Ei. Loomis— United States Weather Maps. 11 
To determine whether a storm is increasing or diminishing in 
intensity. 
In order to determine by what indications it may be known 
whether a storm is increasing or diminishing in intensity, that 
is, whether the barometric pressure at the center is diminishing 
or increasing, I have made a comparison of ‘nearly all the con- 
comitant phenomena which could be supposed to have any 
influence upon the result, and find the following rulé to be 
well-nigh universal: when the barometer rises more rapidly than 
usual as the storm passes by, the pressure at the center of the 
storm is increasing; but when, in the rear of the storm, the 
barometer rises less rapidly than usual, the pressure at the center 
is diminishing, or the storm is increasing in intensity. A com- 
parison of all the observations of two years indicates that when 
the rise of the barometer is 22 per cent greater than usual, the 
pressure at the center of the storm increases one-tenth of an 
inch in 24 hours; and when the rise of the barometer is 22 per 
cent less than usual, the pressure at the center decreases one- 
tenth of an inch in 24 hours. e thus see that a sudden rise 
metimes one of these effects predominates, and sometimes 
the other; but generally, when the storm center is advancing 
with the greatest rapidity, the pressure of the center of the storm 
is decreasing, and when the storm’s center is nearly stationary, 
the pressure at the center remains nearly stationa 
The rate at which the barometer falls in front of a storm 
appears to have very little influence upon the question whether 
the pressure at the center is increasing or diminishing. 
The average of two years’ observations indicates that when the 
winds on the eastern side of the storm’s center are very muc 
stronger than those on the western side, the pressure at the center 
of the storm is increasing; but when the winds on the western 
side are very much stronger than those on the eastern side, the 
Shvetige at the center of the storm is decreasing. This rule is, - 
owever, subject to numerous exceptions. 
Form of the isobarie curves. 
Tn order to determine the average form of the isobaric curves, 
and the position of their longer axes, I selected all those cases 
in which the center of a storm was distinctly indicated upon 
one of the weather maps, and in which at least one isobaric 
curve was shown for not less than one-half of the circuit. The 
number of cases found suitable for this kind of comparison 
was 203. Of these, nearly one-half were so situated that only 
half of one of the axes could be measured. These were gener- 
