388 
On the Potato Disease. 
it will not be completed in six weeks. I believe it has been much 
more rapid in many parts this year than last. 
The same disease is said to have visited St. Helena in 1840, 
and Ireland as well as America in former years. I am also quite 
certain that I had it amongst my crop in 1841, and then it was 
confined to a spot where pits had been filled in. To account, 
however, for its breaking out so universally as it has these last 
two years, is a task which I cannot undertake, believing, as I do, 
that it may arise from transitions of various kinds and degrees, 
which conviction I have come to, not only from the effects of my 
own experiments, or the bare fact that the present season differed 
in many respects from the last, but also from the testimony of old 
residents and writers of acknowledged credit in various parts of the 
world. From these I learn that, in parts of America and South 
Australia, where extreme heat and drought are suddenly suc- 
ceeded by drenching rains, the potato cannot be successfully cul- 
tivated ; and in others, for the same reason, it is never attempted ; 
while in Western Australia, which is not so subject to violent 
changes, the potato does not degenerate. I have no doubt, 
therefore, although I cannot personally account for the peculiar 
effects of which the potato is susceptible in foreign countries, that, 
if the disease can be satisfactorily accounted for in a single sar- 
den, it will be found upon due inquiry that the same general 
cause has existed with more or less severity wherever the disease 
has appeared — I mean a sudden and extreme change in the state 
of the weather. When had we such a summer as that of 1845 ? 
— such extreme heat succeeded by long-continued gloom and 
chilling rains, with a temjierature almost reduced at times to the 
freezing point? Again, in 1846, January passed without a single 
frost ; intense heat prevailed during June and July, on the lOth 
of which last month my thermometer reached 100' in the shade 
of a tree at half-past three p.m., and 83° on another day while 
buried si.x inches deep in the earth in one of my potato-beds, being 
equal to the bottom heat required for the pine-apple ! 
On the 5th of July it was hotter in the shade, I believe, than 
any day on record in this country ; and throughout June and part 
of July the thermometer only ranged Irom 4^ to 8° above the 
average for the last twenty years, according to the register kept 
at the Horticultural Garden ; but we know not how mucli lonpfer. 
I have also been assured by an old resident at Rio Janeiro that it 
has been quite as hot in Yorkshire this year as at Rio, where the 
])Otato is not grown, but imported by the packets. During the 
intense heat the potato-plants flagged in many places, and on the 
5th of July, with the thermometer at *.)5° in the shade, torrents of 
rain descended, accompanied by an unusual storm of thunder and 
lightning ; in the morning succeeding which, so dense an exlialu- 
