70 APPENDIX TO EEPORr OF THE SECRETARY. 



Instruments — Base-measuring apparatus. — The apparatus used in ineas- 

 uriug the base-liues from which the primary triaugulatiou is developed 

 consists essentially of wooden rods aligned and leveled on movable 

 trestles or tripods, the contact being made by coincidence of lines in- 

 stead of by direct abutment. 



The rods are 15 feet long, one-half of an inch square, thoroughly dried, 

 oiled, and varnished ; they are supported in cases made truss-form to 

 prevent sagging, and moved in these cases to make the contact by a 

 rack and pinion motion. Both ends of the rod are shod with a plate of 

 brass firmly fixed upon and half the width of the rod, or one-fourth of 

 an inch, and so arranged that the plate upon the forward end of any rod 

 projects by the plate upon the rear end of the preceding one in such a 

 manner that both rods are in the same straight line. The line of coinci- 

 dence is marked upon both plates and contact is determined by a mag- 

 nifier. 



A delicate spirit-level is attached to each case to adjust it horizontally 

 and a thermometer inserted to determine the temperature of the rod. 

 Two steel pins, by which the rods are aligned, are fixed on the cases 

 directly over the center of the ends of the rods. 



The tripods (or stands upon which the cases carrying the rods rest) 

 have short double legs of the usual construction. Firmly fixed upon the 

 tripod-heads are two uprights upon which a sliding cross-piece is clamped 

 by thumb-screws. Above this cross-piece, parallel to and carried with 

 it, is a second, which can be moved up or down three-fourths of an inch 

 by means of a long slender wedge working between the cross-pieces, 

 and furnishing an easy means of making the final adjustment of the 

 rods in level. The uprights are several inches apart, and give sufficient 

 range to align the rods, which is done by a theodolite placed in advance 

 upon the line to be measured. Two or three rods and six or eight tri- 

 I)ods are used. The rods are kept in a horizontal position, and, when the 

 inequalities of the ground demand, vertical offsets are made with a the- 

 odolite. The line is first ranged out, and stakes set 500 feet apart along 

 its length, then, with six men to work the apparatus, 3,000 feet per day 

 can be measured with all the accuracy the refinements of the triangu- 

 lation demand. 



Theodolite. — The theodolite used in the triangulatiou is of a new pat- 

 tern, embracing a number of improvements demanded by the character 

 of the work. So far as possible the number of parts has been reduced 

 by casting in a single piece parts that are usually combined by screws. 

 In this manner the liability to derangement incident to the vicissitudes 

 of mountain work is greatly reduced. The telescope has been enlarged 

 as compared with the graduated circle, so as to make its defining power 

 bear a greater proportion than usual to the refinement of graduation. 

 The object-glass has an aperture of two inches and a focal length of 

 twent;y. The horizontal circle is ten inches in diameter, and reads by 

 double verniers to five seconds of arc. The vertical circle is five inches 



