CHAPTER IX. 



SYNOPTICAL REPRESENTATION OF THE FIELDS OF FORCE AND 



OF MASS IN THE SEA. 



80. Quasi Static State. The motion of the sea being generally much slower 

 than that of the atmosphere, we may characterize sea-motion with still greater reason 

 than that of the air as slow, and going on near a state of equilibrium. Excepting 

 local phenomena, such as the formation of whirlpools in narrow straits or waves on 

 the surface, we find the conditions of equilibrium apparently fulfilled to a great 

 extent during the motion. 



On the other hand we have, still more than was the case in the atmosphere, small 

 distances in a vertical and great in a lateral direction. Consequently the conditions 

 of the quasi static state (section 60) are fulfilled in the sea and with still greater 

 approximation than in the atmosphere. We therefore state this principle, forming 

 the basis of all practical investigations in oceanic dynamics: 



The condition of equilibrium is apparently fulfilled along every vertical 

 or quasi vertical line. But as -we proceed in a horizontal direction, there is a 

 gradual change in this apparent state of equilibrium from vertical to vertical. 



81. Topography of Isobaric Surfaces. Owing to this principle we can pro- 

 ceed formally as in the case of the atmosphere. Let us suppose first the depth of 

 a given sea-pressure to have been determined by a set of simultaneous soundings. 

 Then on a chart containing the situation of the hydrographic stations, we can note 

 these depths and draw topographic charts, exactly as in the corresponding case of 

 the atmosphere. Fig. 27 below gives examples of such charts. 



It is important, however, to get a clear perception of these charts. For an 

 important difference enters between the results attained in the case of the atmos- 

 phere and in that of the sea. 



First, the surfaces whose topography is represented are surfaces of equal value 

 of the sea-pressure, not of the total pressure. Secondly, the topography is given 

 relatively to the physical sea-level, from which the measurements are made, and 

 not Jrom the ideal sea-level of the gravity potential zero. To be able to draw charts 

 of absolute topography we want to know the topography of the physical sea-level, 

 and this can not be found from the results of the soundings. This is of great 

 importance to keep in mind. For owing to the motions of the sea and to the 

 varying atmospheric pressure, the distance of physical from ideal sea-level will be 

 of the same order of magnitude as that of isobaric from corresponding level sur- 

 faces. Thus, not only theoretically, but also practically, the topographic charts of 

 the isobaric surfaces given in fig. 27 are charts of relative topography, relatively 

 to the unknown topography of physical sea-level. This is an important restric- 

 tion on the completeness of the result, making the discussion of sea-motion much 

 less direct than that of atmospheric motions. 



J 33 



