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220   Life and Letters of Francis Galton

a horizontal scale which is 800 mm. below the point of suspension of the pendulum. Galton provides a table for reading off the distances along the scale from the vertical position of the pendulum in terms of the time the pendulum takes from the vertical position to the position in which the thread is clamped. The reaction time is thus ascertainable on the assumption that the time from pressing the key to the mechanical clamping of the thread is negligible compared with the reaction time. It would however be easy to correct for this, if we arranged occasionally for the pendulum to work the clamping key itself, and so ascertained the time of clamping independently of the living being's reaction and response.

Another instrument designed by Galton was intended to measure the rapidity of a blow, or indeed the rate of movement of any limb'. The principle of the mechanism is that the limb

is attached by a string to a light

mechanism which draws in the

string at a faster rate than the

limb moves. The motion of the

string is checked when the limb

reaches its full extension, but a

light weight on a platform con

tinues to rise freely and measures

by the height it reaches the

velocity of the platform (and of       30. the string) when the string was

checked. The whole scheme is indicated in the accompanying

rubberhand; in the actual machine   10 as worked this was much longer

than indicated in the diagram.

BC is a thin steel wire to which T   11111111   

the conical platform D is firmly attached, an ivory cylinder Erests on the platform and runs loosely on the steel wire. When the platform D is checked, E goes forward with D's final velocity and this velocity is measured on the scale behind BC according to the height to which E rises. A string passes from the vertex

of C and is wound round and ultimately fastened to one wheel F of 'a differential pulley. Another string is wound round the second wheel G of the differential pulley and ultimately fastened to it ; the remainder of this string is made horizontal by being carried over a small pulley. To the

' Journal of the Anthropological Institute,- Vol. xx, pp. 200-204, 1890. "A new Instrument for Measuring the Rate of Movement of the various Limbs."

diagram. AB is a stretched India-   20'