144 Account of the 



hard. The difference in the temper of these two kinds of rolls 

 is so great, that when they are put into the lathe to be turned 

 perfectly true, the turnings from the one will be |th of an inch 

 in thickness, whilst the turnings which come from the other are 

 not larger than very fine needles. The temper of cast-iron thus 

 varying according to the nature of the mould into which it is 

 poured, is a circumstance that appears to me to be deserving of 

 attention in the manufacture of a variety of other utensils em- 

 ployed in the arts. 



These rollers are used without heat, but they are screwed 

 down very close one upon the other, only leaving bare room for 

 the plates to pass, that the utmost attainable degree of pressure 

 may be given to them. This last operation in called cold 



ROLLING. 



When the plates have undergone this process, they are put 

 one by one into troughs filled with a liquid preparation called 

 the lies. 



This is merely water, in which bran has been steeped for nine 

 or ten days, until it has acquired a sufficient acidity for the pur- 

 pose. The desig-n of putting the plates into the troughs, singly, 

 is, that there may be more certainty of the liquor getting between 

 them, and both the sides of every plate being soaked alike in the 

 lies. In this liquor they remain for ten or twelve hours standing 

 on the edges, but they are turned, or inverted, once during that 

 time. This operation is called working in the lies. 



The next operation is that of steeping in a mixture of sulphuric 

 acid and water, in proportions which vary according to the judg- 

 ment of the workmen. 



The trough in which this operation is conducted, is made with 

 thick lead, and the interior is divided by partitions of lead. 

 Each of these divisions is by the workmen called a hole, and each 

 of them will contain about one box of plates. In the diluted sul- 

 phuric acid which is in the diflFerent compartments of this vessel, 

 the plates are agitated for about an hour, or until they have become 

 perfectly bright, and entirely free from the black spots which are 

 always upon them when they are first immersed in it. 



