METEOROLOGY OP THE FIELD EXPLORED. 133 



found. From the crossing of Clark s fork to the Cceur 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 Cceur 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 

 Bcnton, 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 llth, 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 llth 

 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 tempera lure 

 of the pass was 6.6 milder than that of Pembina, but T 9 C 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 Benlon 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 tp 22d January, 10.l 



Pembina, Red River valley 13th to 24th &quot; 17.7 



Fort Ripley 14th to 24th -14 



Lacquiparle, Minnesota , 14ih to 24th &quot; - 9.2 



Fort Snelling 14th to 24th 9.3 



Madison, Wisconsin , 16th to 25th &quot; + 4.4 



Fort Ridgeley 14th to 24th &quot; 7.3 



Pittsburg 14th to 24th +23 



Rochester 16th to 25th &quot; +21.8 



West Point ] 6th to 25th &quot; +25.8 



A mherst, Massachusetts 17th to 26th +20.7 



Oldtown, Maine 18th to 28th &quot; 2.7 



Albion Mines, Nova Scotia 18th to 29th &quot; + 7 



Montreal 18th to 29th - 1 



St. Johnsburg, Vermont 18th to 29th &quot; + l.l 



