174 ELECTROLYSIS. 



of the sides. A graduated scale constantly indicates the 

 energy of the pressure. 



The screw press is perhaps less liable to accidentally get 

 out of order than the hydraulic, but its action is less intense 

 and less uniform. 



When this operation is done, the mould is lifted and the 

 wax is examined; it must represent a faithful sunk copy of 

 the engraving. The impress is sprinkled with fine plumbago 

 and polished with a very smooth brush. 



In America they employ for plumbago coating purposes a 

 machine (Fig. 29), the use of which is more economical and 

 healthy. When working by hand the dust flies everywhere, 



FIG. 29. 



Plumbago Coating Machine. 



whereas the machine is so arranged that the brushes almost 

 cover the mould, and the excess of plumbago drops into a lower 

 vessel. The construction and working of this machine are most 

 simple ; it is a horizontal frame mounted on four legs, with a 

 sliding table composed of a series of bars. A long brush of the 

 same width as the table receives, from a spindle rotating at 

 400 revolutions per minute, a vibratory motion. The machine 



