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mercury is soon covered with a coating of oxide, which either entirely prevents the metallic contact, or at least weakens it. In employing amalgamated surfaces this effect is produced still more rapidly. Besides I have by incontestible proofs arrived at the conviction, that the simple contact of metals with a clean surface is quite sufficient to conduct the electric current, even of the weakest tension. The contact by means of mercury adds nothing to the energy of this current. It is erroneous to judge of this energy by the brilliancy of the spark, proceeding only from the combustion of the mercury. I have thought right to mention these circumstances, though apparently trifling. In a motor, from which we look to obtain an infinitely accelerated motion, the smallest details should not be disregarded; the most trivial are ultimately of importance.

7.

Fig. 3. represents the Commutator, adapted to the magnetic apparatus, so as to produce the inversion of the poles: $a,\,b,\,c,\,d$ are four discs of copper fixed upon the axis of rotation $e\,e$. The discs $a,\,b$ and $c,\,d$ are united by copper tubes $f,\,f$, entirely insulated from the axis by the interposition of a tube $g$, of varnished wood or any other insulating substance.

The periphery of each disc is divided into eight exactly equal parts, of which four $h$ are cut into sectors and filled afterwards by pieces of ebony, forming with the metal an accurate and smooth surface. The discs are arranged upon the axis of rotation, so that the sectors of wood and of metal alternately correspond, as represented by the shaded parts of the figure. $ZZ,\,CC$ are bars of copper, formed as levers, very moveable in their supports: they are intended to conduct the current. The arm of the longest lever forms at its extremity an edge, which rests on the periphery of the corresponding disc. The other arm is bent and plunged into a little jar filled with mercury, $k$. The jars $k\,k$ and $k'\,k'$ are united by plates of copper, as represented in figure 1. The action of this commutator will easily be understood. The levers are always in contact with the discs, and are alternately so with the metallic and insulating parts. By their mobility in their supports they yield to the slightest inequality of the surface. and the friction they occasion is very trifling. The helices which surround the moveable bars are united so as to form a continuous wire, the ends of which $l,\,m$ are soldered respectively to the systems of the discs $a,\,b$ and $c,\,d$. The other helices, wound round the fixed bars, are also united, and the ends $n$ and $o$ immersed, the one in a jar of mercury $p$, attached to the voltaic apparatus, and the other in the jar $k$ of the commutator. Thus all the sixteen helices form only one connecting wire, through the medium of the commutator. The voltaic apparatus consists of four troughs of copper, in which plates of zinc are immersed, all being united as in a pile. The