R. S. Woodward — Iced Bar Base Apparatus. 41 



holds the bar near to bisection under the microscope while the 

 front end observer brings his microscope into position over 

 the front end of the bar ; to do which he can make use of the 

 lateral motion of the trough, of the microscope, or both. 

 When the bar is adjusted at both ends the rear observer brings 

 the rear end graduation accurately to bisection between the 

 micrometer wires by use of his lever without turning the 

 micrometer screw. Simultaneously he gives the signal u read " 

 to the front observer, who brings his micrometer wires to 

 bisect the front end graduation mark by moving the micro- 

 scope, the micrometer wires, or both. The observers then 

 read their micrometers and the recorder notes them down in 

 his book, after which the rear observer turns his micrometer 

 screw a half revolution or less backwards. The observers 

 then exchange positions. The rear observer carrying with 

 him his lever applies it to the front car and brings the front 

 end graduation to bisection without disturbing the micrometer 

 threads from their previous position ; while at the signal 

 " read " the front observer bisects the rear end graduation by 

 moving the threads with the micrometer screw. They then 

 announce the readings as before and the recorder jots them 

 down, notifying the observers at the time if the screw revolu- 

 tions differ from their previous values. This process eliminates 

 the relative personal equation of the observers, and checks any 

 blunders of whole revolutions in reading the microscopes, each 

 of them being read four times, and the four readings being the 

 same within a few microns. The probable error of a bisec- 

 tion is less than ±1A 



While the bar is in position under the microscopes, the third 

 observer measures the distance of the front end (and the rear 

 end at starting) of the axis of the bar from the reference line, 

 and adjusts the sector level bubble to cente«r, taking care at 

 the same time to keep away from the microscope posts when 

 the bar is observed. The grade sector reading is then made 

 and recorded, and the bar is rolled rapidly forward to a new 

 position. 



As soon as the rear end of the bar is brought safely to posi- 

 tion under a microscope the one previously at the rear end is 

 taken up and carried forward by the microscope porter who 

 clamps and adjusts it on a new post. Likewise, as soon as a 

 section of track is passed over it is carried forward to a new 

 position. 



The observers stand on platforms which rest at their ends 

 on the ground at a distance of about one meter on either side 

 of a microscope post. 



At intervals of 20 to 40 minutes fresh ice is supplied, the 

 trough being run to the rear of or ahead of the two micro- 



