34:2 Dynamic Theory. 



is laid horizontally in a shallow basin and covered with a concentrated 

 solution of nitrate of potash, which is poured over it so as to cover it. 

 Then, in the layer of liquid which covers the glass, and along the edges 

 of the plate, a horizontal platinum wire is immersed in connection with 

 the pole of a secondar} r battery of from 50 to 60 elements, then, hold- 

 ing in the hand the other electrode, formed of a platinum wire insulated 

 except at the extremity, the glass covered with the thin layer of saline 

 solution is touched at the points where the letters or drawing are re- 

 quired to be engraved. A luminous track appears wherever the elec- 

 trode touches, and no matter how quickly they are written or drawn the 

 characters become distinctly engraved on the glass. If one writes or 

 draws slowly the lines are deeply marked, their breadth depending on 

 the diameter of the platinum wire serving as electrode ; if it be pointed 

 these characters may be extremely fine. " The engraving may be done 

 with either electrode, but with the negative one, the engraving is more 

 distinct and it requires a less powerful current. These results have 

 been obtained by the use of secondary batteries ( storage batteries ), but 

 better, or at least more continuous work, could be got from a Bun sen 

 battery of a sufficient number of elements, or from a dynamo, or even 

 an alternating current magneto-electric machine. 



1 ' The phenomena of attraction and repulsion which seem so charac- 

 teristic of electricity can be imitated with the aid of a strongly com- 

 pressed jet of air escaping through an extremely narrow opening. Balls 

 of different substances, even metal, may be held in equilibrium attracted 

 or repelled by a jet of air at high pressure according to their distance 

 from the opening, density, &c. The recent works of M. Bjerknes have 

 shown the possibility of also obtaining, by other purely mechanical 

 means, attractions and repulsions similar to those caused by electricity. " 

 Perforations in paper by electrical currents resemble those made by pro- 

 jectiles impelled at a high velocity. "A jet of steam projected under 

 strong pressure against the slag of blast furnaces divides it into number- 

 less threads forming it into a kind of mineral wool. In the same way 

 matter impelled by electric movement sub-divides to an infinite extent 

 all other matter it finds in its wa} r . " Experiments have been made by 

 Prof. Bjerkness, of Christiania, in which he produced in water, "lines 

 of force " which imitate very closely the lines of force produced by mag- 

 nets. He used two little drums with India-rubber membranes for heads, 

 and which he caused to vibrate by means of a pair of air pumps worked 

 by a crank which rapidly forced air in or sucked it out of the drums. 

 They could be worked in the same phase, that is, both bulge out or both 

 cave in at the same instant ; or they could be worked in opposite phases, 

 that is,'' one bulge out while the other caved in. He placed them under 

 water a little ways apart, and made them vibrate in various ways. He 



