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Third Sort.-The third and last sort of rude produce, of which the

price naturally rises in the progress of improvement, is that in

which the efficacy of human industry, in augmenting the quantity,

is either limited or uncertain. Though the real price of this

sort of rude produce, therefore, naturally tends to rise in the

progress of improvement, yet, according as different accidents

happen to render the efforts of human industry more or less

successful in augmenting the quantity, it may happen sometimes

even to fall, sometimes to continue the same, in very different

periods of improvement, and sometimes to rise more or less in the

same period.