R. S. Woodward — Iced Bar Base Apparatus. 39 



brought by a rack and pinion motion to focus under the micro- 

 scope whose position relative to the fiducial point is sought. 

 The scale and level, which are parallel to each other, are 

 placed parallel to the line measured. With the cylinder thus 

 disposed readings of the micrometer on the scale and of the 

 position of the level bubble are made. The cylinder is then 

 turned 180° in azimuth and the scale and level readings are 

 again observed. From these observations and the height of 

 the scale above the bolt head, the horizontal distance (in the 

 direction of the line) between the micrometer zero and the 

 fiducial point may be accurately determined. 



Adjustments of apparatus. — The most important adjust- 

 ment of the apparatus is the alignment of the bar in the 

 Y-trough. This adjustment is made when the ice load is in the 

 trough and after the latter has had time to assume a stable 

 shape. This time does not exceed 15 minutes. 



As already stated, the alignment of the bar requires that the 

 upper surfaces of the alignment plugs be in one plane and 

 that the lines on these plugs be in the same straight line. The 

 former requisite is secured by a striding level whose feet are 

 99 cm apart, so that they reach from any plug to the second 

 adjacent plug. Beginning at one end of the bar the plugs are 

 numbered 1, 2, 3, — 11. By placing the level feet in succes- 

 sion on plugs 1 and 3, 3 and 5, etc., plugs 1, 3, 5,-11 are 

 brought into the same plane by means of the corresponding 

 vertical adjusting screws, the screws under plugs 2, 4, — 10 

 being loosened if need be to secure this end. Having thus 

 adjusted plugs 1, 3, 5, — 11, the level is placed on plugs 2 aud 

 4, 4 and 6, etc., and the vertical screws are brought up to 

 contact with the bar but not raised enough to disturb the 

 previous adjustment of 1, 3, 5 — 11, which are the principal 

 defining plugs in this adjustment. 



To place the lines on the plugs in the same straight line a 

 sharp pointed plumb bob suspended from a fine brass wire 

 stretched over the trough was originally used. This device 

 with the aid of the lateral adjusting screws of the saddles per- 

 mits placing the lines in proper position within 0'l mm when 

 the trough is fully loaded with ice. Experience with the appa- 

 ratus, however, showed that the simpler method of stretching 

 the wire, or better still, a fine thread, close over the plugs 

 when the trough is about four-fifths loaded secures equally 

 good results. 



It was feared before, using the apparatus that the daily tem- 

 perature range might produce an appreciable effect on the 

 length of the bar through change in curvature of the trough. 

 Hence the accurate method of measuring such change by the 

 striding level was provided. But experience shows that the 



