( 566 ) 



through which passes a surface out of each of the pencils. Through 

 the points P, P' , P" and P'" of such a quadruple pass three sheets 

 of the surface L and three I (ranches of the double curve. The 12 

 branches of the double curve through those four points we can call 

 PI, P2, P'S, P'i, P"2, P'4, P"l, P'% P'% P'"2, P"% P'"4, in such 

 a way that the triplet of points is movable along the branches 

 P1,P'1,P'% along P2,P'2,P'"2, along 7*3, P "3, P '"3 and along 

 P'4, i>»4 } P"'4. If the sheet of L passing through PI and P2 is 

 called P12, then the corresponding sheets (i. e. sheets along which 

 the pair of points not lying on the double curve is movable) are 

 P12 and P'12, P13 and P"13, etc. 



Geophysics. — "Current-measurements <<t c<tri<>u< depths in the 

 North Sea." (First communication). By Prof. C. H. Wind, 

 Lt*. A. F. H. Dalhuisen and Dr. W. E. Ringer. 



In the year 1904 accurate measurements of the currents in the 

 North Sea l ) were started by the naval lieutenant A. M. van Roosen- 

 daal, at the time detached to the -Rijksinstituut voor het Onderzoek 

 der Zee", having been proposed and guided by the Dutch delegates 

 to the International Council for the Study of the Sea. 



By him four apparatus were pul to the test, viz. 2 specimens of 

 the current-meter of Pettersson 5 ), one of that of Nansen ') and one 

 of that of Ekman 4 ), all destined to determine the direction and the 

 velocity of the current at every depth. 



The experiments were partly made on the light-ship "Haaks", 

 where Dr. J. P. van der Stok, the Marine Superintendent of the 

 Kon. Nederl. Meteorologisch Instituut, also took part in them. Other 

 experiments were made in the harbour of Nieuwediep and further, 

 from the research-steamer "Wodan", in the open North Sea at a 

 station (H2) of the Dutch seasonal cruises 6 ), situated at Lat. 53°44' N. 

 and Long. 4°28' E. 



*) Cons. Perm. Intern, p. l'expl. de la mer, Publications de circonstance No. 26 : 

 A. M. van Roosendaal and G. H. Wind, Priifung von Strommessern unci Strom- 

 messungsversuche in der Nordsee. Gopenhague, 1905. 



2) Publ. de circ. No. 25. 



3) „ „ „ No. 34. 



4 ) „ „ „ No. 24. 



5 ) Quarterly cruises of the countries taking part in the international study of 

 the sea, along fixed routes, observations being made at definite points or ''stations". 



