42 Mr. J. S. Stuart Glennie on the 
a difference of polar pressures ; and of the motion of a fluid, a 
difference of maximum and minimum pressures; and similarly, 
it is evident that the causes of the other mechanical motions are, 
in fact, conditions of pressure. Thus, rotation is the effect of 
equal, a compound motion of translation and rotation of unequal, 
differences of polar pressures in opposite directions with respect 
to a line joining the points of application of each pair of polar 
pressures. It need hardly be said that it is not proposed that we 
should use the phrase “ differences of polar pressures” instead 
of the term “ forces.”” But, preparatory to a mechanical considera- 
tion of physical and chemical motions, it seems of importance 
clearly to see that, whatever force may be absolutely, whatever the 
unknowable ultimate cause of motion may be, or have been, the 
ordinary mechanical forces* called “a single force at the centre 
of gravity,” ‘a couple,” and “a single force not at the centre of 
gravity,” are, in fact, such conditions of pressure as above stated. 
3. Thus, as the cause of a mechanical motion is, in general, the 
condition of a resultant difference of the pressures on the moved 
body, and as the general law of the motion of the body is, that 
it is in the direction of the greater of the two opposed resultant 
pressures, if the great object of modern physical research is 
accomplishable, it must be attained by showing that physical 
forces are, like mechanical forces, conceivable as differences of 
pressure under certain characteristic but interchangeable con- 
ditions, and that, for instance, the movement of iron towards a 
magnet, or the movement of a paramagnetic body towards a 
place of stronger action, are motions in the direction of least 
resistance. And there is evidently this further condition of a 
general mechanical theory of physical and chemical phenomena 
—that pressure be, in physics and chemics, conceived, as in 
mechanics, as “a balanced forcey,” or “momentum virtually 
and not actually developedt.” Hence it appears that, if a 
general mechanical theory is possible, the ultimate property of 
matter must be conceived to be a mutual repulsion of its parts, 
and the indubitable Newtonian law of universal attraction be 
herefrom, under the actual conditions of the world, deduced. 
4. The general experimental condition of the fitness of the 
mechanical conception of pressure as the basis of a general phy- 
sical and chemical theory, evidently is that there be a plenum. 
did Toutefois, e’est une chose trés-remar quable, qu'un méme livre, écrit 
sur la science des forces, pourrait, sans cesser d’étre exact et de traiter régu- 
lerement laméme science, étre entendu de deux maniéres différentes, selon 
qu’on attacherait au mot de force Vidée d’une cause de translation, ou 
Vidée toute différente d’une cause de rotation.’’—Poinsot, Théorie Nou- 
velle de la Rotation des Corps, p. 13. 
+ Rankine, ‘ Applied Mechanics.’ 
{ Price, ‘Treatise on Infinitesimal Calculus,’ vol. iu. 
