicates the date of the calendar, quite wrongly; and yet, presuming on his own accuracy, he accuses another writer of leaving out the hieroglyph of the winter solstice. What is much more strange is, that Humboldt’s drawing in the small edition of the _Vues des Cordillères_ is wrong in both points. The drawing in Nebel’s great work is probably the best. As to the wax models which Mr. Christy and I bought in Mexico, in the innocence of our hearts, a nearer inspection showed that the artist, observing that the circle of days would divide more neatly into sixteen parts than into twenty, had arranged his divisions accordingly; apparently leaving out the four hieroglyphics which he considered the ugliest. The details made out at present on the calendar are as follows:—the summer and winter solstices, the spring and autumn equinoxes, the two passages of the Sun over the zenith of Mexico, and some dates which possibly belong to religious festivals. The dates of the two zenith-transits are especially interesting; for, as they vary with the latitude, they must have been made out by actual observation in Mexico itself, and not borrowed from some more civilised people in the distant countries through which the Mexicans migrated. This fact alone is sufficient to prove a considerable practical knowledge of astronomy. Besides this, the Mexican cycle of fifty-two years seems to be indicated in the circle outside the signs of days, and also the days in the priestly year of 260 days; but to make these numbers, we must allow for the compartments supposed to be hidden by the projecting rays of the sun.