In 1803, Mr. Jefferson invited the astronomer to visit him at Monticello, but the increasing infirmities of age made it imprudent to undertake the journey. His Almanacs sold well for ten years, and the income, added to his annuity, gave him a very comfortable support; and what was a still greater satisfaction to him was the consciousness of doing something to help the cause of his oppressed people, by proving to the world that Nature had endowed them with good capacities. After 1802 he found himself too old to calculate any more Almanacs, but as long as he lived he continued to be deeply interested in his various studies. He was well informed on many other subjects besides arithmetic and astronomy. He was a great reader of history; and he kept a Journal, which shows that he was a close observer of the vegetable world, of the habits of insects, and of the operations of Nature in general. That his busy mind drew inferences from what he observed is evident from the following entry in his Journal:-- "Standing at my door to-day, I heard the discharge of a gun, and in four or five seconds of time the small shots came rattling about me, which plainly demonstrates that the velocity of sound is greater than that of a common bullet." After the Constitution of the United States was adopted, in 1789, commissioners were appointed to determine the boundaries of the District of Columbia. They invited Banneker to be present and assist them in running the lines; and he was treated by them with as much respect as if he had been of their own color. His Almanacs were much praised by scientific men, and they often visited him in his humble little cabin. But these attentions never made him pert and vain. He rejoiced in his abilities and acquisitions, because he thought they might help to raise the condition of his oppressed brethren; but he always remained modest and unobtrusive in his manners.