#Copper plate engraving and printing# have long been used in chart preparation. A drawing is prepared as a guide for the engraver; this must be correct as to all information to be shown but need not be a finished drawing. A true projection is ruled upon a copper plate. By photography a matrix is made from the drawing and a wax impression taken from this matrix. This is then laid down on the copper to fit the projection, and the impression is chemically fixed on to the copper. The work thus marked out is engraved by hand or by machine. A high degree of skill is required in the accuracy and finish necessary for chart engraving. Machines have been invented in recent years which can be used for portions of the work on copper plates, as for instance for cutting the sounding figures, the bottom characteristics, the border and projection lines, border divisions, compasses, line ruling, and stipple ruling. Stamps and dies have been successfully used for some symbols and notes, and roulettes for shading. By means of these various machines, many of which are American inventions, the process of chart publication from plates has been materially facilitated. [Illustration: FIG. 29. ELECTROTYPING PLANT FOR ELECTROTYPING CHART PLATES.] When the plate is completed an alto, or raised copy, is made by depositing copper on to it in an electrotype vat, and from this alto another basso or sunken copy is made by the same process. This latter basso is used in printing. A copper plate may be used for about 3000 impressions, after which it may become too much worn for satisfactory chart printing. By printing from a duplicate basso the original plate is preserved and additional copies can be made when needed. The use of the alto also greatly facilitates matters when a considerable correction to the chart is required. All the portions of the chart to be changed can be scraped off the alto, and when a new basso is electrotyped from this scraped alto all such areas will of course appear as smooth copper, on which the new work can be engraved. Numerous small corrections are called for on charts, and on copper plates where these are to replace old work the latter is removed either by hammering up the back of the plate or by scraping its face.