1875.| Physvcs. 131 
accomplished by hand. It takes up much time, and injures the workmens’ 
health, on account of stooping all day over powdered materials containing 
copper. M. Vavin’s apparatus consists of two hollow superposed cylinders 
revolving in the same direction, upon which the material to be sorted is 
spread by a hopper. The surface of these cylinders is formed by soft iron bands 
maintained in a continuous magnetic condition by magnets. To the surface 
of these cylinders then the iron particles attach themselves, and at a certain 
time they are detached by revolving brushes, and thrown into a side box, whilst 
the copper and earthy particles fall to the bottom of the apparatus. M. Man- 
gon says he has used this apparatus to search for titanous iron in arable lands 
with such successful results as could never have been attained by any other 
means—chemical or otherwise. 
The following interesting discovery, which is likely to be of considerable 
practical importance in telegraphy, was communicated by Mr. Edison to a 
recent number of the ‘Scientific American.” The salient feature in Mr. 
Edison’s discovery is the production of motion and of sound by the style of the 
Bain telegraphic instrument, without the intervention of a magnet and arma- 
ture. By the motion thus produced, he works any of the ordinary forms of 
telegraph printing or sounding instruments, or relays, and is enabled to send 
messages, by direct tranmission over thousands of miles of wire, at the highest 
speed, without re-writing, delay, or difficulty of any kind. More than this, 
his apparatus operates in a highly effective manner, under the weakest electric 
currents, and he is able to receive and transmit messages by currents so weak 
that the ordinary magnetic instruments fail to operate, or even give an indi- 
cation of the passage of electricity. Thus, when the common instruments 
stand still, owing to feebleness of current, the Edison telegraph will be at full 
work. The author has named his discovery the electro-motograph. The 
instrument attracted considerable notice at the recent soiree of the Society of 
Telegraph Engineers. 
Dr. Ph. Carl has arranged a new tangent-elefrometer for letures, which 
unites very great sensibility with the advantage that the ele@ric phenomena 
can be recognised at a great distance, and can be adapted for the execution 
of absolute measurements. The instrument is represented in the figure one- 
twelfth its size. Upon a wooden support A A, stands an insulating glass rod, 
8, which supports a brass fork, c, provided above with steel bearings. In 
