18: Poor Rates
The "poor rate" is a tax levied upon the farmer with indifference to the ultimate source, but only to the productivity of the farmer, such that it is paid according to the ability of each farmer based on the level of his own production.
(EN: This is a little different from my understanding of the term "poor rate," which is more often used to indicate the share of all tax that is being collected for the express purpose of providing for the idle poor.)
Various consideration about how the funds to pay such taxes could be generated or apportioned, but this is a digression into topics the author has already considered (it makes it the functional equivalent of tax on land, tax on capital, tax on profits, etc.)
Theory aside, Ricardo asserts that the poor rates in his time placed the burden of tax on the farmer to a much more than on other parties - as the farmer was taxed on his production whereas others were taxed on the value of their assets.
(EN: The consequences of this, in theory, would be to discourage farming and encourage manufacturing - which is largely what occurred during the time the book was written: Britain became a manufacturing economy, importing raw goods from the Americas. This was profitable at the time, albeit exploitive of the former colonies, and also left Britain in a vulnerable position when America became industrialized and no longer needed Britain's manufacturing capabilities.)