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



scopes which were last used. The trough is completely un- 

 covered in this operation and the ice stirred up and supple- 

 mented by the amount requisite to replace the waste. This 

 amount is usually 3 to 5 kilograms. 



The speed of measurement has varied somewhat with cir- 

 cumstances. It has usually been about 100 m per hour. 75 m 

 were measured in 7 hours on two different dates ; and a kilo- 

 meter would not be an excessive day's work. 



The Efficiency of the Apparatus. 



Plan of operations with apparatus in 1891. — The plan sub- 

 mitted by me to the Superintendent of the Survey for the use 

 of this apparatus on the Holton Base, of the transcontinental 

 triangulation in Indiana, contained the following recommenda- 

 tions which were approved and earned out during the summer 

 of 1891 : (a) To construct a 100-meter comparator near the 

 Holton Base ; to standardize this comparator by repeated 

 measurements with the iced bar ; and to use this comparator 

 in turn to standardize and study the behavior of 100 m tapes or 

 those of less length, or any other form of base apparatus, (b) 

 To use the iced bar in addition to make several measures of a 

 kilometer at least of the base line ; so that the efficiency of 

 the different forms of apparatus used in measuring the whole 

 base could be tested on the actual ground over which they were 

 applied. 



The plan also contemplated making a determination of the 

 length of the steel bar of the apparatus in terms of one of the 

 International Prototype Meters. This was done, but owing to 

 the small amount of time available before going to the field it 

 was impossible to reach anything better than a tentative value. 



Before giving the results of the measures made with the 

 iced bar it is proper to give a brief description of the long 

 comparator and of the kilometer whereon the apparatus was 

 used. 



The 100-meter comparator. — The 100-meter comparator of 

 the Holton Base was a line 100 m long fitted for measurement 

 with the iced bar apparatus. Twenty-one beech wood micro- 

 scope posts l"8 m long and 15 0ra xl5 cm in cross section were set 

 firmly in the ground 5 m apart on a level plat near the north 

 end of the Base. Alongside of the posts a stationary railway 

 track was laid, the support posts of which were half way be- 

 tween the microscope posts. The ends of the line were 

 marked by brass, spherical headed bolts cemented into the 

 upper ends of stone posts, which latter were well set in beds 

 of concrete. The comparator was covered by a shed 110 m 

 long by 3 m wide. Its length extended nearly east and west. 

 It was covered at the ends and on the south side as well as 



