The Julian calendar, with these changes by Augustus, remained in use until the year A. D. 1582, nearly a century after the discovery of America. Then it was learned that the average year of 365¼ days was still not exactly right according to the motion of the earth around the sun. The exact time is 365 days, 5 hours, 48 minutes and 46 seconds, being 11 minutes and 14 seconds less than 365¼ days. When, therefore, we add a day to the year every four years, as Caesar commanded, we are really adding too much. This excess was corrected by Pope Gregory XII in 1582, when he changed the calendar so that the last year of a century should be a leap-year only when its number could be divided evenly by 400. Thus, 1700, 1800, and 1900 were not leap-years, though the year 2000 will be. This new calendar, which is the one now generally in use in most of the world, is known as the Gregorian calendar. Thus the plan and principle of the calendar, as well as our smaller divisions of time, in spite of the small changes by Caesar and Gregory, have remained from the Babylonian days down to the present; and we have done nothing to their system in all these thousands of years, except, incidentally to correct it. Only once in history have the measures of the ancient calendar been set aside. That was in France at the time of the Revolution, when the French people, in their passionate hatred of all the traditional things that reminded them of their past sufferings, invented a new calendar, in which they changed the names of months and days, and counted the years from 1792, the first of their liberty. They also abolished all Sundays and religious festivals, and divided the day into ten hours. This played havoc with time-keeping, and caused great confusion. Watches and clocks were made with one circle of numbers for the new hours, and another, within, on which were shown the old hours which people could understand. But this complication lasted only a few years, for the traditional system was soon restored.