116 
DETERMINATION OF ALTITUDES BY BAROMETER. 
the head of Des Chutes valley, Oregon Territory, at an elevation of ah > t four thousand feet 
above the level of the sea, and a mean temperature of 50° Fahr. We were again fortunate in 
having little abnormal error, and the table was admirably adapted to the observations taken on 
the elevated plateau east of the Cascade range. 
Table No. 3 was deduced from three days’ observations at Fort Reading, taken about the 
middle of November, with a mean temperature of about 50° Fahr. Here a barometric storm 
rendered it necessary to reject four days’ careful observations, to avoid the effect of abnormal 
error. During the three days, however, there was very little atmospheric disturbance ; and the 
table was useful for the observations taken on my route from Vancouver, where the temperature 
was uniformly low and the elevation generally inconsiderable. 
It is very interesting to compare these tables, especially the two for Fort Reading. As the 
Des Chutes curve was obtained from observations at a place differing widely from the others in 
altitude, mean temperature, and latitude, it is impossible to decide what part of its peculiarities 
is due to each of these causes of variation. The Fort Reading curves, however, are deduced 
from observations taken at the same spot, with a change of mean temperature only; and it is 
fair to suppose that the differences between them are due principally to this cause. These 
differences are, that the November curve has a more rounded form and departs less from the 
horizontal line representing the mean of the day, and that its points of maximum and minimum 
are nearer together by about three hours. These results are precisely what we should expect, 
from the great difference of 33° Fahr. in mean temperature, assuming the heat of the sun to he 
the cause of the horary oscillation ; and they show conclusively that the horary variation is by 
no means constant, even at the same locality, for all seasons of the year. For some hours, the 
difference between the corrections in these two tables would affect the computed height of a 
station more than forty feet. This clearly proves the great error, which has sometimes been 
committed, of applying a table of horary corrections to observations taken at places of very 
different mean temperatures, and far distant from the spot for which it was computed. 
4. For abnormal oscillation .—The abnormal oscillations of the barometric column are princi¬ 
pally caused by general movements in the atmosphere, which are shown by repeated and 
numerous observations to extend rapidly and progressively over very large tracts of country. 
In the first volume of the third edition of Professor J. F. Daniell’s Treatise on Meteorology, 
will he found a very interesting article upon these oscillations. It is illustrated by diagrams, 
constructed as explained above, which represent numerous barometric observations taken under 
the direction of the Meteorological Society of the Palatinate. For Europe, at least, they con¬ 
clusively prove this most important principle of rapid and wide extension. In the third volume 
of this series of reports upon Pacific Railroad Explorations, will he found diagrams, prepared 
by Captain A. W. Whipple, United States Topographical Engineers, to represent barometric 
observations taken at different military posts by the Medical Department of the United States 
army. They show that the same principle is true for our western country. Slight local storms 
do not appear to produce much effect upon the height of the barometric column. The observa¬ 
tions taken on our survey agree perfectly with these well known facts. One of our barometers, 
No. 1068, was left at Fort Reading during our field work, with Dr. J. F. Hammond, United 
States army, who kindly volunteered to have observations taken daily every third hour, from 
6 a. m. to 9 p. m. I obtained a corresponding set of observations-taken at San Diego, distant 
over six hundred miles, and one month’s observations at Benicia, distant about two hundred 
miles from Fort Reading. Both sets were taken under the direction of medical officers of the 
