Proportion to the revenue?  This is most likely a flat consumption tax.  Income taxes did not exist in 1776.

Adam Smith also outlined four canons of taxation, which serve as guiding principles for a fair and effective taxation system:

The Tax Should be Certain: Tax laws and rates should be clear and stable.
The Tax Should be Convenient: Taxation should be easy and convenient for taxpayers.
The Tax Should be Easy to Assess: Taxation should be straightforward and simple to calculate.
The Tax Should be Light: Taxation should be minimal and not excessively burdensome.


Direct quote in fact - 

“The subjects of every state ought to contribute towards the support of the government, as nearly as possible, in proportion to their respective abilities; that is, in proportion to the revenue which they respectively enjoy under the protection of the state.”

Adam Smith