National Geographic : 1897 Jul
GEOGRAPHIC LITERATURE is about 18 by 28 inches in size, and is printed in four colors: black for projection lines, names, and all cultural features; blue for streams; green for the oceans and large lakes, and brown for the hill and mountain fea tures. These relief features are shown by contour lines. The contour interval, from 2,000 feet upward, is 1,000 feet. Below the 2,000-foot con tour the interval is variable. Over this base map the magnetic curves are printed in red. The magnetic declination, popularly called variation of the compass, is subject to several known periodic changes. Of these the most important is the secular change-a change with a period running through centuries; hence its name. As this secular change is progressive from year to year for long periods, and as it amounts in the United States to from 2' to 5' per year, it is for the surveyor and mariner the most important of the periodic changes. Indeed, it is the only one of much practical importance at present. It is to this practically important quantity that Mr Gannett has wisely devoted the greater part of the labor expended on this memoir. The weakness of similar maps hitherto produced has been recognized by both their makers and users to be largely due to defective knowledge of the secular change. Of the 237 pages comprised in the memoir 82 are devoted to datafor secular change. A table of results by counties occupies 135 pages, while the remaining 20 pages are given to introductory matter, discussion, state ment of sources of data, etc. The sources of the data are the Coast Survey, Lake Survey, the Wheeler, Hayden, and Powell Surveys, New York State Survey, New Jersey Geo logical Survey, Boundary Surveys,United States Corps of Engineers, Army Exploring Expedition, National Academy of Sciences, and others; but it is chiefly from the records of the United States General Land Office and from county surveyors that a vast quantity of hitherto unused material has been derived. Indeed, so abundant are data in the General Land Office that it was only needful to select for the older " land office " States such as were desired. The mass is much greater than is needed to pro duce a map sufficient for all practical needs. As to this Mr Gannett says: " I have not attempted to make a complete collection of this material. The amount is too vast to make it worth while. I have, however, col lected all the observations which appear upon the plats of exteriors and standard lines (the Land Office requires that in the survey of all standard and exterior lines the declination be observed), supplementing them wherever needed by observations made in connection with the subdivision of townships. Altogether, I have abstracted from the plats of the Gen eral Land Office nearly 20,000 observations, and these form, perhaps, nine tenths of the material herewith presented." As the work of subdivision and accompanying magnetic observations began a century ago, it is obvious that these Land Office records consti tute a veritable storehouse of information on secular change-a storehouse of which Mr Gannett is the first to make general use. In addition to these data a circular was sent to all the county surveyors in the United States, and from the returns much valuable information was obtained.