170 ANNUAL OP SCIENTIFIC DISCOVERY. 



lurn, commonly employed to study the distribution of tlie electricities of a 

 body electrified by influence, are not suitable to furnish exact indications, 

 because they themselves are effected by the influence of the active body ; 

 and to protect them against this cause of error, he has imagined the expe- 

 dient of interposing a metal plate, communicating -\vith the common 

 reservoir ; the operator then has a body charged with an electricity acting 

 by influence upon a neighboring body through the plate, which is a sort of 

 screen to the organs of exploration. In these new conditions, Monsieur 

 Melloni observes that the electricity on the surface of the body influenced 

 is of the same nature and has the same appearances as that of the acting 

 body, and he concludes : " This experiment dissipates the illusion hitherto 

 formed, touching the contrary electric tensions developed 011 the opposite 

 extremities of the influenced body." 



These new ideas do not meet with the support of many others who 

 have experimented upon this subject ; they cannot admit that the metal 

 plate introduced into the experiment acts simply as a screen ; and some 

 have suggested that the whole controversy rests on a misunderstanding, 

 and that his reasoning follows somewhat the following course : dissimu- 

 lated electricity is not the electricity of tension ; but, in a body influenced 

 within the radius of a given distance, one of the fluids is dissimulated ; 

 therefore, the other fluid alone retains its tension. But in reality, it is said, 

 the two fluids retain their respective tensions, the instruments attest it ; 

 and in the opinion of natural philosophers, the true method of treating the 

 question is invariably to consider the body as symmetrically placed between 

 two contrary influences. 



ELECTRO-MAGNETIC ENGRAVING. 



This machine is somewhat on the principle of the well-known planing 

 machine. The drawing to be copied and the plate to be engraved are 

 placed side by side on the movable table or lid of the machine ; a pointer 

 or feeler is so connected, by means of a horizontal bar, with a graver, that, 

 when the bar is moved, the drawing to be copied passes under the feeler, 

 and the plate to be engraved passes in a corresponding manner under the 

 graver. . It is obvious that in this condition of things a continuous line 

 would be cut on the plate, and, a lateral motion being given to the bed, a 

 series of such lines would be cut parallel to and touching each other, the 

 feeler of course passing in a corresponding manner over the drawing. If, 

 then, a means could be devised for causing the graver to act only w r hen the 

 point of the feeler passed over the portion of the drawing, it is clear we 

 should get a plate engraved, line for line, with the object to be copied. 

 This is accomplished by placing the graver under the control of two electro 

 magnets, acting alternately, the one to draw the graver from the plate, the 

 other to press it down on it. The coil enveloping one of these magnets is 

 in connection with the feeler, which is made of metal. The drawing is 

 made on a metallic or conducting surface, with a rosined ink, or some 



