On detonation, a solid explosive undergoes extremely rapid chemical change with the liberation of a large amount of energy. The rate at which this energy is liberated is governed chiefly by the rate at which the explosive detonates. It is known, however, that the rate of detonation varies with the conditions under which the explosive is fired and is, therefore, presumably dependent on the rate at which the energy liberated on detonation is used up in doing work. It appears, therefore, that the behaviour of, and the results to be obtained from, a detonating explosive under given conditions are intimately connected with its rate of detonation under those conditions are intimately connected with its rate of detonation under those conditions, and that an exact knowledge of the latter is of considerable importance in the study of detonating explosives and their power. The rate of detonation of a solid explosive is usually measured either by the method devised by Mettegang,* or by that due to Dautriche. The former is generally used for lengths of about a metre of explosive. The latter is capable of being used for shorter lengths, but is not an absolute method, as it is dependent on a preliminary determination of the rate of detonation of T. N. T. fuse by the Mettegang method.