50 
The year 1872 apparently forms an exception, but in this case I 
have the record of but a single station, and this of doubtful au¬ 
thority. 
In the records of other western sections which are at hand, this 
year is several inches above the general average. If we commence 
with 1876, w 7 hich is considerably above the average, and count back 
(toward the left) by sevens, we shall find, as before, that each 
seventh year is also above the average, to-wit: 1869, 1862, 1855 and 
1848; 1841 being on the average line. 
If we commence with a year below the average, we find the same 
rule holding true in most cases; thus: commencing with 1842 and 
counting in the same way to the right, we find 1849, 1856, 1813 
and 1870 all below the average line; 1877 is slightly above it. 
Commencing with 1840, w r e find 1847, 1854, 1861 and 1875 agreeing 
w T itli the rule; 1868 forming an exception. 
The evidence of a septenary cycle shown here is too strong to be- 
ignored. 
Secondly .—A careful examination reveals the further fact that this 
septenary period is divided into two sub-periods of four and three 
years. 
This will be apparent if we commence with 1844, w T hich is above 
the average, and count forward (to the right), the fourth and third 
years, alternately, being above the average line as a general rule; 
thus: 1848 and 1851, 1855 and 1858, 1862 and 1865, 1869 and 1872, 
and 1876. According to this rule 1878 should be below the average, 
1879 above it, and 1880 and 1881 dry. 
Thirdly .—The relation of the rain-fall to the appearance of the 
Chinch-bugs. As I have argued elsewhere, and as I believe is gen¬ 
erally admitted, two successive dry years are necessary to the de¬ 
velopment of these insects in injurious numbers. According to this 
theory, as applied to our graphic delineation, the only years which 
could have been serious Chinch-bug years are the following: 1854, 
1857, 1860 or 1861, 1871 and 1874 or 1875. 
The chronological history of the species in the region designated, 
shows that although appearing in the intermediate years in limited 
localities in considerable numbers, the chief Chinch-bug years were 
1850, 1854, 1871 and 1874. 
As this does not sustain the theory with sufficient uniformity to 
establish the rule, let us see if w T e can find another factor which it 
is necessary to consider in arriving at a correct conclusion. 
For this purpose, I now call attention to the line showing the 
annual variation in the temperature. The line showing the general 
average of the rainfall also stands for that of the temperature. Fun¬ 
ning the eye along the curve of temperature, we find that 1850 was 
below the average, 1854 above it, 1857 considerably below it, 
1860 above it, 1864 below it, 1871 and 1874 considerably above it. 
According to this, it appears that the increase of Chinch-bugs de¬ 
pends on the combined influence of a decreased rainfall and high 
temperature. The year 1850 forms an exception, which might pos¬ 
sibly be explained if w 7 e had the monthly or quarterly means of 
temperature and amounts of rainfall, as the low temperature 
and greater amounts of rainfall may have been in the latter 
part of the year. The years 1860 and 1861 were favorable for their 
