The extensive use of rubber as a shock-absorbing and vibration-absorbing material makes necessary a definite criterion for this property of rubber. In his work on the problem of the shock-absorbing quality of rubber, Morrison states: “To make possible an easy calculation of the rubber buffer, it is necessary to know (1) the permissible working strain at the static loading for a given rubber, (2) the maximum strain permissible at the shock loading, and (3) the energy absorbed by a unit volume of rubber in tile transition from static to shock loading.” Reference to static load is made because, in this calculation, it is necessary to consider those static forces which act on the rubber before shock. The energy absorbed by the rubber in the transition from the static to the shock load is supposed by Morrison to be that energy which is absorbed by the rubber in its deformation by the shock. The question of the energy returned by the rubber in resuming its original form is not considered by him. Vetchinkin in his study of the work of shock-absorbing aeroplane cords fails also to take into consideration the energy returned by the cord on resuming its original form, and bases his calculation only on the absorption by the rubber of a definite quantity of energy during stretching caused by the impact of the aeroplane against the ground. He mentions only casually that, by increasing the preliminary tautness of tile cords, their hysteresis losses of energy are increased. However, such a concept of the work of a rubber shock-absorber seem to us inadequate. In fact, let us distinguish clearly between the work of shock-absorption of rubber and a steel spring. Of course it should be noted that for deformations by the highest possible stresses for rubber and for a steel spring, the former requires more energy per unit of weight. Thus, according to the above quoted paper by Morrison, 1 kg. of rubber absorbs 172 kg. of energy and 1 kg. of steel only 119 kg., i.e., 44% less. According to Geer (cf. “The Reign of Rubber” the difference is many times greater, viz., 10,000 kg. for rubber and 230 kg. for steel.