586 abstracts: geodesy 



There is given an account of the remeasurement of the El Paso Base 

 hne in Colorado, which was first measured with the "Secondary Base 

 Bars" in 1872. The remeasuring was done with 50-meter invar base 

 tapes. The difference between the two measurements is one part is 

 about 59,000. As the base ends were very substantially marked, when 

 the base was estabhshed, it is believed that the discrepancy is not due 

 to any shifting of the marks but that it is the result of an error in the 

 first measurement, probably caused by the inability to secure the actual 

 temperature of the metal rods of the bars liy means of mercurial ther- 

 mometers mounted near the rods. 



Three new bases were measured on the 104th meridian with invar 

 tapes and the probable error of each base is smaller than one part in 

 one million. Nearly all of the observing for horizontal angles in the 

 main scheme was done at night, the pointings being made on acetylene 

 lamps posted at the distant stations. These lamps were of sufficient 

 power to be seen easily over the longest lines of the scheme — about 70 

 miles.- 



The introduction of additional precise leveling elevations in the trigo- 

 nometric leveling net made a readjustment of the net necessary. In 

 consequence, a number of mountains are given elevations which differ 

 from the elevations given in Special Publication No. 4. Except in a 

 few cases in the vicinity of Pike's Peak, the new eleyations are lower 

 than the older ones. The changes in California and in western Nevada 

 and in central Colorado are small, being only one foot for Mt. Elbert 

 and Mt. Ouray in Colorado, and two feet for Mt. Grant in Nevada. 

 The maximum change comes in Utah; Pilot Peak, for instance, having 

 its elevation changed by 16 feet. 



A discussion of the errors of triangulation indicates that when the 

 observations are made during the day and in the afternoon, triangula- 

 tion running north and south tends to deviate in azimuth towards the 

 west. Under similar conditions an east and west arc tends to deviate 

 towards the south. This systematic error is not present to any notice- 

 able extent when all the observations are made at night. It^is stated 

 that the error is probably due to some effect of unequal heating of the 

 theodolite. To minimize or overcome any such effect, the theodolite 

 now in use on primary triangulation is fitted with a nickel-iron circle 

 which has a much lower coefficient of expansion than that of the origi- 

 nal circle. No data are yet available as to the accuracy obtained with 

 a new circle. 



W. B. 



