CuAP. VI.] GRAVITATION AND INERTION. 75 



property of vis-inertiae is erroneous : and, therefore — 

 comparing the motion of the moon in its orbit to that 

 of the stone swung round in a sling, and tending 

 constantly to fly off at a tangent from its course — it 

 is evident that an innate property of vis-inertia3, tend- 

 ing to carry the moon onwards in a straight line, is 

 not the force which resists the earth's gravitation 

 drawing towards the earth. 



Now, since the earth's attraction tends to draw 

 the moon to the earth, and the action of vis-inertisB 

 tends to bring the moon to a state of rest, just as in 

 the case of a stone thrown in the air or rolled along 

 the ground ; and as we have shown that the idea of 

 any body having an innate tendency to move uni- 

 formly forwards in a straight line is a mere assump- 

 tion : let us consider whether the fact of the moon 

 not being drawn to the surface of the earth, and the 

 fact of its moving continuously onwards in its orbit, 

 cannot be explained without assuming the existence 

 of any laws or forces whose action cannot be illus- 

 trated by analogy with known phenomena. The 

 stone thrown in the air is drawn to the ground by 

 the earth's attraction, whereas the moon is not. But 

 then, as the moon is farther removed from the action 

 of the earth's gravitation, and is therefore relatively 

 more under the dommion of some other force of gra- 

 vitation, it may, in the absence of any other reason- 

 able cause being assigned, be inferred that the moon 

 is held in equilibrium between the attraction of the 



