50' American Fisheries Society. 



change in the preservative; and (3) changes as influenced by 

 the toxic or biological preserving properties of the preservative 

 used. These will be considered separately. 



1. Immediate changes in tensile strength. The fibers in a 

 cotton line are short, but are made into a long line by being 

 twisted together so that the friction of the fibers against one 

 another causes the line to hold together as a continuous whole. 

 When a line breaks, a force must be applied to the line which is 

 sufficient to overcome the friction of the fibers against one 

 another, or to break them ; if the friction is greater than the 

 tensile strength of the fibers, they will break; if the tensile 

 strength is greater than the friction they will pull out. No 

 doubt both things happen — some of the fibers are actually 

 broken, while others are merely disengaged from the twisted 

 strands. Now the introduction of a foreign material between 

 these fibers and among the strands may greatly alter the prop- 

 erties and behavior of the line. If the foreign substance lubri- 

 cates the fibers or otherwise diminishes friction, the line as a 

 whole is weakened proportionately, so that when a pull is ex- 

 erted on the line the fibers are drawn out or disengaged rather 

 than broken. This weakening, we see, occurs in the case of 

 many of the preservatives. In fact, the only preservative which 

 did not cause an immediate weakening was copper paint. The 

 waterproofing sample, being treated at the factory, was not 

 measured immediately after treatment. 



2. A later change in tensile strength, caused by drying of 

 the preservative. In the case of those preservatives having a 

 volatile and a non-volatile portion, the volatile, or soluble, por- 

 tion evaporates or dissolves in time. The body, or non-volatile 

 portion, then remains, and may markedly alter the strength of 

 the line. Thus, the tars, while at first diminishing tensile 

 strength by their lubricating effect, on drying out cause a marked 

 increase of strength which may persist until the lines begin to 

 deteriorate through decomposition. This is also true of the cop- 

 per paints, waterproofing material, petroleum products, and 

 slightly in some others, and is particularly noticeable in the 

 case of those lines exposed to weather conditions in Washington. 



It is necessary to take these facts into consideration in 

 reading the graphs. The sudden initial drop in tensile strength 

 may convey the idea that a preservative is poor, or a sudden ini- 



