619 
of Edinburgh, Session 1874-75. 
observed pressure, and they are also supposed to indicate real pres- 
sure. But since the Isobars in front are more under the influence 
of the dynamical element than those in the rear, real pressure is 
there represented as being lower than it ought to be. - 
If the difficulties attendant upon the construction of a chart of 
Isorhoics could be overcome, it would exhibit a practical standard 
of reference as to the real amount of inflow of air, which cannot be 
ascertained by the present system of Isobarics. The introduction of 
the dynamical element complicates the forms of the Isobaric curves 
to such an extent as often to render them absolutely uninterpre- 
table : this is done by creating barometric oscillations, and different 
modes of inflow in the various winds, which would not take place 
on a frictionless surface. 
Barometer , liow it represents Pressure . — It is only when the atmo- 
sphere is in a state of perfect rest that the surface barometer 
exhibits the real amount of pressure due to the column of air 
overhead, and it is only then that the normal diminution of 
pressure due to the diminished mass takes place in ascending up- 
wards. 
But when the atmosphere is in a state of motion and the upper 
currents move rapidly, the dynamical element then enters more 
largely into these, than into the slower moving surface currents. 
The consequence is, that the surface barometer will no longer 
indicate real pressure. 
Owing to the lowering of the gradient in front, this diminu- 
tion of surface pressure takes place, most in front of a moving de- 
pression, and least in the rear. It is due to lifting, hence the 
barometer, to a certain extent, represents dynamical or fictitious 
pressure. In the rear it more nearly indicates static or real pres- 
sure. No difference of real pressure, therefore, seems necessarily 
to exist here, setting aside, in the meantime, the effects of conden- 
sation, to which the reduction of pressure in the advancing seg- 
ment is usually attributed. Hence it is to the difference betwixt 
static and dynamic pressure that progress is due. 
When a gradient is lowered by friction, the accompanying 
lowered barometer is an effect, and in so far as it is so, it is in- 
capable of attracting air. The gradient thus lowered is caused to 
extend itself forwards, and its accompanying barometer will con- 
4 l 
vol. vm. 
