618 
Proceedings of the Royal Society 
mass of the column existing overhead. To ascertain this accu- 
rate^, observations would require to be taken by a series of baro- 
meters placed at different heights, and not very far apart. 
This tendency of air to accumulate aloft with the abnormal 
pressure which accompanies it, will be masked by the greater pro- 
portional removal of air w,hich takes place at an upper, as compared 
with a lower station. This is seen in the observations at Geneva 
and St Bernard. The relations betwixt the pressure which exists 
at an upper and a lower station will thus be altered in two ways ; 
first, by the tendency of the air to accumulate aloft, which lowers 
the surface barometer, while it tends to raise the upper barometer; 
and secondly, by the greater proportional removal of air which takes 
place aloft, depending on the height of the upper station. For 
this reason, the surface barometer, although it falls with strong 
winds, will not fall to the same extent as the upper barometer, 
where so much removal takes place. 
Isobarics . — Lifting takes place in front of an advancing depres- 
sion where supply is scarce ; the pressure there indicated is conse- 
quently less than it ought to be. In the rear, where supply is 
more abundant, and where lifting to the same extent does not take 
place, the barometer there will more nearly indicate real pressure* 
than it does in front. Hence an isobar in front is not comparable 
with the same isobar in the rear. An isobar therefore would 
require to be corrected all round, but in different degrees; when 
corrected, it- would extend further forward, and be more widened 
out in the advancing segment where progress takes place. Until 
such a correction is carried out, no uniformity of inflow, either in 
point of force or of direction can be expected from the present mode 
of construction of charts. Instead of isobars, this might be repre- 
sented by a line or curve of Isorhoics, drawn to represent lines 
of equal inflow. Such an Isorhoic Curve would neither coincide 
with lines of equal observed pressure, nor with lines of real 
pressure. 
The Weather Charts are, as at present constructed, drawn from 
*In the subsequent use of the term “real pressure,” the meaning to be 
conveyed is this. — The real amount of pressure due to the height of the 
atmospheric column overhead, but which may not be correctly indicated by 
the surface barometer. 
