We can readily understand why primitive peoples should adopt as units of time these natural periods, but in so doing they incurred much the same kind of difficulty that we should experience in trying to use both English and American money in the ordinary transactions of life. How many dollars make a pound sterling? How shall we make change with English shillings and American dimes, etc.? How much is one unit worth in terms of the other? One of the Greek poets[2] has left us a quaint account of the confusion which existed in his time with regard to the place of months and moons in the calendar: "The moon by us to you her greeting sends, But bids us say that she's an ill-used moon And takes it much amiss that you will still Shuffle her days and turn them topsy-turvy, So that when gods, who know their feast days well, By your false count are sent home supperless, They scold and storm at her for your neglect." [2] Aristophanes, The Clouds, Whewell's translation. 60. DAY, MONTH, AND YEAR.--If the day, the month, and the year are to be used concurrently, it is necessary to determine how many days are contained in the month and year, and when this has been done by the astronomer the numbers are found to be very awkward and inconvenient for daily use; and much of the history of chronology consists in an account of the various devices by which ingenious men have sought to use integral numbers to replace the cumbrous decimal fractions which follow. According to Professor Harkness, for the epoch 1900 A. D.-- One tropical year = 365.242197 mean solar days. " " " = 365d. 5h. 48m. 45.8s. One lunation = 29.530588 mean solar days. " " = 29d. 12h. 44m. 2.8s. The word _lunation_ means the average interval from one new moon to the next one--i. e., the time required by the moon to go from conjunction with the sun round to conjunction again.