with the extent of the Spots on the Sun. 249 
errata, some of them of great importance. All of these ma- 
terials I have endeavored to combine in a single catalogue. 
In attempting to decide whether auroral displays exhibit a 
true periodicity, it is evident that some discrimination should be 
used in selecting our data for comparison. If for each year we 
a therefore I have decided to leave out of the account not 
on 
reports are tolerably complete and continuous. The geographi- 
cal line of division has not, however, been drawn in an irregu- 
lar manner for the purpose of including stations from which the 
observations would favor a pre-conceived theory, and excluding 
Stations from which the observations were unsatisfactory, but it 
is designed to be a line of equal auroral frequency, as determined 
m an article published in this Journal in July, 1860. I have 
chosen for my northern boundary the northern line of the 
State of Massachusetts, and through this boundary have traced 
a line of equal auroral frequency across the Atlantic ocean and 
the continent of Europe. (See Plate 1). 
boundary the meridian of 80 degrees of longitude west from 
Greenwich. The portion of the earth’s surface selected for 
Stockholm : north of Copenhagen ; follows ary the boundary 
between England and Scotland; passes south o : 
follows the northern boundary of Massachusetts ; and divides 
the State of New York by the parallel of 42° 45’. 
