ke into consideration
the resistance of the air, the curvature of the earth, the rigid
connection which exists between particles in the same body, and a host
of other things which are often conveniently neglected in elementary
works, how complicated the various problems become! So we must not be
surprised at some of the difficulties which occur in social science, as
nothing is neglected; the whole problem is before us, and having solved
it we need not make allowances for any falsely assumed _data_.

It is possible that other professors of this science may come to
slightly different conclusions to those which I have arrived at. That
is only to be expected, because their original observations may have
slightly varied. But in physical science allowances are made for
different observers. In astronomy, for example, we find the value of the
'Personal Equation.' One observer on looking through the telescope may
take the meridian of a star rather differently from another watcher of
the heavenly bodies, and the _personal equation_ is used to make
allowances for this quickness, or slowness, of observation. So in social
science there must be a personal equation too, and our object ought to
be, in the ordinary affairs of life as well as in the higher duties of
scientific action, to make our personal equation as small as possible.
But until the old proverb, '_Quot homines, tot sententiae_,' has ceased
to have any meaning, there will be abundant need of this most useful aid
to accuracy.

The close connection which exists between social forces and material
forces is plainly shown by the doctrine of the conservation of energy.
'This doctrine,' says Dr. Tyndall, 'recognises in the material universe
a constant sum of power made up of items among which the most Protean
fluctuations are incessantly going on. It is as if the body of nature
were alive, the thrill and interchange of its energies resembling those
of an organism. The parts of the stupendous whole shift and change,
augment and diminish, appear and disappear; while the total of which
they are the parts remains quantitatively immutable, _plus_ accompanies
_minus_, gain accompanies loss, no item varying in the slightest degree
without an absolutely equal change of some other item in the opposite
direction.' So do the forces in the social world ebb and flow, rise and
fall, carrying on the same universal law which regulates the energy of
material force.

I will now proceed to enum