Major Jackson, who had devised his own simple form of shorthand to keep up with the delegates, pledged to Washington that his request would be ''sacredly observed'' - a promise that he kept but that some other delegates did not.

Madison, a delegate from Virginia, observed and justified the secrecy rules. In a letter to James Monroe dated June 10, 1787, he wrote:

''I think the rule was a prudent one not only as it will effectually secure the requisite freedom of discussion, but it will save both the Convention and the Community from a thousand erroneous and perhaps mischievous reports. I feel notwithstanding a great mortification in the disappointment it obliges me to throw on the curiosity of my friends.'' Washington Family Letter

One of the heretofore unpublished documents by Washington is a letter he wrote on May 17, 1787, to his nephew, George Augustine Washington, who acted as an overseer on his Virginia estate while the general was in Philadelphia. At the start of the Convention, in words more casual than his usual cryptic diary notes, Washington wrote:

''Dear George ''After short stages and easy driving, I reached this city on Sunday afternoon. Only 4 states, viz. Virginia, South Carolina, New York and the state we are in are as yet represented, which is highly vexatious to those who are idly and expensively spending their time here.

''As we have not commenced the business yet, it is impossible to say when it will end. I have not even a hope that it will meet with dispatch.''

By Sept. 9, 1787, Washington still sounded ''vexatious.'' Another letter to his nephew - discovered in a library in Manchester, England - predicted that the delegates were expected to complete their ''business'' that week. The letter concluded: ''God grant I may not be disappointed in this expectation, as I am quite homesick.'' Slave Trade Deliberated

A document discloses that on June 2, 1787, the Pennsylvania Society for the Abolition of Slavery issued a strong statement to its president, Benjamin Franklin, to deliver to the Convention. It was intended to persuade the delegates to include the suppression of the African slave trade in their deliberations. Part of the resolution read:

''It is with deep distress that the members of the Society are forced to observe that the peace was scarcely concluded before the African Trade was revived and American Vessels employed in transporting the Inhabitants of Africa to cultivate as Slaves the soil of America before it had drank in all the blood which had been shed in her struggle for liberty.''