June 29, 1900.] 



SCIENCE. 



1003 



I must mention also a special apparatus 

 invented by astronomers and called a 

 chronograph. It consists ordinarily of 

 a revolving drum about which a paper is 

 wrapped and against which rests a pen. As 

 the drum turns the pen draws a line on 

 the paper. Through an electric circuit the 

 pen is brought under the influence of a 

 pendulum in such a way that at the middle 

 of each swing of the pendulum the pen is 

 deflected, making a mark at right angles to 

 the straight line. The series of marks thus 

 drawn constitutes a time scale. The elec- 

 tric arrangements are so made that the pen 

 will also be disturbed in consequence of 

 some independent event, such as the firing 

 of a gun or the transit of a star ; and the 

 mark caused by such disturbance, being 

 automatically platted on the time scale, 

 records the time of the event. 



N"o attempt has been made to character- 

 ize these various time pieces with fullness, 

 because they are already well known to 

 most of those present, and, in fact, the 

 chief motive for giving them separate men- 

 tion is that they may serve as the basis of 

 a classification. In the use of the clep- 

 sydra and taper, time is measured in terms 

 of a continuous movement or process ; in 

 the use of the pendulum time is measured 

 in terms of a movement which is periodic- 

 ally reversed. The classification embodies 

 the fundamental distinction between con- 

 tinuous motion and rhythmic motion. 



Passing now from the artificial to the 

 natural measures of time we find that they 

 are all rhythmic. It is true that the spin- 

 ning of the earth on its axis is in itself a 

 continuous motion, but it would yield no 

 time measure if the earth were alone in 

 space, and so soon as the motion is consid- 

 ered in relation to some other celestial 

 body it becomes rhythmic. As viewed 

 from, or compared with, a fixed star the 

 period of its rhythm is the sidereal day ; 

 compared with the sun it is the solar day. 



nearly four minutes longer ; and compared 

 with the moon it is the lunar day, still 

 longer by 49 minutes. As the sun supplies 

 the energy for most of the physical and all 

 the vital processes of the earth's surface, 

 the rhythm of the solar day is impressed 

 in multitudinous ways on man and his en- 

 vironment, and he makes it his primary or 

 standard unit of time. He has arbitrarily 

 divided it into hours, minutes and seconds, 

 and in terms of these units he says that the 

 length of the 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, but the 

 sidereal is known only to astronomers. 



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, but as progress in knowledge has 

 shown that the influence of the satellite had 

 been vastly overrated, less and less atten- 

 tion has been paid to the returning cres- 

 cent, 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 cal- 

 endar month ; and the week possibly repre- 

 sents 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 reechoed in innumerable phys- 

 ical 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 complex drama of climate is annually 

 enacted, and the sequence of man's activi- 

 ties is made to assume an annual rhythm. 

 The year is second only to the day as a ter- 



