The Gregorian calendar was immediately adopted in France and Germany, and a little later in England. Now it is in operation in all the Christian countries of Europe, except Russia, where the Julian calendar is still followed. It follows that Russian dates do not agree with ours. In 1582, the difference was ten days, and this difference remained the same till the end of the seventeenth century, when the year 1700 was bissextile in the Julian, but not in the Gregorian calendar, so the difference increased to eleven days, and now in the same way is twelve days. Next to the year, comes the day as the most natural division of time in connection with the earth, though it admits of less difference in its arrangements, as we cannot be mistaken as to its length. It is the natural standard too of our division of time into shorter intervals such as hours, minutes, and seconds. By the word _day_ we mean of course the interval during which the earth makes a complete revolution round itself, while _daytime_ may be used to express the portion of it during which our portion of the earth is towards the sun. The Greeks to avoid ambiguity used the word _nyctemere_, meaning night and day. No ancient nation is known that did not divide the day into twenty-four hours, when they divided it at all into such small parts, which seems to show that such a division was comparatively a late institution, and was derived from the invention of a single nation. It would necessarily depend on the possibility of reckoning shorter periods of time than the natural one of the day. In the earliest ages, and even afterwards, the position of the sun in the heavens by day, and the position of the constellations by night, gave approximately the time. Instead of asking What "o'clock" is it? the Greeks would say, "What star is passing?" The next method of determining time depended on the uniform motion of water from a cistern. It was invented by the Egyptians, and was called a clepsydra, and was in use among the Babylonians, the Greeks, and the Romans. The more accurate measurement of time by means of clocks was not introduced till about 140 B.C., when Trimalcion had one in his dining chamber. The use of them, however, had been so lost that in 760 A.D. they were considered quite novelties. The clocks, of course, have to be regulated by the sun, an operation which has been the employment of astronomers, among other things, for centuries. Each locality had its own time according to the moment when the sun passed the meridian of the place, a moment which was determined by observation.