22 



METEOROLOGY. 



estimate from the amount of moisture in winter as we have clone there, especially since we find 

 that at Fort Benton, with a winter 5° colder than on the Great Plain, the proportion of snow 

 to moisture was only 18.5 to 5 inches, while at Fort Snelling we allow 50 to 5 inches. 



The register of rain and snow at Fort Benton for one j'ear, although imperfect, still shows 

 the less moisture is there deposited than at Fort Laramie ; and although the amount is much 

 greater west of the Rocky mountain divide, as shown by the comparison of the number of days 

 in which rain or snow fell at Cantonment Stevens, yet none of the parties who traversed that 

 part of the route in any winter month found snow enough to interfere with a railroad. 



OBSERVATIONS ON THE ISOTHERMAL CHART. 



The isothermal lines east of the Mississippi are copied from Blodgett's Climatology of the 

 United States, published in 1857. These do not differ much at the sea level from those deter- 

 mined by the Smithsonian, and published in the Patent Office Agricultural Report for 1856, but 

 are curved on the land to conform generally with the undulations of the surface, their correct- 

 i^ess being tested by numerous and long continued observations at fixed points, by which the 

 local influences, independent of latitude and altitude, may be considered pretty well established. 



The direction of these lines west of the Mississippi and north of latitude 50° have been deter- 

 mined partly from the same and partly from data additional to those used by Mr. Blodgett, and 

 they are found to vary so much from his conclusions that a statement and discussion of them is 

 rendered necessary. 



1. Red River of the North. — Mr. Blodgett gives in his table of statistics only seven months' 

 observations at Pembina, latitude 49°, and at such irregular intervals that only part of them 

 can be used for computing the means for spring and summer. A year's observations at Fort 

 Garry, latitude 50' 15', near the mouth of the river, are used in this computation, showing a 

 difference of about four degrees for the two places at corresponding seasons, which, being about 

 that due to latitude in this region, makes the mean for winter reliable as the true winter climate 

 of the place. The following table shows the comparative results thus obtained : 



The lower altitude would account for the mean annual difference being in favor of Fort Garry 

 over Fort Ripley, even supposing that the summer temperture is not too great, which it may 

 very possibly be from local circumstances. 



The table for Fort Garry is copied from the "report of the exploration of the country 

 between Lake Superior and the Red River settlement," Toronto, 1858. 



The next points in the lines in which alteration was found necessary are those where the 

 isothermo for winter (isochimenes) strike the Missouri between the mouth of the Platte and 

 Fort Union. The materials at Mr. Blodgett's command were very imperfect for this portion. 



