358 Special Reports 



Plumb-lines were fixed for declination reference-marks at the north and south 

 ends of the 3 rows of stations; auxiliary marks outside the building were established 

 for the observations with magnetometer No. 26 at station E8. The horizontal-intensity 

 observations were made partly by oscillations and partly by deflections, the values of 

 // being computed, using the mean value of the magnetic moment derived from several 

 complete determinations of intensity preceding and following the survey. The inclina- 

 tion values were determined with the earth inductors. 



The following results were obtained from the magnetic survey of the interior of 

 the Experiment Building. 



1. A steady decrease in westerly declination from the north to the south end of the building, 

 the total range being minutes of arc for stations 1 to 7, the ranges for the high and low stations 

 being practically identical for all stations. The average value of the declination was 4.9 west. 



2. A steady decrease in northerly inclination from the south to the north end of the building, 

 the total range being slightly less than 2 minutes of arc for stations 1 to 7, the ranges for the high 

 and low stations being of the same order. The average value of inclination was 71. 7 north. 



3. A steady increase in horizontal intensity from the north end to the south end of the building, 

 the total range being approximately 0.00025 C. G. S. for stations 1 to 7. The average value of 

 horizontal intensity was 0.1878 C. G. S. 



4. A slight increase in westerly declination and horizontal intensity and a slight decrease in 

 northerly inclination from the east to the west sides of the building. 



The small variation from absolute uniformity of magnetic field within the building 

 arises from the following sources: (a) disturbance of the Earth's normal field by mag- 

 netic materials and equipment in main laboratory (A); (b) small natural local dis- 

 turbance, amounting in a distance of 50 feet in the true meridian, as shown at the Stand- 

 ardizing Magnetic Observatory, to about one minute of arc in declination, about 

 0.00005 C. G. S. in horizontal intensity, and several tenths of a minute of arc in inclina- 

 tion; (c) very slight magnetic impurity of cement and possibly of aggregate; and (d) 

 slight disturbance from the exterior earth embankment at the north end of building. 



Inasmuch as the stations at the lower level were but one foot from the thick concrete 

 floor, it must be concluded that the results of the tests were highly satisfactory. The 

 investigational work (see Plate 8) , performed by Dr. Barnett, in the Experiment Build- 

 ing since its completion, has shown that the desired requirements, as already enumerated, 

 have been fulfilled. 



