242 PREFACE TO THE 



get rid of the necessity of supposing the existence of a 

 ninth and tenth heaven, the former to explain the 

 precession of the equinoxes, and the latter the imag 

 inary phenomenon of their trepidation, he ascribed 

 the motion by which these phenomena are produced to 

 the earth itself. The cause of this motion he sought 

 in the action of the ambient air on the earth s surface. 

 To explain trepidation, the earth s motion was supposed 

 to be in some measure libratory and irregular ; and by 

 being so it produced the tides. 1 



From the theory of Cassalpinus we pass naturally to 

 that of Galileo, seeing that in both the tides are ex 

 plained by the unequal motion of the earth. Galileo s 

 theory was first propounded in a letter to Cardinal Or- 

 sino, dated 1616. He remarks that the libratory mo 

 tion &quot; che alcuno ha attribuito alia Terra,&quot; (alluding 

 of course to CsBsalpinus,) is in several respects not such 

 as to save the phenomena, and maintains that the true 

 cause is to be sought in the combination of the earth s 

 motion in its orbit with its rotation on its own axis. 

 In consequence of this combination, the velocity of 

 any point of the earth s surface varies, going through 

 its different values in the space of twenty-four hours. 

 The waters of the sea, not accommodating themselves 

 to this varying velocity, ebb and flow at any place as 

 their velocity is less or greater than that of their bed. 

 The boldness of the assertions by which Galileo sup 

 ports this theory is remarkable : thus he affirms that 

 the ebb and flow is always from west to east, and vice 

 versa&quot; ; and that the notion that, speaking generally, 

 the interval between high water and low is six hours 

 &quot; stata un ingannevole opinione la quale ha poi fatto 



1 QuaBStiones Peripat. iii. 4. and 5. 



