[70] We cannot too strongly recommend the reading of the excellent works which Professor Marey has published, and which have for their object the study of movements, as well as the exhibition of the procedures which he has employed. E. J. Marey, 'La Machine Animale,' Paris, 1873; 'La Méthode graphique dans les Sciences expérimentales,' Paris, 1884; 'Le Vol des Oiseaux,' Paris, 1890; 'Le Mouvement,' Paris, 1894. It is necessary to understand first of all, in this connection, that which relates to a man's walking pace. The method of Professor Marey rests on the following principle: Suppose two rubber globes connected with one another by a tube. If we compress one of these globes, the air which it contains will be driven into the other, and will afterwards return when the pressure has ceased. Nothing more simple, evidently; but it is necessary to describe it in detail in order the better to comprehend that which follows: The walker who is the subject of experiment is furnished with special shoes (Fig. 115), having thick indiarubber soles, hollowed in the interior, so that the whole thus constituted forms a sort of hollow cushion which is compressed under the influence of the pressure of the foot on the ground. A tube which is attached to a registering apparatus, which the person who is walking carries in his hand, communicates with this cavity (Fig. 116). This apparatus is formed of a metal drum, which is closed at its upper part by a flexible membrane. Each time that one of the man's feet presses on the ground, the air contained in the cavity of the sole of the shoe is driven into the drum, which we have just mentioned, and the flexible membrane of this drum is elevated. To this membrane is attached a vertical rod which supports a horizontal style. [Illustration: FIG. 115.--EXPERIMENTAL SHOES, INTENDED TO RECORD THE PRESSURE OF THE FOOT ON THE GROUND.] When the membrane, as we have just seen, is elevated, the style is lifted, and then descends when the pressure of the foot ceases. It traces these displacements on a leaf of paper, the surface of which is covered with a thin layer of lamp-black, which it removes by its contact; different parts of this surface are successively presented to it, the paper being rolled round a cylinder which is turned on its axis by means of a clockwork movement. It is necessary to add that the inscription is made, in the study of the walk of man, by means of two styles, each corresponding to one of the feet.