RHYTHMS AND GEOLOGIC TIME. 34* 



The pendulum, a comparatively modern invention, excelling the 

 clepsydra and taper in precision, has altogether supplanted them as the 

 servant of civilization. Its accuracy results from the remarkable prop- 

 erty that the period in which it completes an oscillation is almost ex- 

 actly the same, whatever the arc through which it swings. It regulates 

 the movements not only of our clocks, watches and chronometers, but 

 of barographs, thermographs and a great variety of other machines for 

 recording events and changes in their proper order and relation in 

 respect to time. 



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 de- 

 flected, making a mark at right angles to the straight line. The series 

 of marks thus drawn constitutes a time scale. The electric 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. 



No attempt has been made to characterize these various timepieces 

 with fullness, because they are already well known to most of those 

 present, and, in fact, the chief motive for giving them separate mention 

 is that they may serve as the basis of a classification. In the use of the 

 clepsydra and taper, time is measured in terms of a continuous move- 

 ment or process; in the use of the pendulum time is measured in terms 

 of a movement which is periodically reversed. The classification em- 

 bodies the fundamental distinction between continuous 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 spinning 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 mo- 

 tion is considered 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 environment, 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 



