METEOROLOGY OF THE FIELD EXPLORED. 
133 
found. From the crossing of Clark’s fork to the Coeur d’Alene prairie, a distance of sixty miles, 
the route was through a wooded country, and the snow was two and a half feet deep and very 
hard. On leaving the forest for Coeur d’Alene prairie, the snow disappeared, the grass was good, 
and no difficulty whatever was experienced in reaching Wallah-Wallah. I will call attention to 
the influence of the forests in preserving the depth of snow, and to its entire disappearance, from 
being two and a half feet deep, immediately on reaching the prairies, and this on the 23d of Feb¬ 
ruary. On this same route, in January the snow was, in the woods, not over one and a half foot 
deep, and there was little or no snow on the prairies. A track opened for a wagon or a railroad, 
would not have been encumbered at any point with over a foot of snow the entire winter. 
In this connexion it will be well to advert to the large quantities of horses and cattle at Fort 
Benton, in the St. Mary’s valley, and in the several prairies on Clark’s fork, which are alluded to 
by Lieutenants Grover and Mullan as being fat in the middle of winter. 
The average temperature found by Lieut. Grover from January 2d to January 11th, before 
leaving the plains to ascend to the dividing ridge, (he reached the ridge at noon January 12th,) 
ten days, was 20°.9 ; and whilst going through the pass to the Bitter Root valley, from January 11th 
to January 21st, eleven days, was 10°.4 below zero. From January 31st to March 2d, on his 
journey to Wallah-Wall ah, the average temperature was 33°.3. 
It must be remarked, however, that Lieut. Grover crossed the divide and was in the pass du¬ 
ring the coldest weather of the winter ; that on the 22d day of January, after entering the St. 
Mary’s valley, the thermometer at sunrise was only 1° above zero, while at about the same 
point on the 31st of January it was, at sunrise, 55° above zero, and at Cantonment Stevens it 
ranged, from January 27th to January 31st, from 29° to 46°. 
That Lieut. Grover crossed the divide at the coldest season is confirmed by corresponding ob¬ 
servations at many other points. The period of greatest cold marched steadily eastward last 
winter, it requiring some four or five days to reach the Atlantic from the head of the Mississippi, 
and six days from Fort Benton. 
The following tables of comparisons, both for the cold period of eleven days in the pass and 
for the comparatively mild period of ten days approaching the-pass, show that the temperature 
of the pass was 6°.6 milder than that of Pembina, but T 9 S ° colder than that of Lacquiparle, and 
some 7°.4 colder than that of Oldtown, Maine ; and that the temperature of the plateau reaching 
from Fort Benton to the pass was 2°.6 milder than the corresponding mild period of the same 
stations, stretching from the Red river of the North to Nova Scotia. In the appendix will be 
found a temperature chart illustrative of these facts. 
Comparison of eleven days crossing the Rocky mountains with the corresponding temperatured 
periods in a line towards the Atlantic : 
Crossing the Rocky mountains.12th to 22d 
Pembina, Red River valley. ........ 
Fort Ripley.14th to 24th 
Lacquiparle, Minnesota.14th to 24th 
Fort Snelling...14th to 24th 
Madison, Wisconsin. 16th to 25th 
Fort Ridgeley.14lh to 24th 
Pittsburg.......14th to 24th 
Rochester... 16th 
West Point.... 
Amherst, Massachusetts...17th to 26th 
Oldtown, Maine. 18th to 28th 
Albion Mines, Nova Scotia.18th to 29th 
Montreal. 18th to 29th 
St. Johnsburg, Vermont...18th to 29th 
—10°.l 
66 
—17°.7 
66 
— 14° 
66 
— 9°.2 
66 
— 9°.3 
66 
+ 4°.4 
66 
— 7°.3 
66 
+ 23° 
66 
+ 21°.8 
66 
+25°.8 
+20°.7 
66 
— 2°.7 
U 
+ 7° 
66 
— 1° 
66 
+ l°.l 
