January 17, 1895J 



NA TURE 



^11 



are varied accordingly. The scales adopted at the com- 

 mencement of the work were I : 62,500, i : 125,000, and 

 I : 250,000 ; or very nearly i, 2, and 4 miles to the inch 

 respectively. With the progress of industrial develop- 

 ment, the maps came to be in great demand in connection 

 with all sorts of enterprises in which the nature of the 

 ground required consideration, as in the projection of 

 railways, water-works, drainages, and the like. Maps on 

 a larger scale, and showing more detail, have in many 

 instances become necessary, so that it has been deter- 

 mined to altogether discontinue the four miles to the inch 

 map, but only to make new maps of the areas already 

 represented on this scale in cases where they are specially 

 required. It is believed that on the scales of one and 

 two miles to the inch, it is possible to represent with 

 f.iithfulness all necessary details. 



The relief of the maps is represented by contours, or 

 lines of equal elevation ; in the larger scale maps the 

 intervals range from 5 to 50 feet, and in the smaller 

 ones from 10 to 100 feet, according to the nature of the 

 area mapped. For the now discarded scale the intervals 

 are from 200 to 250 feet. 



The methods adopted in the preparation of these maps 

 form the subject of the twenty-second monograph of the 

 Inited States Geological Survey, which constitutes an 

 excellent manual of topography. It is not intended as 

 an elementary treatise on surveying, nor as a general 

 treatise on topographic work, "although it may, to a 

 certain e.\tent, supply the existing need of such a work.'' 

 It is primarily intended for the information of the men 

 actually engaged upon the survey ; but we believe that it 

 will have a much larger field of usefulness. 



We may look upon this manual as consisting of two 

 essential parts : first, that dealing with the methods 

 employed in the surveys ; second, that giving a brief 

 account of the origin of the various topographical 

 features. The latter part we hope to refer to on another 

 occasion, and for the present it is sufficient to say that 

 its object is to act as a guide to correct delineation in 

 filling in the details of the sketching. 



A map, whatever its character, is defined as a sketch, 

 corrected by locations. " The work of making locations 

 is geometric, while that of sketching is artistic, and how- 

 ever numerous the locations may be, they form no part 

 of the map itself, but serve only to correct the sketch, 

 while the sketch supplies all the material for the map." 

 Hence, the education ofthe topographer, as Prof. Gannett 

 tells us, should consist of two parts, the mathematical 

 and the artistic. " The first may be acquired from books, 

 and this book knowledge must be supplemented by 

 practice in the field. The second, if not inherited, can 

 be acquired only by long experience in the field, and by 

 many can be acquired only imperfectly. In fact, the 

 sketching makes the map, and therefore, the sketching 

 upon the Geological .Survey is executed by the best 

 topographer in the party, usually its chief, whenever 

 practicable to do so. 



In making a map, four principal operations are in- 

 volved, (i) Astronomical observations for locating the 

 mip upon the earth's surface; (2) the horizontal location 

 of points ; (3) the measurement of heights ; (4) the 

 sketching ofthe map. 



With regard to the methods now employed, " it is to 

 be understood that they are not fixed, but are subject to 

 change and development, and that this manual describes 

 the stage of development reached at present. ' Five 

 principal instruments have been employed in the Survey : 

 theodolites of a powerful and compact form, for use in 

 the primary triangulation ; plane tables of the best type 

 with telescopic alidades, for secondary triangulation and 

 height measurements ; plane tables of simple form with 

 sighted alidades, used for traversing and minor triangula- 

 tions ; " odometers," for measuring distances ; aneroids, 

 for the measurement of details of heights. 



NO. I 3 16, VOL. 51] 



All these instruments arc described with sufficient- 

 fulness, while other instruments, such as transits, chains, 

 tapes, and telemeters, which are commonly figured and 

 described in all works on surveying, receive no special 

 attention. 



A single instrument of a very convenient form suffices 

 for the astronomical determinations of position. This 

 is a combined transit and zenith telescope, and consists 

 of an ordinary transit instrument provided with a zenith 

 micrometer eye-piece, and resting on a graduated circular 

 base in such a way that the whole instrument can be 

 made to revolve when using it as a zenith telescope. The 

 telescope has an aperture of two and a half inches, and a 

 focal length of twenty-seven inches. We are not ac- 

 quainted with any other instrument so convenient for the 

 double purpose of finding latitudes and longitudes with 

 accuracy. E.xamples ofthe observations made with the 

 instrument are given, and these, with the various steps 

 in the reductions, form an admirable guide to the astro- 

 nomical work. 



Triangulation is employed in preference to primary 

 traversing wherever the country presents sufficient relief 

 for the purpose, as it is more accurate and cheaper. 

 The initial step in this process is, of course, the measure- 

 ment of a base line, and in our British survey this was 

 accomplished by Colby's compensation bars. This 

 method of measurement was also employed in the United 

 States up to 1S87, when it was decided to adopt a system 

 of measurement by steel tapes. The tape in use has a 

 length of 300 feet, and it is claimed that it is easy to 

 obtain the re(|uired degree of accuracy in a far shorter 

 time and at much less expense. A special apparatus for 

 using this tape has been devised, and full instructions 

 for its use and reduction to standard are given. 



The description of the base-line measurement is natur- 

 ally followed by hints as to the selection of stations and 

 the erection of signals for triangulation. .\ very con- 

 venient form of observing tower, or combined mstrument 

 support and signal, for use when surrounding objects 

 have to be overlooked, is figured and described. We 

 learn that vernier theodolites have now been discarded 

 in favour of others in which the circles are read by 

 micrometer microscopes, although the circles are only 

 8 inches in diameter. An excellent and concise account 

 is given of the various errors to which angular measure- 

 ments are liable, and ofthe methods of eliminating them 

 from the final results. Some of these errors are instru- 

 mental, others personal ; and in this connection. Prof. 

 Gannett remarks that, " after learning how to make good 

 oDservations, the observer should place the utmost con- 

 fidence in them, and never yield to the temptation of 

 changing them because they disagree with some preced- 

 ing observations. Such discrepincies are in general an 

 indication of good, rather than poor, work." 



In some districts it is almost impossible to carry on a 

 triangulation, and in such cases primary traverse lines 

 are resorted to, these simply differing from ordinary 

 traverse lines in being more elaborately and carefully 

 executed. These traverse lines, it may be said, consist 

 of a series of measurements of distance and directions, 

 and when they are intended to replace the triangulation, 

 they are made with the steel tape, to which reference has 

 already been made, and theodolites. 



The account of the secondary triangulation is remark- 

 able chiefly for the great prominence given to the plane 

 table. Speaking of this, Prof. Gannett says that " much 

 misapprehension exists, especially in this country [the 

 United States], regarding the character and application of 

 this instrument. This arises, apparently, from the fact that 

 it is little known. For making a map the plane table is 

 a universal instrument. It is applicable to all kinds of 

 country, to all methods of work, and to all scales. For 

 making a map it is the most simple, direct, and economic 

 instrument ; its use renders possible the making of the 



