FISH-NET TKESERVATIVES. 



29 



product, J, lines treated with which are almost entirely covered with 

 hydroids. The tarred lines are also heavily fouled. In the next 

 two months after these samples were taken up the tarred samples 

 were so heavily fouled with ascidians that they broke and were 

 carried away. The lines treated by quercitron and potassium 

 bichromate, I, petroleum products Nos. 1 and 2, J and X, and gil- 

 sonite, S, also failed to be returned. 



Here, again, copper oleate shows to advantage, though no better 

 with respect to prevention of fouling than copper paint. The petro- 

 leum products, gilsonite, quercitron and potassium bichromate, and 

 the tars seem to produce an agreeable anchorage for these organisms, 

 and this fact must be held seriously against them as net preserva- 

 tives. 



TESTS WITH LINEN LINES. 



160 



50 



fe40 



5 

 £30 



:ti2o 



10 



160 



MATERIALS TESTED. 



In this series of experiments samples of Imen lines were included. 

 The linen selected was 10-ply 40 "Irish flax salmon thread," loose- 

 laid, such as is used in gill netting 

 for salmon on the Pacific coast. 

 The tensile strength of the un- 

 treated, unexposed line is 55.2 

 pounds, somewhat greater than that 

 of No. 24 cotton twine. Although 

 its tensile strength is greater than 

 cotton, it is a characteristic of this 

 linen line that it fails completely 

 to resist mechanical wear on the 

 testing machines, even the new un- 

 treated line falling in two in only 

 a half dozen strokes. The exposed 

 samples usually broke on the first 

 stroke, so that it was useless to 

 apply this test. This weakness is 

 apparently due, at least in part, to 

 the fact that the component strands of the line are not twisted to- 

 gether, but each acts separately. When the lines "saw" together, 

 each individual strand is exposed; under such circumstances the life 

 of the whole line is only equal to the life of one strand. 



It would be useless to test for linen thread the preservatives that 

 cause stiffness, such as tar, copper paint, and gilsonite. The preser- 

 vatives and preservative methods physically suitable for gill nets and 

 available at the beginning of these Beaufort experiments were three, 

 quercitron and potassium bichromate, V (when applied to linen this 

 symbol was given), Dutch method, W, copper oleate, U, the un- 

 treated linen, T, being the control. The samples were exposed at 

 Beaufort 180 days, samples being returned each two months. The ex- 

 posure began February 15 and continued to August 15, 1922. 



60 120 



DAYS EXPOSED 



Fig. 20. — Tensile strength of linen lines exposed 

 in sea water at Beaufort, N. C. 



TENSILE STRENGTH. 



The results of the measurements of tensile strength of these samples 

 are given in Table 10 and are shown graphically in Figure 20. 



