Lithographic printing by transfer from engraved plates. An impression on transfer paper may be taken from an engraved plate and this laid down on the stone in a manner similar to that used in laying down the prints from the glass negatives in photolithography. Prints are then made from the stone the same as in photolithography, but with superior results as to clearness. This general process is extensively used in both map and chart publishing in this country, as it combines the advantages of the plate in preservation of the chart record and facility of correction, and the advantages of the lithographic printing in less distortion of the printed chart, ability to print more than one shade, and facility for large editions. As the transfer from the plate can be readily made it is also better applicable to small editions than is photolithography. It is, however, not as convenient in the latter respect as plate printing, and it does not give a resulting impression equal in clearness or durability to the impression directly from the plate.
Etching on copper for chart publication has been recently developed in the Coast and Geodetic Survey. A finished tracing is made, the surface of a smooth copper plate is sensitized, and by exposure to the sun a print is made on the sensitized surface. It is essential to use an air-exhausted printing frame so as to get good contact between the vellum and the plate. The work is then etched into the copper and the plate cleaned and touched up, after which it may be used the same as a hand-engraved plate, either for transfer to stone or direct plate printing. The expense and time required in the etching process are much less than for hand engraving. The process has been successfully used for a number of harbor charts. The etching of course will be of the same scale as the vellum at the time of the print, and vellum varies somewhat in scale with weather conditions and age. Unless overcome by the substitution of some more invariable material in place of vellum, this might be an obstacle to the use of the process for general charts where a true scale on the copper plate is desirable because of future work to be done on the plate. It must also be taken into account that the etching requires a finished tracing in ink, which is not essential for the hand engraver; if, however, the chart is first published by photolithography, as is the usual practice in the Coast and Geodetic Survey, the same tracing is used for both processes.
Distribution of charts. Charts published by the government are sold to the public at a small price, estimated to cover the cost of paper and printing. The charts may be obtained direct from the publishing office or from the chart agents who are to be found in all the principal seaports. Catalogues are published from time to time giving complete lists of the current charts and the main facts regarding them. Index maps show graphically the area covered by each chart. The notices to mariners contain announcement of new charts or new editions published and of charts or editions cancelled, as well as of all corrections.
[CORRECTION OF CHARTS.]
Need for revision. The making of the survey and the printing of the chart do not complete the problem of the chart maker. Both nature and man are constantly changing the facts the representation of which has been attempted on the charts, and also the needs of man are always varying. The original surveys are made to meet the reasonable requirements of the time, but breakwaters and jetties are built, and channels and harbors dredged and otherwise improved, and cities built, and new paths of commerce are opened which bring vessels into waters previously thought of minor importance.
With the increase of commerce and speed of vessels more direct routes are demanded for reasons of economy. Inside routes not originally used are sometimes developed for defensive reasons. The average draft of the larger vessels has also increased remarkably since the modern hydrographic surveys were commenced, and surveys once made to insure safety for the deepest vessels of that time are now not adequate. The average loaded draft of the 20 largest steamships of the world has increased as follows: 1848, 19 feet; 1873, 24 feet; 1898, 29 feet; 1903, 32 feet. The average length of these vessels was 230 feet in 1848, 390 feet in 1873, 541 feet in 1898, and 640 feet in 1903. The number of vessels drawing as much as 2614 feet rose from 36 in 1902 to 185 in 1904. In 1906 there were 17 vessels afloat, drawing 32 feet and upwards. There are now two steamers on the Atlantic 790 feet long, 88 feet beam, and 3712 feet draft when fully loaded, and larger vessels are already planned.
Great natural agencies are also constantly at work effecting changes in features shown on the charts. The action of currents and waves is continually cutting away or building the shore, particularly on sandy coasts exposed to storms. When surveyed in 1849 Fishing Point on the east coast of Maryland was but a bend in the shore line. By 1887 it had built out about two miles in a southerly direction, and in 1902 about two-thirds of a mile further, curving to the westward. Altogether in about half a century this tongue of land has grown out nearly three miles.
Rivers are bearing vast quantities of sediment and depositing these near their mouths, pushing out the coast line and filling in the bottom. The main mouths of the Mississippi are advancing into the Gulf, but at a comparatively slow rate. A break from the main river at Cubit's Gap just above the head of the passes, however, has done an enormous amount of land making, filling in an area of about 50 square miles between 1852 and 1905.