of Edinburgh, Session 187 4 - 7 5 . 621 
remains unchanged. Hence, in these circumstances, a barometer 
does not exhibit real vertical pressure. 
When horizontal movement takes off vertical pressure, this is 
accomplished in two ways, — first, by actual removal of air, and 
secondly, by lifting. Strictly speaking, however, no real horizontal 
movement takes place. 
Addendum. 
Isobarics . — As above stated, these do not indicate the existence 
of real pressure. There are three modes in which a barometric 
chart may be constructed. It may be made to exhibit real pressure, 
it may exhibit dynamical pressure, or it may assume the form of 
isorhoic curves, which will represent the correction of barometric 
pressure. 
1. Charts of Real Pressure. — These are the daily weather charts, 
and the curves are there drawn through figures of equal observed 
pressure, but they do not exhibit the effects of the introduction of 
the dynamical element, and hence do not represent real pressure as 
they are supposed to do. To enable them to do so graphically, the 
curves in front must be widened out and extended forwards to such 
a point that the amount of pressure which they there indicate will 
correspond with an equal amount of observed pressure in the same 
isobar in the rear. Let the observed pressure in the rear of the 
isobar in question amount to 30-00, and let it also be supposed lobe 
real, the front of it on the chart will be exhibited as 3CK)0. Let 
the dynamical lowering there, however, amount to say 0-20. An 
isobar, therefore, drawn through an observed and real pressure of 
30-00 in the rear, will in front require to be extended forwards and 
drawn through an observed pressure of 30*20, to make it exhibit 
one which is real and comparable with that in the rear. All its 
parts will now be graphically comparable. The comparative wide- 
ness of the isobars in front, with the corresponding diminution in 
the steepness of the gradients there, will thus represent a greater 
scarcity of supply there than is to be found on charts of the usual 
construction. 
A curve thus drawn, although its different portions are thus 
rendered graphically comparable, will not be one of isobarics, as it 
is not drawn through figures of equal observed pressure, nor will it 
