10 FERGUSON'S LECTURES. 



distant bodies tend towards one another. Of this we 

 have daily instances, in the falling of bodies to the 

 earth. By this power in the earth it is, that bodies, on 

 whatever side, fall in lines perpendicular to its surface ; 

 and consequently, on opposite sides they fall in opposite 

 directions ; all towards the center, where the force of 

 gravity is as it were accumulated : and by this power it 

 is, that bodies on the earth's surface are kept to it on all 

 sides, so that they cannot fall from it. And as it acts 

 upon all bodies in proportion to their respective quan- 

 tities of matter, without any regard to their bulks or 

 figures, it accordingly constitutes their weight. Hence, 



If two bodies, which contain equal quantities of matter, 

 were placed at ever so great a distance from one another, 

 and then left at liberty in free space ; if there were no 

 other bodies in the universe to affect them, they would 

 fall equally swift towards one another by the power of 

 gravity, with velocities accelerated as they approached 

 each other ; and would meet in a point which was half 

 way between them at first. Or, if two bodies, contain- 

 ing unequal quantities of matter, were placed at any 

 distance, and left in the same manner at liberty, they 

 would fall towards one another with velocities which 

 would be in an inverse proportion to their respective 

 quantities of matter ; and moving faster and faster in 

 their mutual approach, would at last meet in a point as 

 much nearer to the place from which the heavier body 

 began to fall, than to the place from which the lighter 

 body began to fall, as the quantity of matter in the for- 

 mer exceeded that in the latter. 



All bodies that we know of have gravity or weight. 

 For, that there is no such thing as positive levity, even 

 in smoke, vapours, and fumes, is demonstrable by ex- 

 periments on the air-pump ; which shews, that although 

 the smoke of a candle ascends to the top of a tall 

 receiver when full of air, yet, upon the air's being 

 exhausted out of the receiver, the smoke falls down to 



