452 
can be said, in a general way, that the hydrodynamic 
stability increases or decreases depending on whether 
yi increases or decreases. It is the instability corre- 
sponding to the weak positive values and to the nega- 
tive values of vg which seems to be of the greatest 
practical importance [12]. 
Godson has not only calculated the inertial orbits 
in the four cases discussed above [11] but has also 
made a statistical and synoptic study of the four 
corresponding types of stability and instability [12]. 
The hydrodynamic instability which accompanies 
the deepening of frontal cyclones appears in the at- 
mosphere wherever the baroclinity is strongest and the 
static stability, simultaneously, is weakest (see pp. 
438-440), that is, in the tropical air above the polar 
front zone. It is particularly with reference to the 
importance of hydrodynamic instability in the forma- 
tion of cyclonic circulation that new synoptic studies, 
in greater number and on a larger scale, appear to be 
desirable. 
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