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DETERMINATION OF ALTITUDES BY BAROMETER. 
INTERPOLATION, AND APPROXIMATE TEST OF ACCURACY IN THE OBSERVER. 
Before proceeding to discuss the determination of altitudes, I shall explain the mechanical 
method used in examining and studying observations taken at a fixed station. It is to repre¬ 
sent them, the temperature of the mercury having been reduced to 32° Fahr., by a curve, of 
which the abscissas denote the times of the observations, and the ordinates the corresponding 
height of the mercurial column. By this means any great error in observation can be readily 
detected by an abrupt change in the curve, and a very clear and comprehensive idea can be 
obtained of the relations of the different observations to each other. This also furnishes the 
best method of interpolating properly for intermediate readings. 
CORRECTIONS PREPARATORY TO COMPUTATION. 
1. For temperature of mercury. —In preparing observations for computation, the first step 
taken was to reduce the observed readings of the barometer to what they would have been had 
the temperature of the mercury been 32° Fahr. For this purpose the tables of Mr. A. Guyot, 
published by the Smithsonian Institute, were used. 
2. For instrumental errors. —The correction for instrumental errors was then applied, and, 
when more than one barometer had been observed, a mean of the readings thus corrected was 
taken, to eliminate, as far as possible, errors of observation. 
3 For horary oscillation. —The next step was to correct for the oscillations of the mercurial 
column, due to the ever varying weight of the atmosphere. Of these there are two kinds, the 
normal and abnormal. Although a monthly and a yearly normal oscillation, and also one 
depending on the amount of moisture in the atmosphere, are supposed to exist, still, we practi¬ 
cally know but one, called the horary variation. This is a kind of daily atmospheric tide, caused 
principally by the heat of the sun, but greatly affected by the altitude and latitude of the place, 
and, doubtless, by other circumstances. It is far from constant, even at the same locality, as 
will be clearly shown by the result of our observations made at Fort Beading, in July and 
November. 
Observations for the construction of a table of horary corrections should be taken hourly 
with very great care, and continued, if possible, for a long period of time; but this is not 
absolutely necessary. A good one may be constructed from observations taken even for a single 
day, when the mean temperature does not differ much from that of the season, and when there 
is little or no abnormal oscillation. The latter condition is generally fulfilled when the 
reading of the barometer, with the mercury reduced to 32° Fahr., is the same, or nearly the 
same, at the last observation, as it was at the same hour on the preceding day. Even when 
observations are taken for several days, the latter of these conditions must not be neglected ; 
that is, the last observation, with the temperature of the mercury reduced to 32° Fahr., should 
always be very nearly the same as that taken at the corresponding hour immediately preceding the 
first observation used. This is manifestly necessary, as an abnormal change affects the horary 
curve. For instance, if the mercury should, beside the horary change, uniformly descend for 
one entire day and ascend for the next to the same height as before, the descending portion of 
the horary curve on the first day will be lengthened, and the ascending shortened, and vice 
versa on the second day. In a mean curve for the two days, these errors will balance each other. 
To construct a table of horary corrections, the observations, after the temperature of the 
mercury has been reduced to 32° Fahr., should be represented by a curve, as already explained, 
and examined to detect any errors of observation and to reject any portion in which the effect 
