Above is the sketch for Dedham lock and Mill. Constable probably did the sketch on location. I have found the drawing for this, for that is how the picture seemed to begin. It is in a new book called "Constable: The Great Landscapes" written by Sarah Cove, Anne Lyles and Charles Rhyne and published by the Tate gallery in London. So here I am again recommending expensive books. This really IS the Constable book to have theses days. It is full of beautiful illustrations of his paintings but most importantly an intelligent description of the working methods of the artists. If the link to Amazon is below.
Constable did a series of huge and important salon paintings to secure reputation and notice at the Royal Academy ( the painting above, is incidentally not one of them) The book is mostly focused on these blockbuster pictures and their preparatory studies.
I was painting in Paris a number of years ago and saw a Constable show in a museum just off the Champs-Elysees. The show was of the six footers as these big pictures are called and the full size sketches done for them. Oddly enough the show was curated by Lucian Freud, who did a good job. The drawings and studies and all that a painter would want to see to understand the making of these pictures were all grouped together. So I have had the good fortune to actually see a lot of the work that is in the book. I have also seen a great number of this drawings in a show at the Victoria and Albert in London. That is one of the places you hunt Constable, which I was doing. The Tate is of course another.
Constable made at least three versions of the Dedham Lock painting from this sketch and drawing. Tomorrow I will begin on the "great landscapes"