36 



eye of the reader. Provided that the water is moving faster in the 

 surface layers than in the depths, the rule follows: Look in the 



DIRECTION TOWARD WHICH THE CURRENT IS RUNNING, IN THE NORTH- 

 ERN Hemisphere, and the lightest water will always lie on 

 THE RIGHT HAND. The Vertical differences of velocity may be calcu- 

 lated from equation (f), see page 24, which is affected fundamentally 

 by the values of temperature, salinity, and depth, at any two verticals 

 in a plane and which it is important to note lies at right angles 

 across the path of flow. 



GENERAL SUGGESTIONS FOR A PROGRAM OF HYDROGRAPHICAL 



SURVEY 



Ocean currents, it has been pointed out, may be determined by a 

 distribution of temperature and salinity in a plane, the position of 

 which is perpendicular to the flow of the water. Conversely, if no 

 forces are found as represented by the position of the isosteres and 

 isobars in vertical section, then there is no current at right angles 

 to the plane of the section. It is easy to see, on the other hand, that 

 a section parallel with the course of a current contains no informa- 

 tion whatsoever regarding its movement. Hydrographical survey of 

 extensive ocean surfaces involves in any event a large program of 

 time and expense, and the task grows to considerable magnitude, 

 especially when the work devolves upon the efforts of one vessel. 

 An ideal program, of course, includes a maximum number of oceano- 

 graphic stations distributed netlike over the area to be investigated, 

 and wherein the promulgation of the work most nearly approaches a 

 simultaneousness of observation. Unfortunately, the ideal survey 

 rarely occurs, and it is usual that resort is made to lines of stations 

 along a vessel's track. Under such conditions it is apparent that 

 before commencing the observational work the particular area should 

 be studied carefully with respect to all previous, available knowledge 

 of a hydrographical nature, remembering that the lines of stations 

 in a program of dynamic investigation should run in such a manner 

 that the sections secured approach most nearly to right angles across 

 known or suspected currents. As an example let us take the region 

 around the tail of the Grand Banks where there are two main move- 

 ments. (1) The Labrador Current is the inshore set, which flows 

 southward along the east side of the Grand Banks and to a variable 

 distance around the "Tail." (2) Offshore in the Atlantic basin the 

 easterly moving masses of the Gulf Stream, guided by the trend of 

 the bottom configuration, progress in a generally opposite direction 

 to the cold water inshore. A program of dynamic investigation in 

 this region should be based upon a series of lines of stations rimning 

 offshore in a direction normal to the Grand Banks' slopes as shown by 

 Figure 18. Stations should be taken as close together (and repeated 

 as often) as practicable in order that the influence belonging to tem- 



