Page:Collected Physical Papers.djvu/373

Rh left, by which the rate of compensating fall is retarded or accelerated. In this way the rate of growth becomes exactly compensated, and the recorder now dots a horizontal line instead of the former curve of ascent. The turning of the adjusting screw of the Balanced Crescograph also moves an index against a circular scale (not shown in the figure) so graduated that its reading at once gives the rate at which the plant is growing at that instant. When balanced, the recording apparatus is extraordinarily sensitive. Any change, however slight, in the environment is at once indicated by the upset of the balance with up- or down-movement of the curve. This method is so extremely sensitive that it is possible to detect variation of rate of growth so excessively minute as 1/1500 millionth of an inch per second.

As an illustration of the delicacy of this method, a