INTRODUCTION. 19 



distinctly adduces the tides as evidence* that the attractiv& 

 force of the moon (virtus tractoria) extends to the earth , 

 and that this force, similar to that exerted by the magnet 

 on iron, would deprive the earth of its water if the forme] 

 should cease to attract it. Unfortunately, this great man 

 was induced, ten years afterward, in 1619, probably from 

 deference to Galileo, who ascribed the ebb and flow of the 

 ocean to the rotation of the earth, to renounce his correct 

 explanation, and depict the earth in the Harmonice Mundt 

 as a li ving monster, whose whale-like mode of breathing oc- 

 casioned the rise and fall of the ocean in recurring periods 

 of sleeping and waking, dependent on solar time. When we 

 remember the mathematical acumen that pervades one of the 

 works of Kepler, and of which Laplace has already made 

 honorable mention,t it is to be lamented that the discoverer 

 of the three great laws of all planetary motion should not 

 have advanced on the path whither he had been led by his 

 views on the attraction of the masses of cosmical bodies. 



Descartes, who was endowed with greater versatility of 

 physical knowledge than Kepler, and who laid the founda- 

 tion of many departments of mathematical physics, under- 

 took to comprise the whole world of phenomena, the heav- 



* " Si Terra cessaret attrahere ad se aquas suas, aquae marinse omnes 

 elevarentur et in corpus Luna? iufluerent. Orbis virtutis tractoriae, qua? 

 est in Luna, porrigitur usque ad terras, et prolectat aquas quacunque 

 in verticem loci incidit sub Zonam torridam, quippe in occursum suum 

 quacunque in verticem loci incidit, insensibiliter in maribus inclusis, 

 sensibiliter ibi ubi sunt latissimi alvei Oceani propinqui, aquisque spa- 

 ciosa reciprocationis libertas." (Kepler, 1. c.) " Undas a Luna trahi 

 ut ferrum a Magnete." .... Kepleri Harmonice Mundi, libri quinque, 

 1619, lib. iv., cap. 7, p. 162. The same work which presents us with 

 so many admirable views, among others, with the data of the establish- 

 ment of the third law (that the squares of the periodic times of two 

 planets are as the cubes of their mean distance), is distorted by the 

 wildest flights of fancy on the respiration, nutrition, and heat of the 

 earth-animal, on the soul, memory (memoria animce Terra), and crea- 

 tive imagination (anima Tdluris imaginatio) of this monster. This great 

 man was so wedded to these chimeras, that he warmly contested his 

 right of priority in the views regarding the earth-animal with the mys- 

 tic author of the Macrocosmcs, Robert Fludd, of Oxford, who is report- 

 ed to have participated in the invention of the thermometer. (Harm. 

 Mitndi, p. 252.) In Kepler's writings, the attraction of masses is often 

 confounded with magnetic attraction. " Corpus solis esse magneticum. 

 Virtutem, quae Planetas movet, residere in corpore solis." Stella Mar 

 tit, pars iii., cap. 32, 34. To each planet was ascribed a magnetic axis, 

 which constantly pointed to one and the same quarter of the heavens. 

 CApelt, Joh. Kepler's Astron. Weltansicht, 1849, 8. 73. 



t Compare Cosmos, vol. ii., p. 327 (and iiole 



