H. A. Hazen—Projection of Lines of Equal Pressure. 361 
Art. XLVI.—On the Projection of Lines of Equal Pressure 
in the United States, west of the Mississippi River;* by 
Henry A. Hazen. 
over more than half the country. The late Chief Signal Officer 
in his annual reports for 1877, ’78 and 79, has stated in substance, 
that the reduction of pressures, at elevations west, of the Missis- 
sippi River is greatly to be desired. 
he’ solution of this problem is an exceedingly complicated 
one for the following reasons: 
Ist. The elevations of stations are not accurately known. In 
order to eliminate this error, the method of “isabnormals” or 
“departures” has been proposed. This system may be briefly 
described as follows : 
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lows,” the pressure would remain constant. The algebraic 
difference between the mean pressure for’a month or year‘and 
of describing a “ high” or “low.” 
This method is open to serious objections and in fact may 
give results directly contrary to the truth. If all stations 
Were at sea-level the results would be approximately accurate, 
but it will be seen that this would not be the case at elevations. 
‘or example, in winter the atmosphere condensed by the cold 
sinks and the pressure at a high station diminishes, whereas at 
the same time at sea-level the pressure increases. Since the tem- 
perature is constantly rising and falling it would be impossible 
to compare “departures” at different altitudes with each other or 
With those at sea-level. 
n Table I are found “departures” as computed from the 
actual observations and also from the same reduced to sea-level. 
* The term pressure is used to denote atmospheric pressure. Nearly every daily 
te in the country is publishing “Indications” in each i in which the ex 
should be uniformity. Why may not “pressure” and “ temperature” be t 
Am. Jour. Sor.—Tairp — Vou. XXI, No. 125.—May, 1881. 
