448 Mr. Thomas H. Blakesley on 



Finally, these observations and deductions have, it seems to 

 me, an important practical application not only as regards a 

 living creature like the soaring bird but still more as regards 

 a mechanically constructed body, whose specific gravity may 

 probably be many hundred or even many thousand times 

 that of the atmosphere. We may suppose such a body to be 

 supplied with fuel and engines, which would be indispensable 

 to sustain it in a calm, and yet which we now see might be 

 ordinarily left entirely inactive, so that the body could sup- 

 posably remain in the air and even maintain its motion in 

 any direction without expending its energy, except as regards 

 the act of changing the inclination or aspect which it presents 

 to the wind, while the wind blew. 



The final application of these principles to the art of aero- 

 dromics seems then to be, that while it is not likely that the 

 perfected aerodrome will ever be able to dispense altogether 

 with the ability to rely at intervals on some internal source of 

 power, it will not be indispensable that this aerodrome of the 

 future shall, in order to go any distance — even to circum- 

 navigate the globe without alighting, — need to carry a weight 

 of fuel which would enable it to perform this journey under 

 conditions analogous to those of a steamship, but that the 

 fuel and weight need only be such as to enable it to take care 

 of itself in exceptional moments of calm. 



Smithsonian Institution, 

 Washington, D. C, August 1893. 



XLII. A neiv Electrical Theorem. 

 By Thomas H. Blakesley, M.A.* 



THE very short paper which I shall read to the Society 

 contains the account of a Theorem which, though ad- 

 mitting of easy proof, appears, so far as my inquiries have 

 gone, to have hitherto escaped notice. 



In order to state the matter briefly, it will be well to adopt 

 the following definition : — 



If in any system of conductors, however reticulated, two or 

 more modes of disposition of sources of electromotive force 

 produce in every part of the network the same current, such 

 systems of disposition are called equivalent systems. Then 

 the theorem is as follows : — In any system of conductors, 

 possessing seats of electromotive force at any number of 

 points, if any of these sources be supposed to move con- 

 tinuously along the various bars of the conducting system, 



* Communicated by the Physical Society : read February 23, 1894, 



