Apparent Solar Time is shown by the sundial, and therefore depends upon the motion of the sun. Mean Solar Time is shown by a correct clock. The difference between Mean Time and Apparent Time, that is, between the time shown by the clock and the sundial, is called the Equation of Time, and in the _Nautical Almanack_, a Government publication, there are tables showing these differences. =Day and Night.=--Obviously the hours of darkness offered a greater problem to the horologist than the hours of light. His sundial was of no use at night and of little use on cloudy days. The hour-glass was not a piece of mechanism a man would wish to employ to record the night watches. Some other self-acting mechanism had to be devised. The interval between sunset and sunset, or sunrise and sunrise, or noon and noon, was divided by the Babylonians, who had a love for the duodenary system, into twenty-four hours. It is curious to read that "until the eighteenth century in England the hour was commonly reckoned as the twelfth part of the time between sunrise and sunset, or between sunset and sunrise, and hence was of varying durations" (_Webster's New International Dictionary_, 1914). The hour was further divided, also by the Babylonians, into periods of sixty minutes. It was the Babylonians who first divided the circle into 360 degrees, and Ptolemy followed this division. The dial of a clock was at first termed the _hour-plate_, as only hours were engraved upon it and only one hand was employed. Later, another hand was added, the minute hand, which travelled a complete circuit while the hour hand was travelling between two hour numerals. Later, again, a new sub-dial was added, and a seconds hand recorded the sixty seconds which made the minute. The term "second" was at first called "second-minute," denoting that it was the second division of an hour by sixty. The learned John Wilkins, Bishop of Chester, that extraordinary old savant, writes in 1650: "Four flames of an equal magnitude will be kept alive the space of sixteen _second-minutes_, though one of these flames alone, in the same vessel, will not last at most above twenty-five or thirty seconds." These dry facts may serve to whet the curiosity of the student in regard to the measurement of time and its origin. They add a piquancy to the clock dial as we now know it. Scientific it is, as one of man's most exact recorders of natural phenomena. That an exact timekeeper should be found in the pocket of every schoolboy would seem an astounding miracle to our ancestors two hundred years ago, or even less than a hundred years ago: 'Tis with our judgments as our watches, none Go just alike, yet each believes his own, writes Pope in his _Essay on Criticism_ in 1725. This is a damning indictment of the accuracy of watches in the early eighteenth century, but Dickens in _Dombey and Son_ suggests equally faulty mechanism not in true accord with the mean solar day: