Observations of the United States Signal Service. 7 



the thermometer should rise 20° above the mean temperature 

 of the hour and place. 



The accompanying Chart, Plate II, shows the isobaric curves 

 for the same date as the temperature curves of Plate I. We 

 perceive that throughout the entire United States, with the 

 exception of the Pacific coast and the vicinity of the Gulf of 

 Mexico, the pressure of the atmosphere was less than 30 inches; 

 and in Minnesota the pressure was 29-30. This barometric 

 minimum would lead us to look for southerly winds through- 

 out the valley of the Mississippi. The actual direction of the 

 winds observed is shown by arrows on Chart I. The following 

 table shows the number of stations at which the wind blew 

 from each of the eight principal points of the compass. 

 Direction of the winds, June 18th, 4 1 ' 45 ra P. m. 



w*a. 



<£&&. 



W ta i 



sSiol 



North 



Northeast 



| 



nM: 



1 



.• that at o4 s 



- the 



ind blew from some southern 

 did it blow from a northern 



t only 12 

 Quarter. The following a,e lu 



North wind at Cleveland, Mobile and Santa F« 



Northeast wind at Duluth and Yankton. 



Northwest wind at Eastport, Fort Garry, Fort Sully, Mt. 

 Washington, Norfolk, Oswego and Pembina. 



At three of these stations': viz: Cleveland, Duluth and Os- 

 wego, the northerly wind was probably the result of the cooler 

 air of the Lakes moving toward the wanner land. Three of 

 the stations, viz: Fort Sully, Fort Garry and Pembina, were 

 situated on the north side oi the area which was most heated; 

 and two other stations, viz: Eastport and Mt. Washington 

 were on the extreme eastern border of the heated area. At 

 velocity of the wind was only one mile per hour; at 

 Norfolk and Yankton it was four miles ; and at Santa Fe five 

 miles per hour. 



We thus see that throughout the entire heated area (with a 

 few trifling exceptions which are easily explained) the move- 

 ment of the atmosphere was from the south. This general 

 movement from the south had prevailed for more than - 24 hours 

 with an average velocity of about ten miles per hour, which 

 should cause a rise in the t lennometer of from 6 to 10 degrees 

 in the vicinity of the western Lakes: that is, this cause seems 

 to be sufficient to account for nearly (if not fully) half of the 

 rise actually c" 



