Essay on Motion and Force. 



263 



other change transpires, it has the following motions ; a 

 motion toward the earth's center of gravity ; a motion in 

 the direction of the earth's rotation ; a motion correspond- 

 ing to the motion of the Poles of the earth called nutation ; 

 a motion around the center of gravity of the earth and the 

 moon taken together ; a motion around the center of grav- 

 ity of the earth and the sun taken together ; a motion around 

 the sun with the earth in its annual revolution ; a motion 

 around the center of the great nebula of which the solar sys- 

 tem is only, so to speak, an atom ; and without doubt, many 

 other motions, of which we are totally unconscious, and 

 which we may never detect by experiment or induction. 

 Now one of the conditions which are necessary in order 

 that a body may impart the whole of any particular motion 

 to another body, is, that the direction of the motion 

 which is to be imparted, shall be in a right line joining 

 the centers of the two bodies. All motions in curves 

 are to be regarded as resultants. All the so called radiant 

 forces act in lines radiating from some point within a mass 

 of matter, which point may be considered as the center of 

 that particular mode of motion. With regard to gravity 

 heat, light, &c, this manner of action causes a remarkable 

 variation in the amount of motion imparted at different 

 distances from the center of either of these motions, the 

 amount varying inversely as the squares of the distances 

 from their centers. This is of course a necessary conse- 

 quence of such a mode of action ; to give a reason for the 

 variation in the quantity imparted, would be to give a 

 reason why motion is thus imparted from a center ; or what 

 is the same thing, to show why the resultant of the motions 

 of the particles of a mass, is a motion in a right line to 

 or from its center of motion. Of course it will remain 

 impossible to demonstrate this until the peculiar nature of 

 these molecular motions shall have been discovered. It 

 may indeed be possible to determine something of their 

 nature from the two data already established, and to which 

 I have alluded viz., that motion is imparted in right lines 



