NATURAL PHILOSOPHY. 117 



on a vertical axis, and carrying on its circumference eight bars of soft iron, 

 which, in going round, come very nearly in contact with the electro-magnets. 

 Now, from what has been said, k is easy to understand, that whenever one 

 of these bars approaches an electro-magnet, it is attracted laterally until it 

 comes in front of it, when the action of the electro-magnet ceases ; but at 

 that instant the attractive power of the next electro-magnet commences, and 

 so on. Now, as each of the bars thus receiA'C six impulses in one revolution, 

 and as there are eight bars, the number of impulses received in all by the 

 revolving drum is forty -eight, which impulses occurring in a few seconds, 

 produce, in point of fact, a continuous motion. Now, if the place of the 

 spokes be occupied, not by cylinders of induction, but by magnetized iron 

 bars, the cylinders being fixed on the revolving drum, and their hollows 

 filled with cores of unmagnetized soft iron; then each time a cylinder comes 

 in front of a magnet, the soft iron becomes magnetized, and generates a 

 current in the coil, which ceases as soon as the cylinder changes its place. 

 Now, as the motion is continuous, and the revolution rapid, each ceasing 

 current is replaced by another, and so on, ad infinitttm. These currents are 

 concentrated into a common conductor, and by this means an amount of 

 electricity is obtained which will melt an iron wire three yards long and 

 one-fourth of a line in diameter. An apparatus, consisting of thirty-two 

 cylinders and twenty-seven magnets, and made to revolve two hundred and 

 thirty-eight times in a minute, produced a permanent and regular light, 

 equal to that of two hundred and thirty tapers. Such, indeed, was the in- 

 tensity of the light produced, that a lighted candle being held against a 

 white wall, not only the shadow of the candle, but the shadow of the flame, 

 was projected on the wall by the electric light. The cost at which a light of 

 this intensity is produced, is stated not to exceed fifteen centimes per hour, 

 for each apparatus." 



CURIOUS ACTION OF ATMOSPHERIC ELECTRICITY;. 



In front of the Bibliotheque Impe'riale, at Paris, there exists an open space 

 upon which the Opera-house formerly stood. The place is ornamented with 

 a bronze fountain, which has been coated with copper by the electrotype 

 process. The operation was carried on in a workshop built for the purpose, 

 at the neighboring village of Auteuil. While the upper basin, from which 

 the water flows, through sixteen tigers' mouths was in the bath of sulphate of 

 copper, a violent thunder storm burst over Paris, and the lightning fell close to 

 the workshop in question. Immediately after the storm had subsided, M. 

 Oudry had the copper solution poured off, in order to examine the vase, 

 and to assure himself that the electric fluid had not deranged the deposit : he 

 was extremely surprised to discover that the copper had been deposited on the 

 tigers' heads in streaks or lines about the twenty-fifth of an inch in height, 

 separated by equal intervals, and so happily arranged that they form a veri- 

 table tiger's skin, covered with hair, in as perfect a manner as if they had 

 been produced by the hands of a skilful engraver. This curious effect of the 

 electric fluid has accordingly been alloAved to remain, and the result is a great 

 addition to the expressive character of the work. 



EFFECT OF PRESSURE ON ELECTRIC CONDUCTIVITY. 



Prof. "VTartmann, of Geneva, Switzerland, has recently made a series of 

 highly interesting experiments on the effect of pressure on the electric con- 



