71 



to the succeeding ones, and the induced magnetism after a certain 

 point, proceeds entirely by communication from particle to particle, 

 until the whole power is expended. When, on the contrary, the 

 retentive power of the given substance is small, little or no screen- 

 ing energy exists between its particles, in which case the magnetic 

 excitement will depend upon the influence of the magnet on each 

 individual particle : hence it is only by the succession or multipli- 

 cation of effect resulting from a great number of particles, that we 

 at length render the controlling power of such a substance sensible. 

 The diminished action of a magnet on a disc of copper, when inter- 

 sected by radiating grooves, seems to be owing to this cause, since 

 a portion of the substance, requisite to the full development of the 

 magnetic energy, is removed. In confirmation of this reasoning it 

 was found that the number of oscillations of a delicately suspended 

 bar, made in vacuo, in a given arc, surrounded by a mass of copper 

 formed into rings, did not sensibly differ when, in the one case, that 

 mass was made up of concentric rings, and, in the other, was entirely 

 solid : while, on the contrary, by removing a very thin external 

 lamina from the former, the number of vibrations was sensibly 

 changed. 



The concluding part of this paper is occupied by speculations on 

 the nature of magnetic action : the author being disposed to regard a 

 magnet as rather in a passive than an active state, when exhibiting 

 the phenomena of magnetic attraction. This attraction he con- 

 siders as the result of an impression first made on the magnet by the 

 iron which appears to be attracted by it: because he finds that with 

 different masses of iron of the same quality, the force at the same 

 distance is unequal ; being with some pieces very sensible, whilst 

 with others it is altogether inappreciable. He views a magnet as a 

 substance put into a peculiar state or condition, in consequence of 

 which it exhibits certain properties when subjected to external ex- 

 citation ; in a way analogous to the elastic force of a spiral spring, 

 which is not called into action unless that spring is stretched by a 

 weight suspended to it, or by some other extraneous force. In the 

 case of magnetism, the exciting substance is likewise affected in a 

 similar manner with the magnet which it excites ; and the analogy 

 of the spiral spring may be further pursued, in order to render the 

 two cases corresponding, by supposing the weight which elongates 

 the first spring to be itself another similar spiral spring, which is 

 also elongated while exerting its force on the first. Under these 

 circumstances the separation of the coils will be greatest at the 

 upper end of the whole combination of springs, at least at the lower 

 part, presenting a contrariety of states at the two extremities, ana- 

 logous to the opposite polarities of the two ends of a magnet. 



A paper was read, " On the Atmosphere of Mars." By Sir James 

 South, F.R.S. 



The author refers the origin of the hypothesis of the " Extensive 

 Atmosphere of Mars" to the observations of Cassini and Rcener, 

 made at Briare and Paris in the year 1672. By the former it would 



