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Proceedings of the Royal Society of Edinburgh. [Sess. 
kinematic equilibrium towards which all atmospheric motions tend, and 
have tended either since the earth began to rotate as it does now, or the 
atmosphere was first formed, whichever of those events is the later in time. 
Any deviation from the equilibrium state is by infinitesimal steps during 
which readjustment to the equilibrium condition has been taking place 
automatically. Hence any finite difference from the equilibrium state can 
only occur in quite exceptional conditions. Consequently if there is an 
ascertained difference from the equilibrium condition it requires explanation 
just as the divergences from the uniformity contemplated by the First Law 
of Motion require explanation. 
An allowance for “ curvature of path ” is one of the differences of which 
account may have to be taken. Its importance depends upon the latitude. 
For* the half of the globe north of 30° N. and south of 30° S. it is generally 
negligible, but near the equator it becomes the paramount consideration in 
the question of the persistence of distribution. Thus rotary systems, small 
or large, are the only possible isobars for a synchronous chart of an 
equatorial region, if one were drawn. The long sweeps of “ parallel isobars ” 
with which we are concerned in this paper would be inadmissible there. 
Near the surface there is always a component of motion along the 
gradient from high pressure to low pressure. In this region the friction 
due to obstacles and to the viscosity of the air prevents the steady state 
being reached, and in consequence the centrifugal force due to the 
velocity of motion is not adequate to balance the pressure. 
This modification of the general principle in the case of surface air may 
be inferred from the fact that in all maps of the distribution of pressure 
and wind at the surface there is evidence of a flow across the isobars. 
The maps are not always conclusive, as they are for sea level and not 
station level ; but no person of experience will doubt the general truth of 
the statement, which in books often takes the form of postulating con- 
vergence towards centres of low pressure and divergence from centres of 
high pressure. 
2. The Law of the Computation of Pressure and of the Application of 
the Gaseous Laws. 
The pressure at any point in the atmosphere and at any instant is 
the weight of the column of air which stands upon one unit of horizontal 
area containing the point. 
This principle assumes that the motion of the air is so slow that the 
hydrostatical forces are not interfered with. Explosion or elastic wave- 
motion would invalidate the law. It therefore assumes that the 
