222 On the Measure of 
pressure. Although pressures are balanced by 
pressures relatively at rest, under an almost 
infinite variety of circumstances; their most 
intricate combinations are distinctly explained 
and estimated by the application of a small 
number of general principles; and upon that 
subject no difference of opinion exists. 
If pressure be applied to a mass of matter 
at rest, but free to move in any direction, the 
mass is put in motion. But that motion of 
the mass implies motion of the pressure ; for 
unless the pressure follow and act upon the 
mass through some portion of space, no motion 
can be produced.’ If it be clear that the 
motion of a mass of matter must be produced 
by the action of pressure through a portion of 
space, it is not less obvious that the mecha- 
nical compression, or the mechanical separa- 
tion, of the parts of a mass of matter, must be 
produced by the same means; and when we 
speak of the resistance of inertia in one case, 
or of that of repulsion or cohesion in the other, 
we only mean that the exertion of pressure 
through some portion of space is necessary to 
overcome the resistance in either case. Al- 
though we suppose the resistance in the differ- 
ent cases to proceed from different causes, we 
find no difference in the means by which the 
vesistance is to be overcome; and by taking 
a 
