342 POPULAR SCIENCE MONTHLY. 



sidereal day is a little more than 23 hours, 56 minutes and 4 seconds, 

 and the average length of the lunar day is a little less than 24 hours 

 and 49 minutes. The lunar day finds expression in the tides and is of 

 moment to maritime folk, hut the sidereal is known only to astron- 

 omers. 



Next in the series of our natural time units is the month, or the 

 rhythmic period of the moon regarded as a luminary. By our savage 

 ancestors, who credited the moon with powers of great importance to 

 themselves, much use was made of this unit, hut as progress in knowl- 

 edge has shown that the influence of the satellite had been vastly over- 

 rated, less and less attention has been paid to the returning crescent, 

 and it is only in ecclesiastic calendars that the chronology of civilization 

 now recognizes the natural month. Its shadow survives, without the 

 substance, in the calendar month; and the week possibly represents an 

 early attempt to subdivide it. 



In passing to our third natural unit, the year, we again encounter 

 solar influence, and find the rhythm of the earth's orbit echoed and re- 

 echoed in innumerable physical and vital vibrations. As the attitude 

 of the earth's axis inclines one hemisphere toward the sun for part of 

 the year and the other hemisphere for the remainder, the whole com- 

 plex drama of climate is annually enacted, and the sequence of man's 

 activities is made to assume an annual rhythm. The year is second 

 only to the clay as a terrestrial unit of duration; and as the day is man's 

 standard for the minute division of time, so the year is his standard for 

 larger divisions, and the decade, the century and the millenium are its 

 multiples. 



But the rhythms of day and night, of summer and winter, are not 

 the only tides in the affairs of men. At birth we are small, weak and 

 dependent, we grow larger and stronger, we become mature and inde- 

 pendent, and then by reproducing our kind we complete the cycle, 

 which begins again with our children. The cycle of human life is the 

 generation, a time unit of somewhat indefinite length and varying in 

 phase from family to family, but holding a place, nevertheless, in hu- 

 man chronology. 



Still less definite is the rhythm of hereditary rulership, progressing 

 from vigor through luxury to degeneracy, and closing its cycle in usurp- 

 ation; yet it makes an epoch in the life of a nation or empire, and so the 

 dynasty is one "of the units of the historian. 



The generation and the dynasty are of waning importance in human 

 chronology, and they can claim no connection with the problem of 

 geologic time; but here again I have turned aside for a moment in 

 order to illustrate a principle of classification. The daily rhythm of 

 waking and sleeping, of activity and rest, does not originate with man, 

 but is imposed on him by the rhythm of light and darkness, and that 



