MOTION. 7 



and mercury, though fluids, have not this property 

 of adhering to bodies in contact with them. 



Elastic fluids, on the contrary, are those which 

 are reduced into less space by pressure, and which 

 return to their former volume when the pressure is 

 withdrawn; hence they are called also compressible 

 fluids: such are the different kinds of air. 



As all bodies seem to possess attraction, in a 

 greater or less degree, it has been considered by 

 some as one of the properties of matter; but this 

 quality does not necessarily enter into our idea of 

 body or substance, as distinguished from mere 

 space ; and while we are ignorant of the cause of 

 attraction, and also of the nature of many species 

 of bodies, it would be too much to assume this as 

 necessary to the existence of matter: and we are 

 uncertain whether it be an adventitious quality, or 

 whether it forms one of the essential properties of 

 substance. 



MOTION. 



Motion has been defined to be a change of place, 

 or the act by which a body corresponds with differ- 

 ent parts of space at different times. It is by motion 

 alone that we know the existence of bodies, and 

 that a relation is established between them and 

 our senses : all the phenomena of nature, all the 

 changes that happen in the system of bodies, de- 

 pend immediately upon it ; hence modern philoso- 

 phers have applied themselves with peculiar ardour 

 to investigate its laws. 



In considering motion, several circumstances 

 must be attended to. 



1. The force which impresses the motion. 



2. The direction of the motion. 



3. The space passed over by the moving body. 



b 4 ♦ 



