67 
1918-19.] The Origin of Anticyclones and Depressions. 
is deflected further from an “ undisturbed path ” owing to its mass being 
less and its acceleration therefore greater. Thus it follows a path PR, 
which brings it nearer to the low pressure. Similarly, M is subject to a less 
acceleration on account of its greater mass, and its path PS must bring it 
nearer to the high region. (In applying this principle to the winds it should 
be constantly borne in mind that the “ undisturbed path ” for a body held 
by gravitation to a horizontal surface on a rotating spheroid is not a great 
circle but a curve that deviates continually to the right in one hemisphere 
and to the left in the other.) Thus the effect of inertia in a moving fluid 
of varying density is to cause the denser portions to accumulate in the 
regions of higher pressure, and the more rarefied portions to accumulate in 
the regions of lower pressure. This is the principle of action of the centri- 
fugal separators used in modern dairies. It also accounts for the relatively 
abrupt wind changes behind the southern portion of the trough line of a 
typical depression. In this region (see fig. 4) the circulatory motion is 
tending to bring the cold dense air of the N.E. current towards the low- 
