414 
Proceedings of the Royal Society 
which takes place when the atmosphere is at rest would be greatly 
altered when its upper portion is in rapid motion. 
Ketarded surface currents and rapid upper currents which move 
in inclined columns and produce these effects, can only be found 
with an imperfect fluid, and on a resisting surface, into which the 
element of friction enters. On a frictionless surface this could 
not take place. The atmosphere would there move in vertical, not 
in inclined columns, no “ lifting” would take place, pressure would 
be real or statical ; its upward diminution would be normal, as 
when it is at rest, and horizontal movement would not take off 
vertical pressure. 
Barometric pressure must hence be regarded in two points of 
view — 1st as being a cause ; and 2d, as being an effect. When, as 
in the first case, it is real or statical, it operates as a cause due to 
gravitation, which is unresisted. When, as in the second instance, 
there is an introduction of the dynamical element, surface gravita- 
tion is diminished by “ lifting,” and it must then to a certain 
extent be regarded as an effect. The practical conclusion from 
this is obvious. On weather charts the constant rise and fall of 
the barometer, which is there reported, is to a large extent simply 
due to the passage of air over a resisting surface. Over a surface 
devoid of friction these mechanical effects would be entirely 
removed : its rise and fall would be greatly reduced, and might 
be considered as being solely dependent on the effects of heat and 
vapour. The gradients and isobars which are represented by these 
movements of the barometer would consequently require also to be 
similarly corrected. 
The barometer does not indicate the real weight of the atmo- 
sphere, it only exhibits the amount of its elasticity, from which its 
real weight can only be deduced when the dynamical element of 
motion does not enter into its currents. The two cases above 
mentioned may illustrate this point. The surface barometer there 
indicates fictitious pressure, or in other words, the amount of 
pressure due to the elasticity of the air , hut not to that of its real 
weight , which is there diminished by “lifting,” and as lifting can 
increase or diminish in amount, so also can the elasticity of the air, 
while its real weight remains unaltered. 
As a general rule, in the British Isles, equatorial winds are 
I 
