70 ECONOMIC ESSAYS IN HONOR OF JOHN BATES CLARK 
tion.® Such verification will, of course, always reveal the presence 
of other forces than the purely static ones, modifying the results 
in any given case. Inductive studies will deal, not only with the 
trend-values around which actual values fluctuate, but also with 
the forces setting limits on their oscillations. Here the static 
forces, corresponding to the force of gravity in mechanics, are at 
work, but under conditions which differ from the complete static 
picture, and require correspondingly different methods of study. 
And finally, in the inductive study of actual conditions, there 
will always arise the difficulty that a mere description of facts 
does not afford an explanation or interpretation of them. The 
question will still remain why they behave as they do. And here 
again the static approach will prove useful and effective, chiefly 
in the form of inverse deduction, which has already been men- 
tioned. The reasoning takes the following form. If the facts 
were found to behave in certain simple ways, we should infer the 
presence of static forces only, acting under static conditions only. 
Since the facts behave differently, we infer the joint action of 
static and dynamic forces, and attribute the departures from the 
static model to the dynamic elements in the situation. And the 
nature of these departures are, if properly understood, such as we 
should expect from the nature of the dynamic forces. Thus brief 
reversions to the static method of isolation will help us to 
separate out the forces acting under actual conditions, and to 
make of dynamics an explanation, rather than a mere description 
of economic behavior. 
1 See “Partial Elasticity of Demand,” Quar. Jour. Econ. XL, 393-401, 
May, 1926; “A Theory of Economic Oscillations,” XLI, 1-29, Nov., 1926.