"Congress" - Singular or Plural?

Friday was tax-filing day, so I thought - what better time to reaffirm one's hatred for the income tax than the day when jackbooted IRS agents come knocking at your door, demanding the federal guvmint's cut of your hard-earned wages?

As I was sifting through various materials on the web and from my vast collection of right-wing propaganda, I came across the following passage from Federalist #30:

The present Confederation, feeble as it is, intended to repose in the United States an unlimited power of providing for the pecuniary wants of the Union. But proceeding upon an erroneous principle, it has been done in such a manner as entirely to have frustrated the intention. Congress, by the articles which compose that compact (as has already been stated), are authorized to ascertain and call for any sums of money necessary in their judgment to the service of the United States; and their requisitions, if conformable to the rule of apportionment, are in every constitutional sense obligatory upon the States.

You catch that? Congress...are authorized.

At first I thought...so maybe Alexander Hamilton didn't have a great command of the English language. I mean, he was a bastard child who grew up in the West Indies. But, I searched around a little more, and it seems that Hamilton wasn't alone in using "Congress" in the plural. From Article I Section 4 of the U.S. Constitution:

The Congress shall assemble at least once in every Year, and such Meeting shall be on the first Monday in December, unless they shall by Law appoint a different Day.

From Article I Section 7:

If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.

From Article II Section 2:

...but the Congress may by law vest the Appointment of such inferior Officers, as they think proper, in the President alone, in the Courts of Law, or in the Heads of Departments.

From Article II Section 3:

He shall from time to time give to the Congress Information of the State of the Union, and recommend to their Consideration such Measures as he shall judge necessary and expedient...

From the ninth anti-federalist article written under the pseudonym "Brutus,"

I shall not undertake to enquire whether or not Congress are vested with a power to keep up a standing army in time of peace...

The present Congress are restrained from an undue exercise of this power, from this consideration, they know [that] the state legislatures, through whose authority it must be carried into effect, would not comply with the requisition for the purpose, if it was evidently opposed to the public good: the proposed constitution authorizes the legislature to carry their determinations into execution, without the intervention of any other body between them and the people.

From the first anti-federalist paper written by "Centinel,"

By sect. 8, of the first article of the proposed plan of government, "the Congress are to have power to lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defence and general welfare of the United States, but all duties, imposts and excises, shall be uniform throughout the United States." ...

The plain construction of which is, that when the state legislatures drop out of sight, from the necessary operation this government, then Congress are to provide for the election and appointment of representatives and senators.

OK...you get the point.

So, here's a question for everyone to ask their English-major friends. Is "Congress" singular or plural? If it's supposed to be plural, an issue on which Federalists and Anti-Federalists seem to have been in agreement, then why do we use it in the singular?