INTRODUCTION. 19 



ascribed the movement of the celestial bodies to " a primitive 

 impulse, and the continued tendency to fall ; " and while, 

 as we have already observed, Copernicus defined only the 

 general idea of gravitation, as it acts in the sun, as the centre of 

 the planetary world, in the earth and in the moon, using these 

 memorable words, " Gravitatem non aliud esse quam appe- 

 tentiam quandam naturalem partibus inditam a divina provi- 

 dentia opificis universorum, ut in unitatem integritatemque 

 suam sese conferant, in formam globi coeuntes;" Kepler in 

 his introduction to the book, De Stella Martis^ 1 was the first 

 who gave numerical calculations of the forces of attraction 

 reciprocally exercised upon each other, according to their rela- 

 tive masses, by the earth and moon. He distinctly adduces the 

 tides as evidence 32 that the attractive force of the moon (virtus 



31 " Si duo lapides in aliquo loco Mundi collocarentur pro- 

 pinqui invicem, extra orbem virtutis tertii cognati corporis ; 

 illi lapides ad similitudinem duorum Magneticorum corporum 

 coirent loco intermedio, quilibet accedens ad alterum tanto 

 intervallo, quanta est alterius moles in comparatione. Si luna 

 et terra non retinerentur vi animali (!) aut alia aliqua 

 aequipollente, quselibet in suo circuitu, Terra adscenderet ad 

 Lunam quinquagesima quarta parte intervalli, Luna descen- 

 deret ad Terram quinquaginta tribus circiter partibus inter- 

 valli ; ibi jungerentur, posito tamen quod substantia utriusque 

 sit unius et ejusdem densitatis." Kepler, Astronomia nova, 

 seu Physica ccelestis de Motibus Stellce Martis, 1609. Introd. 

 fol. v. On the older views regarding gravitation, see Cosmos, 

 vol. ii. p. 691. 



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

 marinas omnes ele\arentur et in corpus Luna3 influerent. 

 Orbis virtutis tractorias, quae 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, sensi- 

 .biliter ibi ubi sunt latissimi alvei Oceani propinqui, aquisque 

 spaciosa reciprocationis libertas." (Kepler, 1. c.) " Undas a 

 Luna trahi ut ferrum a Magnete." .... Kepleri Harmonics 



c2 



