Moving Force. 185 
body cannot be produced by mere pressure. 
Unless the pressure act through some portion 
of space, no motion can be produced ; and if, 
together with the pressure, we take into con- 
sideration the space through which the pres- 
sure acts, we shall find that, while the motion 
of C has been transferred to D, the whole 
force of the spring has also been communi- 
cated to D. This will become more chvious in 
examining the 8thcase. It should be observed 
too, that when Mr. Maclaurin sets out with 
supposing the bodies to be in motion, and the 
spring to be in a compressed state, he refers 
to a previous application of force, of which he 
takes no farther notice, although a part of 
this previous force is.afterwards expended, or 
given out, in producing the changes which he 
describes. 
It is true our researches must be limited 
chiefly to relative motion—Of absolute motion 
we know but little. But is not the motion of 
the space EF GH with the velocity as 1, 
relative motion with regard to some supposed 
point, as much as the final motion of D is 
relative motion? 
if the compressed spring be disengaged 
while the bodies are at rest, the motion of the 
bodies is acknowledged to be produced by 
the -force of the spring: but when the space 
Aa 
