> 
58 ABNORMAL MOVEMENTS OF THE BAROMETER. 
bia ; at Fort Gibson, on the Arkansas ; at Great Salt Lake City; at camps upon the Colorado - 
Chiquito; and, though in a less degree, at San Diego, California. Hence, it appeafs that the 
same tide of low atmospheric pressure covered at once a large portion of the western part of 
this continent. It would be of interest to compare other observations made at the same period, 
and determine whether, upon distant points of the globe, correspondingly high pressures were 
produced. The phenomena commenced at Fort Benton and St. Mary's one day earlier than at 
Great Salt Lake, and at other places further south. This fact is particularly noticeable in the 
secondary oscillation of the 11th and 12th. 
The meteorological record at Great Salt Lake conforms closely with the observations at Flax 
river. At both places the weather continued ‘clear and pleasant”” during the first rise and 
part of the gradual fall of the barometer. Clouds were then formed. On the 10th, light showers 
occurred. This disturbance of the atmosphere having restored the equilibrium of pressure, the 
mercury rose, although heavy masses of threatening clouds were still visible. On the 12th, 
the tide of pressure turned, and the mercury fell rapidly until the night of the 13th, when oc- 
curred a light snow-storm. From this period the barometer remained almost stationary for a 
day and a half, and then commenced a gradual and uniform rise, notwithstanding the contin- 
uance of a series of violent storms of wind, rain, sleet and snow that swept from the San Fran- 
cisco mountains. 
This comparison leads to the following opinions regarding the abnormal movements of 
the barometer :* 
st. They are of great magnitude, and, if not taken into account, may produce an error in 
the deduced altitude of several hundreds of feet, 
2d. They are but slightly affected by local storms. 
3d. They may occur almost simultaneously over the whole interior portion of the continent. 
4th. They are actually identical within certain areas of great extent. 
Our line, from the Mississippi valley to the Pacific, crosses three distinct barometric regions. 
The first covers the extensive prairies east of the Rocky mountains; the second lies within the 
great basin and the valley of Rio Colorado; and the last occurs upon the Pacific slope of the 
Sierra Nevada. These different countries may be equally affected by heat and vapor, and pro- 
duce similar changes of the barometric column. Usually, however, their waves are non-con- 
formable, except when they meet and mingle near the immediate ranges of mountains. 
The idea of applying to our barometric observaticns a correction, to be obtained by the plan 
alluded to above, was first entertained in the field; otherwise, it would have been necessary to 
have divided our party into two barometric divisions—one to be stationary, while the other was 
en route—a system which would have considerably retarded the operations. But upon our 
return to Washington, while hastily preparing the material for a profile, I was informed that 
no records of observations could be obtained suitable for a comparison. Hence, under the direc- 
tion of L. Blodget, esq., then meteorologist of the Smithsonian Institute, our notes were com- 
puted in the manner described by him in a preface to the tables. Afterward, when called upon 
to review the work, I found that, notwithstanding the careful manner in which the observations 
had been reduced, there might be errors, due to the abnormal atmospherie changes, remaining 
uncorrected, 
In the effort to determine these errors, and whether some practical method might be found 
for eliminating them, the idea of making a diagram of the observations was suggested. Those 
9 From observations made, for ten days together, by F. Laval, upon the top of St. Pilen, a mountain near Marseilles, 
which is 960 yards high, [see Russell's lectures, previously alluded to,] John Dalton remarks that **it may be inferred that 
the fluctuations of the barometer are occasioned chiefly by a yariation in the density of the lower regions of the air, and not 
by an alternate elevation and depression of the whole superincumbent atmosphere." This theory, if true, would diminish 
the value of the barometer as an instrument for determining altitudes. However, the barometric waves in the elevated 
regions of the Rocky mountains are not appreciably less than at Fort Gibson, with a difference in altitude of 5,000 feet. 
Hence, while the observations ar o coniinal to the mee near the general surface of the earth, it seems to be evident that 
the amount of the barometric oscil t of the height aboye the sea. 
