Professor Marcy has recently discussed, in a lecture on Living Locomotors (_Moteurs Animés_), the principles of propulsion. Had he been an Englishman he would probably have found some of his most striking illustrations among different cases of propulsion through water. But, although he limited his discussion of animated motors to those which work on land, he yet laid down the fundamental principle of all propulsion, which is that as little as possible--and therefore, if possible, none at all--of the energy employed to produce propulsion should be expended in injurious work. Even with the best carriages, he pointed out, there remain vibrations and shocks which must be attacked and destroyed to render the conditions of traction more perfect; they are veritable shocks, which use up part of the work of the horse in giving only hurtful effects, bruising the animal's breast, injuring his muscles, and, in spite of the padding of the collar, sometimes wounding him. Then he showed a simple experiment suggested by the able dynamician, Poncelet. To a weight of five kilos, (about 11lb.) a string is attached by which the weight can be lifted, but not much more. Then the experimenter tries to lift the weight rapidly with the string, which breaks without moving the weight, while the fingers are more or less hurt by the sudden shock. If now, a cord of equal strength, but slightly elastic, is substituted, the experiment ends differently. The sudden effort of elevation is transformed into a more prolonged action, and the weight is raised without bruising the fingers or breaking the cord. Yet a still more sudden movement would break the cord in this case, though a yet more extensible cord would resist even a yet more sudden jerk. According to the strength of the cord, its extensibility, and the weight to be lifted, must be the nature of the upward pull in order that the greatest possible velocity may be communicated without injury to the cord or to the lifter's hand. This simple series of experiments involves the essential principles of effective propulsion, where, at least, great velocity is among the results to be attained.