rnojECTioNS. 129 



ranges from "20 to 50 feet, the later work having the smaller contour inter- 

 val. In the country west of the one hundredth meridian the contour 

 interval is frequently changed with the alternation of mountain and valley, 

 and intervals of 25, 50, and 100 feet are employed, the interval frequently 

 changing upon the same sheet, East of the one hundredth meridian the 

 same necessity for making frequent changes in contour interval does not 

 exist, and in the work throughout that region the contour interval is uni- 

 form upon each sheet. 



The projection used is the polyconic, each sheet being projected sepa- 

 rately. 



Upon originals to be published upon a scale of 1 : 62500 the projection 

 interval is 5 minutes, while single minute lines may be drawn if desired. 



The construction of a projection upon a scale of 1 : 62500 for a limited 

 area is a simple matter, but requires care and accuracy and the use of the 

 best drafting instruments. The process will be described for this scale, for 

 which, as well as all other scales heretofore in use, tables are appended to 

 this volume. 



First draw a line down the middle of the sheet. Lay oif on this line 

 the length of the several projection spaces in latitude. Take from the pro- 

 jection table for the scale 1:62500 the length of 5 minutes of latitude and 

 lay it off repeatedly, thus establishing the points of intersection of parallels 

 at 5 minutes with the middle meridian. Through these points (haw lines 

 across the sheet at right angles to the middle meridian, using beam com- 

 passes for this purpose. Lay off on these lines the din's for 2' 30" and 7' 

 30" from the middle meridian, corresponding to the latitude on each side, 

 and at these "points erect short perpendiculars. On these lay off the dp's 

 corresponding to the din's and through the points thus obtained draw and 

 ink the projection lines. 



For other scales and areas the process is quite similar, but when a 

 large area such as that of the United States is to be projected, the mechan- 

 ical difficulties greatly increase. 



Original sheets must conform in size and shape to equal parts of square 

 degrees — i. e., each sheet should comprise 15' of latitude by 15' of longitude, 

 or 30' in each dimension, according to the scale. 

 MON xxii 'J 



