of cases, we can calculate beforehand what will take place, and we are under no necessity of trying actual experiments.
Thus, a portion of our knowledge of electro-magnetism is very much in the condition of our knowledge of what may be called geometrical astronomy, in distinction to physical astronomy, "We can calculate what will take place with small errors, which arise merely from the faults of observation, and not from a want of knowledge of conditions, or from the errors of a defective theory. It is probable, for instance, that the correct form of a dynamo -machine for producing the electric light can be calculated, and the plans drawn with as much certainty as the diagrams of a steam-engine are constructed. There is a department of electricity corresponding, perhaps, to hydraulics, in which the electrical engineer can find full employment in subjecting perfectly definite conditions to exact calculation. We can congratulate ourselves, therefore, in having a large amount of systematic knowledge in electricity, and we see clearly how to increase this systematic knowledge; for we have discovered that a man, to become an electrician, can not expect to master the subject of electricity, who has not made himself familiar with thermo-dynamics, with analytical mechanics, and with all the topics now embraced under the comprehensive title of physics.
Some may think that an electrician is a narrow specialist. I can only invite such persons to engage in the study of "What is electricity?"
In standing upon our scientific Mount Pisgah, we can survey the beaten roads by which we have advanced, and can see partially what has been good and what has been bad in the theories which have stood in the place of the leaders of the Israelites and have conducted us thus far. Out of all the theories—the two-fluid theory, the one-fluid, or Franklin theory, and the various molecular theories—not one remains to-day under the guidance of which we are ready to march onward. The two-fluid theory serves merely to fix the ideas of the student whose mind is new to the subject of electricity, I think I can safely affirm that no scientific man of the present believes that there is even one electric fluid, to say nothing of the existence of two. We have discovered that we can not speak of the velocity of electricity. We do not know whether the rate of propagation of what we call an electrical impulse is infinitely slow or infinitely fast. We do not know whether what we call the electrical current in a conductor is due to molecular motions infinitely faster than those of outlying molecules, or whether there is a sudden comparative cessation of molecular motion in the wire through which the current manifests itself, compared with the molecular motions outside the wire, for this might produce the electrical phenomena we observe. We do not know whether any molecular motion is concerned in the manifestation of energy which we call electrical. All that we can truly say is, we have
