88 
On Irrigation in Tasmania. 
the summer. That the country is capable of sustaining 
a very large population, even with its natural irregular 
supply of moisture, is undoubted; but it is scarcely less 
certain that, if artificial means are used to secure the 
application of the winter rains to supply the deficiencies 
of the summer, it will bear three times as great a 
population as it would without them. 
Respecting the second effect of irrigation, viz. its 
producing all the results of other manure, I have before 
mentioned the undeniable proofs of this seen in India, 
where excessively poor land, not worth cultivating 
without irrigation, is enabled by means of it to bear a 
constant succession of white crops without intermission 
for centuries, and without any other aid. But on this 
point little need be said, because most Englishmen have 
seen or heard enough of the results of irrigation even in 
the mother-country (without the presence of a powerful 
sun) to know that all the effects of manuring arc pro¬ 
duced by it. 
The third effect mentioned, viz. the preventing the 
summer frosts, is very remarkable, and also of the 
greatest importance. This subject of summer frosts,, 
indeed, seems well worthy of examination, for it ap¬ 
pears to be by no means thoroughly understood : but 
it seems well ascertained that they cannot take place 
excepting with the concurrence of these three circum¬ 
stances—a perfectly cloudless sky, the presence of higher 
ground in the neighbourhood of that which suffers from 
them, and the almost perfectly dry state of the soil. I 
was prepared to find this to be the case, by the means 
used to manufacture ice in India. To effect this it is not 
necessary that the air should be lower than from 40° to 
43° of Fahrenheit: but it cannot be produced if there is 
moisture in the neighbourhood of the water exposed to 
be frozen ; and, therefore, the practice is, to put the 
