56 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Table XXXII. — Maximum temperatures observed at Fort Sully. 



We see that in eight of these cases the theruionieter rose to 100° aud upwards; in thirteen 

 of the cases the thermometer rose as high as 95°; aud in seventeen cases it rose as high as 90°. 

 Of the five cases in which the thermometer did not rise as high as 90° two occurred in March, 

 two in May, and one in September. In three of these cases there was a considerable fall of rain 

 at Fort Sully, and in the two remaining cases the temperature was above the average for that 

 season of the year. 



82. Besides the forty cases already examined Table XXXI contains twelve others, of which 

 nine occurred near the northeastern portion of the United States ; two occurred near the Gulf of 

 Mexico; and one near Lake Erie. In all of these cases the most noticeable feature was a nearly 

 stationary condition of the barometer throughout an extended region on the western side, reaching 

 as far as the Rocky Mountains, and sometimes to the Pacific Ocean. In each case there was a 

 second area of low pressure, at a distance generally of about 1,500 miles on the western side^ 

 which made very slow progress eastward ; and in each of the cases (except Nos. 47 and 50) there 

 was an area of high, or moderately high, pressure, generally situated between the two low areas, 

 which high area was also nearly stationary for one or more days, or moved slowly towards the 

 south without disturbing the low area on its eastern side. In the case of No. 14 this high area 

 made no considerable progress for four days, and in Nos. 16 and 17 the high area remained 

 nearly stationary for six days. These facts seem to indicate that the slow movement of these 

 twelve low areas was not principally due to any local cause; it was not wholly due (probably it 

 was not mainly due) to any thing occurring within the limits of the given low area; but a 

 like stationary condition extended to a distance of several thousand miles on the western side, 

 and must, therefore, have been the result of causes which had a very wide extent, perhaps com- 

 prehending a large portion of the northern hemisphere, and possibly portions of the southern 

 hemisphere. We also perceive that the causes which deternnned the slow movement of these 

 twelve low areas are in many respects similar to the causes which operated in the forty cases 

 which occurred in the northwestern portion of the United States. 



83. In order to study the movement of areas of low pressure under the greatest possible 

 variety of circuu)stances, I have endeavored to obtain information from European observations. 

 In the Uebersicht der Witterung for 1881, publi-shed by the Deutsche Seewarte, is given a table 

 showing the mean velocity of movement of the barometric minima for the five years 1876-'80, as 

 deduced from the monthly charts of storm tracks. The following table shows the average results 

 deduced from the observations of these five years : 



Table XXXIII. — Rate of progress of storm centers in Europe. 



January . 

 February 

 March . . . 



April 



May 



June .. .. 

 July 



Myriam.in 



twenty-four 



hours 



67.3 

 69.4 

 67.6 

 62.6 

 56.9 

 60.9 

 54.9 



August ... 

 September 

 October . . . 

 November 

 December. 



Year... 



Myriam. in j Miles 

 twenty-four per 

 hours. hour. 



I Miles 

 in U. S. 



54.1 

 66.7 

 73.2 

 72.0 

 69.3 



14.0 

 17.3 

 19.0 

 18.6 

 17.9 



64.6 



16.7 



22.6 

 24.7 

 27.6 

 29.9 

 33.4 



28.4 



Ratio. 



1.61 

 1.43 

 1.45 

 1.60 

 1.87 



1.70 



