40 R. S. Woodward — Iced Bar Base Apparatus. 



change in shape of the trough gives rise to quite insignificant 

 changes in length of the bar. Indeed, the alignment of the 

 bar may be maintained so perfectly that the correction for its 

 curvature will not exceed a few tenths of a micron. 



The grade sector of the apparatus is adjusted to zero when 

 the graduated surfaces of the bar are in the same horizontal 

 plane. To secure the latter condition an engineer's level is 

 used ; and with appropriate care the difference in height of 

 the ends of the bar can be made zero with a probable error 

 not exceeding ±0\L ram . 



The microscopes are provided with fixed levels, which, when 

 once adjusted, enable the operator to make the axes of the 

 microscopes vertical. They are also provided with clamp 

 screws so that they may be rigidly held in proper position. 



When posts are used to support the microscopes, as has 

 been the case with this apparatus thus far, they must be set 

 in their proper positions within a centimeter or two. It is 

 easy and convenient, however, to adjust their sides facing the 

 line to be measured with much greater precision. When 

 firmly set, a line parallel to the base may be deliberately 

 ranged out with a theodolite of high magnifying power,. and 

 this line may be defined by suitable marks on each post. Then 

 by simply noting the distance of the axis of the bar during 

 measurement from this reference line an accurate correction 

 for deviation of the bar from parallelism with the base may 

 be obtained. This adjustment of the posts though not essen- 

 tial to the use of the apparatus has been followed. 



Another convenient adjustment which the use of posts per- 

 mits is that of making the tops of several or many posts con- 

 form to one grade. By this means, since the four microscopes 

 used are closely alike, the grade angles for several or many 

 bar lengths are nearly the same — a condition favorable to pre- 

 cision in determining grade corrections. As an additional 

 precaution in the use of this apparatus the relative heights of 

 the alternate post tops have been determined with an engi- 

 neer's level. 



Method of measurement. — To conduct the measurement of 

 a line with this apparatus eight men are required, to wit : 

 three observers ; one recorder ; one man to move the micro- 

 scopes ; and three men to move the car tracks, the microscope 

 shades, and the ice and ice crusher. 



The operation of measurement proceeds as follows : The 

 position of the microscope relatively to the fiducial point at 

 the end of the line having been observed as explained above, 

 the rear end of the bar is brought to focus under that micro- 

 scope by the rear end observer. By means of a lever which 

 grips into the track and hinges on the car, the latter observer 



