78 THE POPULAR SCIENCE MONTHLY. 



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 geo- 

 metrical 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 subject- 

 ing perfectly definite conditions to exact calculation. We can con- 

 gratulate ourselves, therefore, in having a large amount of systematic 

 knowledge in electricity, and we see clearly how to increase this sys- 

 tematic 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 embi*aced under the compi-ehen- 

 sive 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 elec- 

 tricity ? " 



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 j^resent 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 elec- 

 tricity. 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 conduct- 

 or is due to molecular motions infinitely faster than those of outlying 

 molecules, or whether there is a sudden comparative cessation of mo- 

 lecular 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 



