increased at a faster rate than corresponds to the power of the battery, but by making a very sharp break it may be stopped very rapidly. Still, however rapidly the circuit is broken, self-induction causes a spark to fly across the gap until the energy of the current is used up. The introduction of the condenser, consisting of a number of sheets of tinfoil insulated by paper steeped in paraffin wax, and connected alternately with one end or the other of the primary coil, serves to increase the rapidity with which the primary current died away, by rapidly using up its energy in charging the condenser, and produces a corresponding diminution in the spark at the contact-breaker. This rapid destruction of the primary current causes a correspondingly great electro-motive force in the secondary coil, and thus very long sparks are produced between the terminals of the secondary coil when the primary current is broken, though no such sparks are produced when the primary current starts. If the secondary coil be connected up with a galvanometer, so that there is a metallic circuit throughout, it will be found that just as much electricity flows in one direction through the circuit at the break of the primary as flows in the other direction at the make, the difference being that the first is a very strong current of great electro-motive force but lasting a very short time, the second a feebler current lasting a correspondingly longer time. * * * * * But though the recent advances in electrical science have been very great, the grandest triumph of this century is the establishment of the principle of the conservation of energy, which has settled for ever the problem of "the perpetual motion," by showing that it has no solution. This problem was not simply to find a mechanism which should for ever move, but one from which energy might be continuously derived for the performance of external work--in fact, an engine which should require no fuel. But in spite of all that has been proved, numbers of patents are annually taken out for contrivances to effect this object.