464 



SCIENCE. 



[N. S. Vol. I. No. 17. 



A Manual of Topographic Methods. 'By Heney 

 Gannett, Chief Topographer U. S. Geo- 

 logical Survey. Washington, Government 

 Printing office. Quarto, xiv+ 300pp. 18 

 plates. 



Whatever may be thought of the advisa- 

 bility of the publication of scientific manu- 

 als or text-books by the government, there 

 is probably little question but that a bureau 

 is justified in issuing volumes or bulletins 

 which are in the nature of instructions to 

 its officers and employees. Some publica- 

 tions of this kind, issued as parts of the re- 

 ports of scientific bureaus, have been of 

 great value to surveyors and engineers on 

 account of the new facts and methods that 

 they contain. The preface of this work 

 states that it was primarily prepared for 

 the information of employees, and further- 

 more that it ' describes the stage of develop- 

 ment reached at present.' Hence it should 

 presumably be of interest and value to all 

 topographers who are acquainted with the 

 excellent maps issued by the Geological 

 Survey. Of the eighteen plates in the vol- 

 ume twelve give beautiful illustrations of 

 types of topography, and these form its 

 most useful and attractive feature. 



The 300 pages of the manual include 130 

 pages of text, 168 pages of tables and 2 

 pages of index. Although the form is 

 quarto, the size of the printed page is only 

 5J X 7^ inches, and being in large type it 

 includes but little more matter than a com- 

 mon octavo page. Chapter I. devotes 14 

 pages to historical and general information, 

 chapter II. has 26 pages on astronomical 

 determinations, and chapter V. is an inter- 

 esting geological essay of 25 pages on the 

 origin of topographic features. Thus only 

 65 pages remain for the discussion of meth- 

 ods of topography, a space entirely inade- 

 quate to do justice to the subject. 



On base line measurements with the 

 steel tape the corrections due to inclination, 

 temperature and elevation above sea level 



are explained, but nothing is said about the 

 sag of the tape, which as well known always 

 makes the recorded distance too long, and- 

 the effect of varying intensity of pull is also 

 unnoticed. The subject of primary trian- 

 gulation is presented more fully tlian any 

 other topic, the general methods of the Coast 

 and Geodetic Survey being adopted, with 

 somewhat different but excellent instruc- 

 tions for measuring angles. No statement 

 as to the allowable probable errors of an- 

 gular measurements is made, and the re- 

 mark that the average length of lines in 

 primarj^ triangulation is 12 or 16 miles, 

 leaves a confused idea as to what class of 

 work is really under discussion. 



On topography proper 5 pages are de- 

 voted to the plane table, 3 to traverses, 1^ 

 to stadia measurements and 9 to barome- 

 ters. It is difficult to ascertain fi-om these 

 the details of the methods recommended or 

 used, and it is safe to saj^ that the excellent 

 maps now being issued by the Geological 

 Survey were not made without the applica- 

 tion of principles and methods of which 

 this volume gives no adequate explanation. 

 It abounds, however, in useful generalities, 

 such as " Stations for sketching should be 

 selected with the utmost freedom ;" " Un- 

 der certain circumstances it is found advis- 

 able to use the stadia method for measuring 

 distances instead of the wheel ;" " Constant 

 communication must be had between the 

 chief of party and his assistants," etc. 



The main feature of a small-scale topo- 

 graphic map is, of course, the contours. In 

 chapter IV. references to the determination 

 of heights by the barometer and stadia are 

 made, but no forms of field notes are given, 

 and the fact that these heights are to be 

 used for locating contours is scarcely men- 

 tioned. In chapter V . , however, one page is 

 devoted to the subject, the essence of which 

 is that contours are sketched in the field by 

 the chief of party. It is stated that this 

 'is artistic work,' that "it is impossible 



