6 INSTRUCTIONS FOR METEOROLOGICAL OBSERVATIONS. 
EN ROUTE OBSERVATIONS. 
Observe all the instruments at every halt of the party. 
Observe the barometer and thermometer at every point of considerable elevation or depres- 
sion, and at every point of abrupt change of climate in passing a mountain range. 
Observe at some certain hour, whenever possible, and not at points, between hours. Take 
the temperature of large springs and streams en route, and of the earth, when practicable, at 
one foot below the service. Measure every fall of rain during a halt, whether partial or com- 
plete, as in starting or halting during rain. 
Enter all observations precisely as taken, with proper note of locality, circumstances, &c. 
Special parties from a camp to determine heights must arrange hours for simultaneous baro- 
metric observations, and compute heights by the compared observations. 
Measure the amounts of rain, at different heights, when it may be done in surveys. 
CAMP OBSERVATIONS, 
Observe all instruments in camp hourly, when the force is sufficient; bi-houwrly, at the even 
hours, with less force; three times at 7 a. m., 2 p. m., and 9 p. m., with the least camp force. 
Observe the barometer simultaneously with any party leaving camp, or known to be taking 
elevations near. 
Camp and field observations to be made in the blank-book forms. Refer to the directions on 
the monthly blanks. 
POST OBSERVATIONS. 
Observe all instruments at 7 a. m., 2 p. m., and 9 p.m. Ifa few more can be taken, observe 
at sunrise, 9a. m.,and6p.m. If sufficient force can be applied, give night observations at 
12 p.m., 2 a. m., and 4 a. m., and hourly observations during the day. 
Use the blank books for hourly observations, and the sheet forms for the less numbers and 
the monthly results. 
Observe extremes of temperature and of barometer whenever they occur differing materially 
from the regular observations. 
INSTRUMENTS. 
Barometers.—Use the open cistern or Alexander’s barometer for a standard; the syphon and 
Wurdemann’s for common field-work; and the aneroid for difficult surveys only, and where great 
accuracy is not required. Compare all the others used with the standard, before leaving a post 
and on the return to the same. 
Adjust the float or tangent point of the open-cistern barometer carefully before each observa- 
tion. 
Fill the tube perfectly on taking the instrument down, by screwing up the bottom of the 
sack, or by inclining the tube in the Alexander’s barometer. The instrument remains perfect 
when a small vacuum gives a clear concussion in throwing the mercury to the end of the tube. 
Wurdemann’s steel barometer should not be completely filled after an observation; leave a 
globule of mercury, as large as a rifle-ball, in the short tube, to permit the expansion of the 
forcing it through the joints of the tube in the changes of temperature. A slight shock of the 
column should be felt on shaking the tube. , 
Adjust the aneroid barometer to the mercurial before each separate use of the aneroid, (by 
the screw at the back.) This form of barometer cannot be used for variations greater than one 
inch, or for a time beyond one day, without re-adjustment. 
Suspend all barometers vertically, for observation, and read the verniers from the top or apex 
of the convex surface of the column of mercury. 
All barometers, except the aneroid, must be carried by a man, and cannot be packed safely. 
