nomenon varies in any manner whenever another
phenomenon varies in some particular manner, is either a
cause or an effect of that phenomenon, or is connected with it
through some fact of causation._

This simple principle is constantly applied by us in connecting and
disconnecting phenomena. If we hear a sound which waxes and wanes with
the rise and fall of the wind, we at once connect the two phenomena.
We may not know what the causal connexion is, but if they uniformly
vary together, there is at once a presumption that the one is causally
dependent on the other, or that both are effects of the same cause.

This principle was employed by Wells in his researches into Dew. Some
bodies are worse conductors of heat than others, and rough surfaces
radiate heat more rapidly than smooth. Wells made observations on
conductors and radiators of various degrees, and found that the
amount of dew deposited was greater or less according as the objects
conducted heat slowly or radiated heat rapidly. He thus established
what Herschel called a "scale of intensity" between the conducting and
radiating properties of the bodies bedewed, and the amount of the dew
deposit. The explanation was that in bad conductors the surface cools
more quickly than in good conductors because heat is more slowly
supplied from within. Similarly in rough surfaces there is a more
rapid cooling because heat is given off more quickly. But whatever
the explanation might be, the mere concomitant variation of the
dew deposit with these properties showed that there was some causal
connexion between them.

It must be remembered that the mere fact of concomitant variation is
only an index that some causal connexion exists. The nature of
the connexion must be ascertained by other means, and may remain
a problem, one of the uses of such observed facts being indeed to
suggest problems, for inquiry. Thus a remarkable concomitance has been
observed between spots on the sun, displays of Aurora Borealis, and
magnetic storms. The probability is that they are causally connected,
but science has not yet discovered how. Similarly in the various
sciences properties are arranged in scales of intensity, and any
correspondence between two scales becomes a subject for investigation
on the assumption that it points to a causal connexion. We shall see
afterwards how in social investigations concomitant variations in
averages furnish material for reasoning.

When two v