Jhn Rennie the Elder writes about an embankment and sketches his concept. Rare

Autograph Letter Signed, with illustration, 3 pages on bifolium measuring 7 x 9 1/4 inches with sketch on page one, London, March 10, 1821. Rennie writes to Admiral Viscount Keith (1746-1823) giving details about constructing an embankment for his estate, Tullliallan, in Scotland. At the time Rennie wrote this letter, he was renown for designing and building bridges, canals and harbors throughout England. Here he provides design details for the proposed embankment and sketches the slope he prefers on the first page. He provides greater detail in accompanying drawings rendered in another hand. Towards the end of the second page, Rennie refers to his construction, the Southwark Bridge. "The tolls of the Southwark Bridge are not increasing so fast as could be wished, the bridge itself is in a perfect state of security more so even than I expected...." He signs, "John Rennie," and continues in a post script. "Unless the earth of which the bank is to be made is Clay or of a retentive nature a puddle about 2 feet thick should be made in the middle of the bank to prevent the water from getting through it as in the dotted line on the Sketch." John Rennie died about seven months after he wrote this letter. Condition: Two small holes along center margin, another along horizontal fold on third page, slight tear at horizontal fold on first and second page, not affecting text. Letters of the elder John Rennie seldom appear for sale and those with drawings are particularly uncommon. Autograph