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. 
