PHYSIOLOGY AND DISEASES OF VEGETABLES. 117 
this period, but often remain thirty or forty days, go off 
and come again, the first race appearing to have lived their 
time. 
This successive attack is wholly dependent on the meteoric 
state of the atmosphere ; to that, and electric influence, we 
must ascribe the early or late attacks of these insects. 
It does sometimes occur that the aphis is not seen on the 
hop plants at all during the whole of the summer. 
The sun is the great invigorator of the insect tribe, and we 
shall find that by referring to this source, we may explain the 
effects of the aphides on the hop plants. 
In the year 1836, the hops in this part of Surrey and 
Hants, extending over a cultivation of three thousand acres, 
were nearly free from the attacks of this pest ; the weather 
was exceedingly cold and wet throughout March, April, and 
May, therefore was most ungenial to the existence of the 
aphis fly. 
The year 1835 was very hot in the spring, and indeed was 
so throughout the year; there was, therefore, much infection 
on the hops in the early period of their growth ; but these 
run their race early, they were all cleared away, and the duty 
came out at a high figure, the old duty being £235,207. 
The spring of the year 1834 was also hot, and there was 
much infection ; so much were they attacked, that the old 
duty was, in June and July, calculated at only £60,000 in 
consequence thereof; but the attacks of aphides were early, 
and occurred before the plants had thrown out their lateral 
shoots; hence they lived their day and passed away, and 
there was a productive return, the old duty having paid 
£189,713. 
It was to the attack of aphides we must attribute the 
failures in the growth of hops in the following years ; at any 
rate they were the chief cause in some years, and in others 
they w ere the sole cause of the failure. 
In 1793 the old duty 
was o 
nly £22,619 
1798 
» 
99 
56,032 
1802 
93 
99 
15,463 
1805 
j » 
>9 
32,904 
1812 
99 
99 
30,561 
1823 
39 
99 
26,057 
1825 
93 
99 
24,317 
1829 
99 
39 
39,866 
1840 
99 
99 
34,091 
1846 
it was equal to 
242,929 
1850 
99 
99 
223,393 
