R. S. Woodward — Iced Bar Base Apparatus. 43 



overhead, but the north side was left open in order to permit 

 free access of daylight and air. 



This comparator was built by Assistant A. T. Mosman after 

 plans drawn up by Mr. Siebert. It answered its purpose very 

 satisfactorily. An efficient auxiliary ajjplied by Assistant 

 Mosman was a sawdust covering to the ground along the com- 

 parator. This covering absorbed dust and moisture, and pre- 

 vented the transmission of disturbances through the ground to 

 the microscope posts. The stability of these posts may be 

 inferred from the measures of the comparator interval given 

 below. 



The standard kilometer, — A nearly level portion one kilo- 

 meter in length of the Holton Base was selected by Assistant 

 Mosman for measurement with the iced bar apparatus. The 

 base line, whose entire length is 5*5 kilometers, runs in a 

 nearly north and south direction across the Crawfish Flats of 

 southern Indiana. The portion selected for the iced bar 

 measures passes for 600 m of its length through a dense forest 

 growth, leaving about 200 m at either end in open fields. The 

 whole kilometer is on low ground and the part within the 

 forest is, in a wet season, subject to partial inundation. The 

 soil along the kilometer is a stiff clay which is very firm when 

 dry but which assumes a jelly-like mobility and elasticity 

 when saturated with water. 



The way through the forest was cleared and the end stones 

 of the kilometer were set under the direction of Assistant 

 Mosman during May and June, 1891. During the latter half 

 of the following August and early September the microscope 

 and track posts were set along the line. Owing to frequent 

 and heavy rains this was a tedious operation. Many of the 

 posts were set in the water which filled the post holes as fast 

 as they were dug. It is impossible, therefore, to present any 

 statistics as to the speed with which this work can be done 

 under usually favorable circumstances. It may be remarked, 

 however, that it is a work which requires but little skilled 

 labor. In addition, it should be said that the microscope posts 

 were set with considerable precision. Accurate spacing of the 

 posts to 5 m apart was secured by means of 100™ and shorter 

 steel tapes ; while the posts were aligned by means of a 

 theodolite. The probable error in position of a post face with 

 respect to the kilometer line does not, I think, exceed ±3 mm ; 

 while the probable error of the reference line fixed on the 

 posts as explained above does not exceed ±l mm . 



The bolts marking the termini of the kilometer were 

 cemented in the end stones by Assistant Mosman early in 

 August, after their proper relative positions had been deter- 

 mined by Assistant O. H. Tittmann with the Survey secondary 



