182 DISCUSSION OF THE ANNUAL FLUCTUATION 



1 day and by the Pacific for about 15 days, the later eflfect being necessarily the 

 greater, owing to the prevalence of westerly winds over the whole area under con- 

 sideration. In the interior, on the contrary, the epochs appear about 4 days earlier 

 than the average values. Our data are yet too scanty to allow of any precise 

 estimate respecting the effect of elevation on these epochs, but they appear to 

 occur earlier for greater elevation. 



The result arrived at respecting the shifting of the epochs in different longitudes 

 may also be stated as follows : The seasons occur 5 days earlier in the valley of 

 the Mississippi and the western plains than on the Atlantic seacoast, and 19 days 

 earlier than on the Pacific coast. 



We may arrive at a tolerably fair estimate of the annual mean temperature at 

 any place by observing for a few days the temperature about the two epochs when 

 the mean is reached, and still better by observing in addition about the epochs of 

 maximum and minimum. The least labor will be spent by observing only at 8 

 P. M. (8'' 05"' may still improve the result), an hour Avhich has the advantage 

 of convenience for the observer and which produces equally good results in all 

 months of the year, the values will probably keep within a half degree, during any 

 month, and within one-tenth of a degree, for the year, of the true value. 



If we now turn our attention to the annual range, we find it to vary between the 

 limits of 80°, nearly, at Port Kennedy (in approximate latitude 72°) and of 10°, 

 nearly, at San Francisco. The next smallest annual range is attained at Key West, 

 of about 15°, next follows San Diego with 19°, and lUoolook (approximate lati- 

 tude 64°) with 20|°. The smaller ranges are due almost entirely to the proximity 

 and equalizing effect of the sea. 



The magnitude of the annual range depends principally on the latitude and the 

 distance from the ocean, apparently less on the altitude of the station ; it is greater 

 in the higher latitudes and appears to reach its maximum value in the region about 

 the Great Bear and the Great Slave Lakes ; from the vicinity of Lake Athabasca 

 high values extend towards Lake Winnipeg and even within the northern boundary 

 of the United States. Our four Arctic stations in the average latitude of 77|° show 

 an average amplitude of 701°, at Peel Eiver in latitude 67° 32' the amplitude 

 probably exceeds^ 83°, Fort Simpson in latitude 62° 10' has an annual amplitude 

 probably greater than 75°, our stations Nos. 5 and 6 in the average latitude of 62° 

 have an amplitude of nearly 72|°, Norway House in latitude 53° 50' shows nearly 

 71°, while at Fort Stevenson, Dakota, in latitude 47° 36' the observed amplitude 

 is as high as 72°, and at Fort Pierre, Dakota, in latitude 44° 23' a range above 70° 

 is indicated; these last two stations exhibit a range of a truly arctic character. 



The rigor of a climate may be supposed measurable by two factors, viz. : the 

 mean annual temperature and its range, which latter is approximated by the value 



2 ^1 (provided B.^ B-^ . . . are small in comparison). The values of A in our table 

 fluctuate between the extreme limits of — 2°.2 at Van Eensselaer Harbor, and of 



" A still greater range of about 90° probably occurs at Fort Yukon, Alaska, in latitude 66° 34', 

 but our observations are too limited to give an exact value. 



