34 BULLIITIN OP THE 



THE EMBRYOLOGY OF LIOGULA AND THE SYSTEMATIC RELATIONS 

 OF THE BRACHIOPODS, 



and illustrated the comparative characteristics of the Brachiopods 

 in their embryonic condition with those of the Polyzoans. 



160th Meeting. April 12, 1819. 



The President in the Chair. 

 Fifty-one members and visitors present. 

 Mr. G. K. Gilbert made a communication 



ON THE KANAB BASE-LINE, AND A PROPOSED NEW METHOD OF 

 BASE MEASUREMENT. 



(abstract.) 



1st. The Measurement of the Kanab Base. 



The base lines of the Powell Survey have been measured with 

 wooden rods. The base at Kanab, Utah, was measured in 1818 

 with apparatus prepared originally by Mr. A. H. Thompson. The 

 measurement was performed under the direction of Mr. Gilbert 

 by Mr. J. H. Renshawe. The apparatus consists essentially of 

 two 15-foot rods applied to each other, end to end, in alternation. 

 They are provided with suitable accessories to regulate their 

 alignment and height, and to record their inclination. They were 

 compared before and after use with standard steel rods furnished 

 by the Coast Survey. Two measurements were made, and the 

 second result was found to exceed the first by 0.84 of an inch. 



It has been stated by Mr. Hayden that the results attained 

 by Mr. Powell by the use of wooden rods are less accurate than 

 those given by the steel tape (45th Congress, 2d Sess., House 

 Mis. Doc. No. 55, p. 25). It is desirable to test the truth of 

 this statement, for if wooden rods do not give greater accuracy 

 than steel tapes they should he discarded on the score of econ- 

 omy. A satisfactory comparison of the two methods is not yet 

 possible for the reason that the results with the steel tape have 

 not afforded data for the computation of probable error, but a 

 first impression may be derived from the comparison of the dis- 

 crepancies between duplicate measurements. The following table 

 includes the only base twice measured by the Powell Survey, 

 and the only bases published by the Hayden Survey. The last 

 column shows the estimated probable discrepancy for a common 

 distance of 4.3 miles. 



