21 
that the surface of the earth was veering to the left in the Northern 
Hemisphere. It is more natural to regard the inverse perspective— 
that is, the earth and resting bodies as stationary—then the paths 
of inertia are apparently being continuously deflected to the right. 
Earth rotation exerts no effect on a water mass free from circulation 
relatively to the earth, but on the other hand no true conception of 
free-moying currents can be had unless this great. influence is con- 
sidered. In this connection it should be realized, from the foregoing 
remarks on motion on a rotating sphere, that currents can not be 
traced solely to a provocative force at their source, but they are 
only to be observed as a resultant of a force, the effect of which is 
constantly being deformed by the earth “‘sliding” beneath it. If 
a water particle moves solely due to inertia, without being acted 
upon by any force, it will follow a course “‘cum sole” (clockwise with 
the sun). As the latitude increases the tendency which drives a 
water particle to the right of its course becomes more and more 
intensified, and the faster it moves, the greater becomes the quasi 
force tending to deflect it. © 
In order to study this quasi force in detail, it is convenient, similar 
to the procedure employed in the investigation of varying mass and 
pressure (see fig. 4, p. 12) to regard the circulation of the curve in a 
plane between any two verticals. We may take, for example, stations 
A and B (fig. 6, p. 22), with their verticals AC and BD forming the 
plane ABDC. The development of an equation for expressing the rota- 
tion effect demands too great a digression into mathematics and is not 
warranted here, but it has been evolved by V. Bjerknes as equal to 
ds 
Dene 
a) a 
where w represents the angular velocity of the earth, viz, 0.0000729; 
and is the projection of the closed curve of the circulation, as illus- 
trated here by the rectangle ABDC, on the equatorial plane; and 2 
represents the rate of change of the projection on the plane of the 
equator. 
In Figure 6, page 22, if the curve of circulation ABDC, which is 
being investigated, is projeted upon the equatorial plane, it is evident 
that a change of the proportional area is effected only by components 
normal to the plane and not by those tangential to it. Also the 
vertical movements can be considered negligible, since they are in- 
significant as compared with horizontal magnitudes. Helland- 
Hansen and Sandstrom have, by this means, found the value for 
Bjerknes’ equation in terms of the projection on the plane of the sea 
surface 
dsaudowe 
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