SCIENCE IN THE INDUSTRIAL WORLD 



and it will always remain, a delicate substance, inas- 

 much as it may be torn easily between the fingers when 

 held in certain positions, but it has most amazing 

 strength when even tension is applied. "In evidence 

 of this," says Butler, "may be cited an instance that 

 seems almost beyond belief. Through some curious 

 mishap a web of heavy paper, in fact, bristol-board, 

 which had been thoroughly formed, was suddenly 

 superheated and then cooled while still on the driers. 

 This was caused by a difference in temperature of the 

 driers, and resulted in the sudden contraction of the 

 web of bristol; the strain on the machine was so great 

 that not only were the driving-cogs broken on two of the 

 driers around which the paper was passing at the mo- 

 ment, but the driers themselves were actually lifted out of 

 place, showing a resisting power in the paper of at least 

 several tons." 



When the paper comes from the driers it may be 

 regarded as finished for certain purposes; but if it is 

 to be a smooth-surface paper there still remains the 

 process of "calendering." The "calender" is an up- 

 right set of rolls, steam-heated, through which the 

 paper is whirled at the same speed at which it passes 

 through the other cylinders. In fact the entire process 

 is a continuous one, the paper traveling at a uniform 

 speed from the time it leaves the pulp-vat until it 

 emerges from the last cylinder as finished paper. This 

 speed varies, of course, with different machines and with 

 the different kinds of paper, but a speed of from four to 

 five hundred feet a minute is not uncommon the trot- 

 ting-gait of the average coach-horse. 



[170] 



