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same velocity as in the previous experiment, the curve would be something like this— Fig. 23. and if it travelled slower in going up than in coming down it would vary its form to— Fig. 24. Thus by recording rapid movements on a quickly moving surface, such as the surface of a drum or cylinder, or on a glass plate travelling horizontally, we get information regarding phases or variations of movement which we could not otherwise obtain. This illustrates the essential principle of the graphic method, a method of great value in all sciences dealing with movement, and not least to physiological science. It is not a modern method, although in later times its use has been enormously extended. In 1734, the Marquis d'Ons-en-Bray described an anemometer, an instrument for