is visited by the Gulf Stream, whilst the shore of Newfoundland is traversed by a cold current from Greenland.  Again, when in 1841 the railway from Rouen to Paris was being built, gangs of English and gangs of French workmen were employed upon it, and the English got through about one-third more work per man than the French.  It was suspected that this difference was due to one other difference, namely, that the English fed better, preferring beef to thin soup.  Now, logically, it might have been objected that the evidence was unsatisfactory, seeing that the men differed in other things besides diet—­in ‘race’ (say), which explains so much and so easily.  But the Frenchmen, having been induced to try the same diet as the English, were, in a few days, able to do as much work:  so that the “two instances” were better than they looked.  It often happens that evidence, though logically questionable, is good when used by experts, whose familiarity with the subject makes it good.

Sec. 4.  THE CANON OF CONCOMITANT VARIATIONS.

Whatever phenomenon varies in any manner whenever another phenomenon (consequent or antecedent) varies in some particular manner [no other change having concurred] is either the cause or effect of that phenomenon [or is connected with it through some fact of causation].

This is not an entirely fresh method, but may be regarded as a special case either of Agreement or of Difference, to prove the cause or effect, not of a phenomenon as a whole, but of some increment of it (positive or negative).  There are certain forces, such as gravitation, heat, friction, that can never be eliminated altogether, and therefore can only be studied in their degrees.  To such phenomena the method of Difference cannot be applied, because there are no negative instances.  But we may obtain negative instances of a given quantity of such a phenomenon (say, heat), and may apply the method of Difference to that quantity.  Thus, if the heat of a body increases 10 degrees, from 60 to 70, the former temperature of 60 was a negative instance in respect of those 10 degrees; and if only one other circumstance (say, friction) has altered at the same time, that circumstance (if an antecedent) is the cause.  Accordingly, if in the above Canon we insert, after ‘particular manner,’ “[no other change having concurred,]” it is a statement of the method of Difference as applicable to the increment of a phenomenon, instead of to the phenomenon as a whole; and we may then omit the last clause—­“[or is connected, etc.].”  For these words are inserted to provide for the case of co-effects of a common cause (such as the flash and report of a gun); but if no other change (such as the discharge of a gun) has concurred with the variations of two phenomena, there cannot have been a common cause, and they are therefore cause and effect.