XII.-3PECIAL COLLECTIONS. 339 



the upper rarefied layers of air to the earth ; this current, during its passage 

 through the rarefied air, produces light phenomena that cannot arise in denser 

 layers of air. 



The polar-light apparatus now exhibited shows that an electric current 

 passing from an insulated hody does not produce any light in air of normal 

 pressure, but as soon as it rises to the rarefied air in the Geissler's tubes, 

 there is directly produced a phenomenon very like the real polar-light. la 

 the apparatus the upper end of the tubes is in union with the earth ; this 

 is by no means necessary, for the light phenomenon is also produced if this 

 union be removed, provided that in such case the tubes be brought a little 

 nearer to the insulated sphere. For the rest, the earth represents here the 

 wide space of rarefied air that we find beyond the limits of the air conductor, 

 and which serves here as an electric reservoir. 



To consider now how the polar-light on a large scale is formed in nature. 

 As before said, the earth and the air conductor hold to each other the posi- 

 tion above mentioned, and the two electricities, the negative electricity of 

 the earth and the positive electricity of the air conductor, endeavour with 

 a certain force to unite in a belt around the north pole. The insulating 

 power of the denser air prevents this reunion ; but if we assume that the 

 equilibrium is attained, the union will instantly take place as soon as this 

 insulating power is diminished or the electricity on the conductor augmented. 

 The first case, which probably is the most ordinary, happens if a southerly 

 wind carrying a quantity of vapour attains the polar regions ; for instance, 

 the belt, where the vapour, in consequence of the cold, is condensed into a 

 fluid form, reduces considerably the insulating power of the air and enables 

 the electric current to flow through it. The same thing would occur if a layer 

 of clouds happened to enter into this belt ; the upper end of the cloud 

 would become negatively electric, the lower one positive, and thus the dis- 

 tance between the two conductors would in fact be diminished. The electric 

 current would go from the air conductor to the cloud, and through this 

 latter to the earth. Similar phenomena are observed in the polar regions, for 

 the upper edges of the clouds are not unfrequently seen shining with a 

 yellowish light stretching considerably upwards, whilst no light is discernible 

 under the cloud because of the air there having attained a density sufficient 

 to prevent the current from producing light. 



For further details of the polar-light and its theory, see Archives des 

 Sciences Phys. et Natur. de Geneve, 1875 (Sept. and Oct.), and in January 

 1876, as well as to two essays published in the years 1869 and 1873 in the 

 same scientific journal, all which articles are more or less the result of obser- 

 vations made in the arctic regions. Besides these may be mentioned the 

 works upon polar-light of the American natural philosopher Loomis, Hep. 

 of Smithsonian Ins., 1865, &c. 



4558. Photograph of a Dielectrical Machine, system of 



Prof. Cecclii. Prof. Filippo Cccchi, Florence* 



4559. Photograph of an Electric Motor, on the plan of 

 Prof. Cecchi, of Florence. Two rectilineal electro-magnets with 

 polar masses of iron are alternately magnetised, and by means of 

 levers and eccentrics produce the motion. 



Prof. Filippo Cecchi, Florence* 



The electro-magnetic motor of Prof. Cecchi, of Florence, is set in move- 

 ment by two rectilineal and parallel electro-magnets placed horizontally upon 



