45 



Various places, as a result of land and shoal formations, may be 

 under the influence of two tidal waves in varying phases. If these 

 latter are similar and both at a maximum, then follows a maximum 

 range of level and a minimum velocity of current. On the other 

 hand, where two tidal waves meet in opposite phases, a minimum 

 range of level results but a maximum strength of current is attained. 

 Tidal currents on soundings are usually determined by the aid of 

 current meters, which in the open sea are often illustrated by an 

 elliptical form of diagram when the current is purely tidal. The 

 water over shelves and near continental slopes is usually in progressive 

 movement as well as being under a tidal influence, and in those cases 

 the current diagram will be a resultant of the two different types of 

 movement. Another method of illustration of current observations 

 is that of a number of vectors radiating from a common center and 

 where the position and relative length of the successive vectors in- 

 dicates the direction and velocity respectively of the current reckoned 

 in moon hours. 



VARIATIONS IN ATMOSPHERIC PRESSURE 



Among the causes of currents ascribed to external forces, there was 

 included (see p. 1) that of the atmosphere as it pressed down upon a 

 sea surface unequally. Some very interesting observations have been 

 collected which deal with this subject where the bodies of water are 

 partially inclosed by basins and the currents thus produced are 

 forced. An example of such geographical qualifications is illustrated 

 by the Mediterranean Sea and its connection — the Strait of Gibral- 

 tar — ^with the Atlantic. Knudsen has found that atmospheric 

 pressure differences between the Baltic and the North Sea can be 

 traced in the acceleration (or retardation) of the current through 

 the Belts and Oresund. Since a difference in atmospheric pressure 

 of 1 centimeter of mercury is equal to about 13 centimeters of sea 

 water, it is not difficult to appreciate that the volume of a water 

 mass which has this dimension as a thickness and an area equal to 

 the Baltic will cause a considerable current when forced through 

 such an opening as Oresund. Apparent natural difficulties have 

 prevented the collection of scientific observations which will throw 

 light upon the degree of this influence in the open sea. Because of 

 (a) the absence of boundary surfaces against which the variations 

 in atmospheric pressure may react; (6) the compensating effect 

 which results from the progressive movement of such maxima and 

 minima areas over the sea's surface; and (c) a counteracting drift 

 current which tends to flow as a result of the accompanying system 

 of winds make it safe to state that in general the relative importance 

 of variations in atmospheric pressure causing currents in the open 

 sea is small indeed. This phenomenon can be totally disregarded 

 in hydrodynamic computations of the ocean. 



