166 THE GENESIS OF SCIENCE. 



eclipses. The Chaldeans were able to predict these. 

 " This they did, probably," says Dr. Whewoll in his useful 

 history, from which most of the materials we are about to 

 use will be drawn, " by means of their cycle of 223 months, 

 or about eighteen years ; for at the end of this time, the 

 eclipses of the moon begin to return, at the same intervals 

 and in the same order as at the beginning." Now this meth- 

 od of calculating eclipses by means of a recurring cycle, — 

 the Saros as they called it — is a more complex case of pre- 

 vision by means of coincidence of measures. For by what 

 observations must the Chaldeans have discovered this 

 cycle ? Obviously, as Delambre infers, by inspecting their 

 registers ; by comparing the successive intervals ; by find- 

 ing that some of the intervals were alike ; by seeing that 

 these equal intervals were eighteen years apart ; by discov- 

 ering that all the intervals that were eighteen years apart 

 were equal ; by ascertaining that the intervals formed a 

 series which repeated itself, so that if one of the cycles of 

 intervals were superposed on another the divisions would 

 fit. This once perceived, and it manifestly became possi- 

 ble to use the cycle as a scale of time by which to measure 

 out future periods. Seeing thus that the process of so j^re- 

 dicting eclipses, is in essence the same as that of predicting 

 the moon's monthly changes by observing the number of 

 days after which they repeat — seeing that the two difter 

 only in the extent and irregularity of the intervals, it is not 

 difficult to understand how such an amount of knowledge 

 should so early have been reached. And wo shall be less 

 surprised, on remembering that the only things involved 

 in these previsions were time and number ; and that the 

 time was in a manner self-numbered. 



Still, the ability to predict events recurring only after 

 so long a period as eighteen' years, implies a considerable 

 advance in civilization — a considerable development of geu- 

 ural knowledge; and we have now to inquire what progress 



