38 Prof. E. Loomis on the action of Electrical Currents, 



The following table shows in how many cases the maximum 

 deviation of the magnetic needle occurred earlier at Cambridge, 

 Philadelphia or Washington than it did at Toronto ; in how 

 manj cases it occurred at the same instant ; and in how many 

 cases it occurred later than at Toronto. From May, 1840, to 

 Dec. 1841, the observations at each station were made at inter- 

 vals of 5 minutes ; but in 1842 the observations at Toronto and 

 Philadelphia were made at intervals of 6 minutes. The differ- 

 ences therefore in the times of maximum deviation are generally 

 0, or 5 or 10 minutes ; but when at one of the stations the indi- 

 cations of the needle were sensibly the same at two successive 

 observations, we may obtain differences of 2^ or 71- minutes. 

 In the few cases in which other differences are obtained, they 

 have been classed with the numbers in the table to which they 

 approached nearest. 



Maximum deviation of the magnetic needle. 



: Can.hridge. 





Washington, 



■I2imii,ute3 earlier, 



1 



11 





Ave^eresd. 



Sm-ll earlier. 



1-40 earlier. 



0'»15 later. 



These observations indicate that the maximum deviation of 

 the magnetic needle advances like a wave over the earth's sur- 

 face; and that the direction of its motion is from N. 68° E. to 

 S. 68° W. A comparison of the Cambridge and Toronto ob- 

 servations indicates a velocity of progress amounting to 113 

 miles per minute; while a comparison of the Philadelphia and 

 Toronto observations indicates a velocity of only 75 miles per 



The following table shows in how many cases the minimum 

 deviation of the magnetic needle occurred earlier than at Toronto; 

 in how many cases it occurred at the same instant ; and in how 

 many cases it occurred later than at Toronto. 



Minimum deviation of the magnetic needle. 







PhiIadelphi,L 



Wa>bin^n. 



15 nu«utes earlier, 



n 



I 



3 



A..r^r,«U. 



2«-29 earlier. 



1-10 earlier. 



O^O* later. 



