LECTURE XV. 



EXPERIMENTAL PHILOSOPHY. 



THE LAWS OF MOTION. 



EVERY thing in mechanics depends upon very 

 simple principles, and may be resolved ultimately 

 into the power of gravity and the laws of mo- 

 tion. 



In treating of gravitation, in our second lec- 

 ture, it was shown to be that kind of attraction 

 which subsists between the mass of the earth and 

 all those bodies which are on its surface. It 

 is that which, in the stated revolutions of this 

 planet, prevents us, and all the bodies which 

 surround us, from falling into infinite space ; and 

 which draws so forcibly every thing whatever 

 towards the centre of the earth. 



That this attraction is greater or less at different 

 distances is generally allowed ; a body which at 

 one semidiameter of the earth weighs one pound 

 will have four times less weight at two semidia- 

 meters, and nine times less at three. At small 

 distances, however, we are not sensible of this 

 difference in weight; for though we could be 

 elevated a mile above the earth's surface, when 

 we consider that its diameter is about eight thou- 



