Observations of the United States Signal Service. 9 



ness, the heating effect of the sun's rays was greatly increased, 

 and this cause combined with the prevalent southerly winds 

 will |>n>l>al>lv explain the extreme heat observed in that region. 

 The heat experienced in the neighborhood of the Great Lakes 

 may be ascribed partly to the same causes operating in that 

 locality, and partly to a mass of heated air drifted eastward 

 from the dryer regions of the West. 



The remarkable heat recorded at Fort Sully and mentioned 

 on page 2, seems to indicate some peculiarity of that locality. 

 In order to discover the nature of this peculiarity I have col- 

 lected all the eases in which a temperature of 100° and up- 

 ward is mentioned in the annual Report* of the Signal Service 

 lor 1*78. 4, and 5. The toll,, wing table shows the number of 

 such cases for each year. For the year 1875 the observations 

 include only the first six months of the year. The mark (*) 

 shows that "no observations of self -registering thermometers 

 were received from the stations indicated. 



Mm 



nr Oj 



«« 



yfah 



mperature of 100° Fahr. 







1873. 



1874. 



1873. 





1873. 



1874. 





Port Sully. .... 

 Omaha. 



2 o 



^ 



18 



Dodge City 



Indianola '."."." 

 Yankton ...... 



* 



\ 





( ,„- na . 

 Memphis 



These observations show that the thermometer seldom rises to 

 100 : in the vicinity of the Atlantic Ocean, the Gulf of Mexico, 

 or the Great Lakes. Cases of extreme heat are of most fre- 

 quent occurrence between the Mississippi River and the Rocky 

 Mountains. They are the effect of the dryness of the soil and 

 of the atmosphere in that region ; and they serve to intensify 

 the dryness from which they result. If we compare the high 

 Mn, mires observed it Fort Sulh with the obsei tious at 



■ stations, we shall perceive t\r. 

 exceptional The first column of the following table shows a 

 serie.- o! statioi - on the Missouri River; column second shows 

 the latitude of the stations : column third shows their elevation 

 (in feet) above the ocean; and column fifth shows the number 

 of cases in which the thermometer, in 1873, 4, and 5, was ob- 

 sen I to rise as high as 100°. 



