12 THE MOTION OF THE EARTH. 



The evidence of the motion of the Earth around its 

 axis, as afforded by the swinging of a pendulum or the 

 rotation of a sphere, is too interesting to be omitted. 

 In mechanical philosophy, we have two terms of the 

 same general meaning the conservation of the plane of 

 vibration and the conservation of the axis of rotation. 

 For the non-scientific reader, these terms require 

 explanation, and in endeavouring to simplify this as 

 much as possible, we must ask the indulgence of the 

 Mechanical Philosopher. Let us fix in the centre of a 

 small round table an upright rod, having an arm ex- 

 tending from its top, to which we can suspend a tolerably 

 heavy weight attached to a string. This is our piece of 

 apparatus : upon the table draw a chalk line, along 

 which line we intend our pendulum to swing, and con- 

 tinuing this line upon the floor, or by a mark on the 

 wall, our arrangements are complete. Raise steadily 

 the bob of our pendulum, and set it free, so that its 

 plane of vibration is along the line which has been 

 marked. As the pendulum is swinging firmly along 

 this line, slowly and steadily turn the table round. It 



tions made at the three observatories of Dorpat, Abo, Pulkovu. 

 and which is expressed in the following thesis: The motion of 

 the solar system in space is directed to a point of the celestial 

 vault situated on the right line which joins the two stars TT and p. 

 Herculis, at a quarter of the apparent distance of these stars, 

 reckoning from TT Herculis. The velocity of ihis motion is such, 

 that the sun, with all the bodies which depend upon it, advances 

 annually in the above direction 1*6^3 times the radius of the 

 earth's orbit, or 33,550,000 geographical miles. The possible 

 error of this last number amounts to 1,733,000 geographical miles, 

 or to a seventh of the whole value. We may then wager 400,000 

 to 1 that the sun has a proper progressive motion, and 1 to 1 that 

 it is comprised between the limits of thirty-eight and twenty-nine 

 millions of geographical miles." Etudes d" Astronomic Stellaire ; 

 Sur la Voie Lactee et sur les Distances des Etoiles Fixes : M. F. 

 W. G. Struve. [A report addressed to his Excellency M. Le 

 .Oomte Ouvaroffj Minister of Public Instruction and President of 

 the Imperial Academy of Sciences at St. Petersburg.] 



