CANADIAN FISHERIES EXPEDITION, 1911,-10 285 



Avill be dependent upon tlie phase of the osciUation. The variation in the depth of the 

 isotherm gives this phase, and thus also furnishes a possibility of correcting for the 

 same . It is an unsatisfactory matter of fact having frequently been noticed that 

 liydrographical measurements repeated from the same station at a few hours' interval 

 may give entirely diiferent results, as well for velocity, temperature and salinity. 



Obviously, these periodical oscilla<tions arel the exact opposite of stationary 

 conditions, and the solenoids thus arising {vide, e.g., fig. 56) cannot therefore be used 

 to calculate velocities according to formula (14). By so doing, entirely erroneous 

 velocities would be obtained, with serious breaks to either side. Such breaks are, 

 however an excellent indication of wave movements of this sort. The distribution 

 of velocity in the boundary between the Labrador current and the Gulf Stream, 

 vide plates XIV and XV, distinctly indicates that the sea is there in a state of violent 

 oscillation. 



Observations as to ebb and flow, and the currents occasioned thereby, should be 

 treated according to the principles set forth in this chapter. 



10. OX THE TOPOGRAPHY OF THE SEA'S SURFACE, THE DISTRI- 

 BUTIOX OF PRESSURE, AND TRAXSFORMATIOX OF ENERGY 



IX THE SEA. 



In order to illustrate further the enormous practical utility of Bjerknes' cir- 

 culation theory in oceanographical work, I will here briefly touch upon one or two other 

 examples. 



Where a sea is in a state of perfect calm and equilibrium, all isobars and 

 isosteres therein will coincide with the level surfaces of gravity. Such a state of 

 equilibrium may be supposed to exist at great depths, so that the sea there would 

 exhibit the same pressure at the same level. We can then, by means of the differ- 

 ential formula for measurement of barometric height, 



, vdp (15) 



dz = 



9 



vvliicli is equally applicable to the sea, arrive at the topography of the sea's surface. 

 By integrating (15) from the surface down to the level of greatest pressure p„ from 

 which observations are available, we obtain 



Pi 



vdp (16) 



9 J po 



where z represents the height of the sea's surface above the isobaric surface p,. If 

 this calculation be carried out for several hydrographical verticals of measurement, wo 

 obtain the formation of the sea's surface p=Po relative to the isobaric surface 

 p=Pi, i.e., the actual topography of the surface of the sea. 



Owing to the great variation in the depths of the sea, the calculation may in 

 practice best be carried out by reckoning the difference in height of the sea's surface 

 at adjacent hydrographical stations, taken in pairs, and subsequently setting out all 

 the results together in the form of a topographical map. According to (16), the 

 difference in height between two hydrographical verticals will be as follows : — 



9 J \ 



1 /-P, 



vj dp (17) 



This integral operation is, however, identical with that carried out in tables 4 and 5. 

 Using the terms there obtained, we get : 



1 

 2,— 2.= — 2A (IS) 



9 

 i.e., we have here the data requisite for calculation of the topography of the sea's sur- 

 face, by dividing the number of solenoids by tlie force of gravity. 



