40 EXPLORATION OF THE COUNTRY BETWEEN 
The observations at Fort Pierre were made mainly by Mr. J. Hudson ~ j 
Snowden, being relieved from 12 o’clock at rosy till 6 in the morning 
by Mr. Paul Carrey and myself. Our short stay, and numerous other 
and fatiguing duties during this eaen season, prevented the observa- 
tions being extended over a longer per 
e psychrometer was made by tes Green, of New York, and is 
the same as that adopted by the Smithsonian Institution. The ob- 
servations made with it at Fort Pierre are in every way reliable, as the 
circumstances were favorable to eecuins the proper air temperature, 
the air having free circulation, and there being no sun’s rays or re- 
flecting bodies to influence the ott Sane the wet bulb was kept 
constantly moist, by being encased with a thin linen rag dipping into 
a vial of water, and care was taken to keep the moisture in the proper, 
quantity over the 
The barometers recat were James Green’s syphons, Nos. 1013 and 
1014. At the time of comparison his standard read 30.107, and the 
height of the barometric column given by syphon No. 1013 was 30.106, 
and by syphon No. 1014 was 30. 116. These instruments remained in 
perfect condition till they were accidentally broken—No. 1013 near 
Fort Pierre, and No. 1014 near Fort Laramie 
Table I gives the observations, altitudes, and distances-on the route 
from Fort Pierre to Fort Kearny; and table II from Fort Kea pane to 
Fort Laramie. i table IIT the temperature of evaporation i 
as observed with the wet bulb thermometer, and the diffevente st this 
and the dry bulb thermometer similarly exposed, both Fahrenheit. 
From the readings of the wet bulb thermometer and these differences 
the elastic force of eather aqueous vapor is deduced, and the per 
- centage of the saturation existing at the time, both by aid of Regnault’s 
tables. The elastic force of vapor, being the share of the barometric 
hekgh made up by the vapor cf the atmo osphere, ranges here from .15. 
0 .80 ofaninch. In tables IV and V, the mean of ‘this elastic force 
fas the several hours of successive days, i is obtained. The last of these — 
- shows a small curve of force through the day, attaining - maximum 
at 2p. m., and falling to its minimum at the colder hour 
~The relative humidit , or per centage of saturation, is oe, several 
se falling below thirty per cent., and only once or twice rising to 
satu 
