_Symbols on charts._ Fair uniformity as to general principles, with differences as to details in carrying them out, exists on the various series of charts regarding their general arrangement and the more important symbols, such as in the shading of land to distinguish from water, the use of depth curves, the representation of hills by shade or contour, the indication of shoals and dangers, and of lighthouses and buoys. #Desirability of uniformity in charts.# Ships engaged in international commerce must enter foreign ports. As the information is constantly changing and charts are being corrected or improved, it is sometimes desirable for the navigator to consult the local foreign charts, and it may often be necessary for him to carry in his chart room the charts of several different countries. There are therefore important advantages in international uniformity in chart publication. There should be a common initial longitude, and as the longitude of Greenwich has been so extensively adopted, it appears quite probable that its use may some day become universal. A common unit for soundings and heights would be very desirable, but the fact that a large group of nations has united on the metric system, while a small group with great commercial interests retains another system, makes the attainment of uniformity difficult. Substantial agreement as to the use of symbols on charts, particularly such as represent aids or dangers to navigation, would be desirable and doubtless feasible. #Privately published charts.# Many of the earlier charts were prepared and published by private enterprise, and such charts are still published, as, for instance, the so-called "blue-back" charts printed in London. These charts have usually differed from those published by the various governments either in representing the main features in a very bold manner with little detail or in including a considerable area with many plans on a single large sheet backed for permanency. An objection to the latter is that the durability together with the high price tends to keep an old chart in use long after it is out of date. It would be financially difficult for a private firm to give the service that a government does in the matter of correcting the charts and issuing new editions, and this is an important consideration in the selection of charts.