APPENDIX. 197 



both to the centre and to the revolving body, aflfect their position relatively to each 

 other. Now, in the case of the atmosphere, the motion in opposite directions just 

 spoken of, is caused solely by the force of gravity, which retains the air about the 

 earth, and prevents it from flying off in a tangent, by virtue of its centrifugal foi'ce, 

 but has no effect whatever upon its horizontal motion, nor any tendency to change 

 the relative position of a place on the earth's surface and the superincumbent air. 

 A musket-ball, discharged horizontally with a velocity of about five miles per second, 

 would, if the air were removed, travel round the earth with a uniform velocity, 

 and yet would move in opposite directions at opposite points of its orbit. Nor 

 would its relative position in regard to the surface of the earth be in any way 

 affected by the revolution of the earth around the sun. 



