38 B. S. Woodward — Iced Bar Base Apparatus. 



the measuring bar as with most forms of apparatus, this rather 

 delicate and difficult operation is supplanted by that of mov- 

 ing the portable tracks. 



The micrometer microscopes. — To define the successive posi- 

 tions of the bar in measuring a line, micrometer-microscopes 

 are used. Through the courtesy of General Casey, Chief of 

 Engineers, U. S. A., the Survey was enabled to borrow the 

 four microscopes and the cut- off cylinder of the Repsold base 

 apparatus- used on the U. S. Lake Survey. These are espe- 

 cially well adapted for use with any line measure apparatus. 

 As designed by the Eepsolds and as used on the Lake Survey 

 the microscopes were mounted on iron tripods. These latter 

 having been destnyyed by fire while stored at the Engineer 

 depot at Willett's Point, N. Y., it was essential to replace 

 them by some equivalent device. In view of the economic 

 and other features of the special work contemplated with the 

 iced bar apparatus it was decided to mount the microscopes on 

 wooden posts set firmly in the ground. To connect the micro- 

 scope with the post a cast-iron post cap is provided. It fits 

 like a box cover on the end of the post and is clamped rigidly 

 to it by means of a screw. 



The microscopes are provided with levels and leveling 

 screws so that their axes may be made vertical. They are 

 mounted on slide rests which give a motion of 2 cm in the 

 direction of the line measured or transverse to it. To secure 

 additional displacement in the direction of the line a small 

 rotary motion is provided for in the connection of the micro- 

 scope with the post cap. The micrometer heads of the micro- 

 scopes are divided to read microns directly, one revolution of 

 the screws corresponding to 01 mm . When used in the field 

 the microscopes are shaded from the sun by large umbrellas. 



With this method of mounting the microscopes it is advan- 

 tageous if not essential to set the microscope posts and those 

 supporting the portable track before beginning measurement. 



End marks and method of reference thereto. — The method 

 of marking the end of a line is essentially that of the 

 Eepsolds and fully described in the Lake Survey Report 

 referred to above. It consists in the use of a metallic 

 bolt terminating in a spherical head, the bolt being em- 

 bedded in a stone or other stable mass set in the ground. 

 The center of the bolt head is the fiducial point. To 

 refer to this point a cylinder called a cut-off cylinder is 

 used. It terminates at one end with a conical hole which 

 fits over the spherical head. The other end is provided 

 with a transverse level and graduated scale. The scale is 



* Fully described in General Cornstock's report referred to on page 33. 



