1915] Michael, ct al.: IliifJi-ogvaphic Rcronh of Scvipps institution 23 



concluded that the net has opened and closed at the same time even 

 though, when thoroughly examined and tested, the net seems to work 

 perfectly and the cause of its failure to open in tliat particular case 

 cannot be discovered. 



Should the reader think it to lie a simple matter to discovi'r the 

 cause for such failure, his attention is called tc the following list of 

 causes which were discovered during the work of July, 1912, some 

 being easily ascertained while others, like 2, 4, and 6 were exceedingly 

 baffling : 



1. Tension of opening spring insufficient to open the net when sub- 

 merged, although it will do so on deck. 



2. Tension of spring so great that impact of the first messenger, 

 when falling through water, is not sufficient to operate the trip, but 

 when augmented by that of the second messenger would cause tripping 

 of both jaws. 



3. Spring of opening jaw loose or broken. 



4. First messenger slightly worn along one side so that, when strik- 

 ing in a certain way, it slips past the opening trip, and the second 

 messenger then trips both jaws. 



5. First messenger slightly flattened and spread in consequence 

 of repeated use .so that, with a cable slightly inclined in the right 

 direction, it strikes and operates both trips. 



6. First messenger, in descending, scrapes tar off the cable which 

 becomes jammed in the groove of the messenger, thus retarding its 

 descent so that it does not strike with sufficient force. Impact of second 

 messenger then trips both jaws. 



7. Three cases in which the cause was not any of the above six, and 

 could not be ascertained. 



(6) The Nansen nets. — Any one having occasion to work over our 

 plankton collections from a quantitative point of view will discover 

 that some vertical hauls made with these nets reveal an absence of 

 certain organisms at depths where, judging from hauls made with 

 horizontal closing nets, an abundance would be expected. The general 

 impression produced is that, for some reason or other, the Nansen nets 

 act in an erratic manner, frequently, though not always, failing to 

 capture such organisms even though they may have been abundant. 

 While this has not, as yet, been proven, it is advisable to point out 

 how such erratic action might arise. 



The "Agassiz's" excessive stern overhang, while of undoubted 

 advantage in protecting the collecting apparatus from the propeller- 



