We often receive letters containing sketches of machines, respecting which, information is asked by our correspondents; many of these— in fact the most of them—are written with the description on the first page and the diagram inside. It is difficult to read and understand such letters, because a leaf has to be turned over to find every letter of reference. Correspondents will save us much trouble, and enable us to understand their sketches better and more rapidly, if they will write the description on one inside ptfge, and make the diagrams on the opposite one, or on a separate sheet. Such letters have but to be opened out and placed before us, to enable us to read the descriptions and examine the sketches with facility and without confusion.