TRANSACTIONS OF THE SECTIONS. 11 



observed) : this smoke traverses from pole to pole, from the negative to the positive 

 side, showing, unless there be some latent optical deception, a mechanical effect of 

 the discharge under the circumstances. — The phsenomenon was exhibited to the 

 members of the Section in the committee-room, which had been darkened for the 

 purpose. 



On the Law of Electrical and Magnetic Force. By Sir W. S. Harris, F.R.S. 



The author prefaced the exposition of the views he himself had adopted, after 

 elaborate experimental research on the subject, by stating that the discovery of the 

 beautiful and comprehensive law of universal gravitation by Newton had predisposed 

 all physical inquirers to entertain the notion that every other force associated with 

 ordinary matter was subject to a similar law. The forces of electricity and magnetism 

 were especially considered as coming under a like law, and a great variety of expe- 

 rimental inquiries were instituted to verify the conjecture. Cavendish, after QEpinus, 

 was certainly the first philosopher who investigated experimentally and threw light 

 on this question. This appears by his celebrated paper in the ' Philosophical Trans- 

 actions' for 1772, and likewise by his unpublished manuscripts, which had descended 

 to the Earl of Burlington, and had been placed by that nobleman in the hands of the 

 author ; and, he might add in passing, were open to the inspection of any inquirer 

 engaged in these researches, and contained matter of the most important kind. The 

 author then pointed out several well-known and acknowledged truths in these 

 sciences which were due to the researches of Cavendish. He then pointed out the 

 influence which the researches of Coulomb had exercised on the universal philoso- 

 phic world, particularly after the writings of the celebrated Poisson, Laplace, Biot. 

 and others had given form and currency to his views and principles. Such a 

 galaxy of eminent names, and so wide a reception of Coulomb's theoretical views, 

 the author considered to be calculated to discountenance and discourage much critical 

 inquiry as to their soundness, and to immerse us in a kind of philosophical ortho- 

 doxy very unfavourable to a more complete knowledge of these unseen, yet astonishing 

 powers of Nature which we daily experience. The author then went on to illustrate 

 the law of the inverse square of the distance as relating to forces emanating from 

 one central point and to other emanations from a centre, and to point out how far 

 this might safely be relied upon as applicable to the electrical and magnetic forces 

 of attraction and repulsion ; and stated that the object of the present communication, 

 which the author submitted with all due diffidence, was to investigate the physical 

 condition under which these forces manifest themselves, — what are the general laws 

 of the operation of such forces, — how far we may safely consider them as central 

 forces, such as gravity, or whether they are to be considered more in the light of 

 forces, operating between surfaces distinctive in their character and in their ordinary 

 relations to common matter. He then pointed out one essentially distinctive cha- 

 racter of these forces. In gravitation, the attracted body, as far as we can observe, 

 remains in the same physical condition before and during all the changes of distance 

 and force to which the bodies are naturally subjected. But in the phaenomena of 

 electrical and magnetic attraction and of repulsion, the very first step was that the 

 body acted upon had its physical condition changed ; and this change again, by a 

 kind of reflex influence, affected what had been the instant before the physical 

 condition of the body producing the change ; and thus, during the action and its 

 changes, new physical conditions of both had to be investigated and taken into con- 

 sideration, that is, if we wish truly to interpret the facts. The author then, with 

 well-arranged apparatus, proceeded to illustrate, by some striking experiments, both 

 electrical and magnetic, the truth and importance of these general views: he endea- 

 voured to explain the peculiar electrical conditions under which the forces of elec- 

 tricity and magnetism might be expected to vary in the inverse duplicate ratio of the 

 distances, but which conditions being interfered with, other laws of force might 

 become developed, as found by many eminent philosophers of the last century, dis- 

 tinguished by their great skill in experimental physics. The author concluded by 

 some observations on the use of the proof plane and the torsion balance, and showed 

 with what great caution the proof plane should be applied as a means of deducing 

 results to serve as data for mathematical analysis. 



