8 £. Loomis—Reduction of Barometric Observations. 
The average of the differences shown in this table (paying — 
3 18 O° ; 
attention to the algebraic signs) for 7.35 A. M. 0°-54, for 
4.35 P. M. it is +2°°88 and for 11 P.M. it is +0°-05. In six 
cases we find differences as great as five degrees. These large 
‘differences occur only at 4.35 P. M. and appear to depend 
mainly upon other circumstances than the height of the barom- — 
eter. Theaverage of all the differences when the barometer on 
Mt. Washington was considerably below the mean is +0°:84; 
and the average of all the differences when the barometer was 
considerably above the mean is +1°-22, showing a difference 4 
of only 0°-38 depending upon the height of the barometer on 
Mt. Washington, and this is only five per cent of the difference 
shown in table III for the warmest season of the year. We 
hence conclude that the error arising from assuming that the 
mean temperature of the air column is equal to the half sum of 
the temperatures at the upper and lower stations, is quite inap- 
_ preciable if the observations embrace a considerable period of | 
time and are made at all hours of the day. The observations — 
made on Mt. Washington in May, 1873, and those made in May, 
¥2 4 
1872, and published in the Report of the Chief Signal Officer _ 
for 1872 lead to similar conclusions. 
column. The question then remains unanswered, what is the 
cause of these discrepancies? In my fifteenth paper, on page 6 
I have given a table of forty cases in which the observed reduc- 
number of observations. They represent the cases in which 
the cause or causes which gave rise to the anomalies shown in 
table IIT acted with their greatest energy, and they are there- 4 
_ fore well adapted to indicate what this cause was, ‘These cases 
are enumerated in table V, and I haye endeavored to free them 
from certain errors which may have affected the results as pub- 
lished in my fifteenth paper. In each case I have deduced the 
ressure and temperature at sea level from the observations at 
urlington and Portland by the formula $(2B+3P), and the a 
results are given in columns three and four. The four suc-  — 
_ ceeding columns are obtained in the manner explained in my 
fifteenth paper, and the discrepancies. are shown in the ninth 
_ column under the heading O—C. It will be perceived that these 
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