184 DIURNAL MOTION OF THE EARTH* 



Bcton i the mo- In the beginning of last summer two gentlemen in Rich- 

 mond laid a considerable wager on the following question : 

 " Do the top and bottom of a cart or carriage wheel, when 

 in motion, move with equal or unequal velocities r" 



The top has When the question was first proposed to me, I certainly 



greater veJoci- c • . ' , „ .« , ,. ,.~, . 



ty than the was ot P ml o»> that there could be no difference in the velo- 

 bottom. city of any point in the same ciroumference of the wheel : 



but upon reflecting, that every point of a carriage wheel 

 moving along a right line in a horizontal plane describes a 

 , cycloid, a leading property of which curve is for the generat- 

 ing point to describe unequal arcs in equal times, I was 

 convinced of my errour ; and perceived, that any point in 

 the upper semicircle of the wheel must move with greater 

 velocity than the corresponding and opposite point in the 

 under semicircle. 

 This applica- This truth immediately suggested the application of the 



hie t.. the mo- saa)e principle to the motion of the Earth ; for it is evident, 



tion of the . 



E'wth. that the motion of any point on the Earth's surface, with the 



exception of the two poles, being compounded of two mo- 

 tions, a rotary motion round the axis of the Earth, and a 

 progressive motion along the plane of the ecliptic, will also 

 describe a curve of the eycloidal, or rather epicycloidal spe- 

 cies, possessing a similar property with the common cycloid 

 generated by a carriage wheel. 

 Important con- The eycloidal motion on the points of the Earth's surface 

 du^bT? S m" being once established, several important consequences obvi- 

 this, ously present themselves. For it is manifest, if every 



point in the same parallel of latitude vary its velocity in re- 

 as the ccntri volving round the axis of the Earth, the centrifugal force of 



fugal force t ^ t point must also vary ; that is, when the velocitv of the 

 must vary. r * , . 



point is greatest, the centrifugal force will also be greatest ; 



and on the contrary, when the velocity of the point is least.. 



Thk accounts the centrifugal force will also be least. This variation in 



:or the tides the centrifugal force of every point on the Earth's surface, 



&c * during a diurnal rotation, necessarily affecting the fluids 



, which encompass the Earth ; it appeared to me, that the 



phenomena of the tides, the trade winds, and several other 



phenomena in nature, might thereby be explained. On this 



subject I published, in the month of May, several essays in 



the Inquirer and Virginia Argus of Richmond. These 



esiays 



