THE AZTEC CALENDAR STONE, another monument of Mexican antiquity, was found in December, 1790, buried under ground in the great square of the capital. Like the idol image of Teoyaomiqui, and the sacrificial stone, it is carved from a mass of basalt, and is eleven feet eight inches in diameter, the depth of its circular edge being about seven and a half inches from the fractured square of rock out of which it was originally cut. It is supposed, from the fact that it was found beneath the pavement of the present _plaza_, that it was part of the fixtures of the great Teocalli of Tenochtitlan, or that it was placed in some of the adjoining edifices on palaces surrounding the temple. It is now walled into the west side of the cathedral, and is a remarkable specimen of the talent of the Indians for sculpture, at the same time that its huge mass, together with those of the sacrificial stone and the idol Teoyaomiqui, denote the skill of their inventors in the movement of immense weights, without the aid of horses. * * * * * The Aztecs calculated their civil year by the solar; they divided it into eighteen months of twenty days each, and added five complimentary days, as in Egypt, to make up the complete number of three hundred and sixty-five. After the last of these months the five _nemontemi_ or "useless days" were intercalated, and, belonging to no particular month, were regarded as unlucky, by the superstitious natives. Their week consisted of five days, the last of which was the market day; and a month was composed of four of these weeks. As the tropical year is composed of about six hours more than three hundred and sixty-five days, they lost a day every fourth year, which they supplied, not at the termination of that period, but at the expiration of their cycle of fifty-two years, when they intercalated the twelve days and a half that were lost. Thus it was found, at the period of the Spanish conquest, that their computation of time corresponded with the European, as calculated by the most accurate astronomers.