Reductions to Standard Instruments 



47 



Lloyd-Creak Dover circle C. 6" G. S. JS {Dover No. i68) continued. 



Subsequent to the overhauling of the circle in June, 1908, it was used for land work in 

 Greenland and Newfoundland. The correction adopted from standard intercomparisons, 

 for the range of dip covered in this work, was +o'.4 for mean value observed with needles 

 Nos. I and 2. 



The tabular designations are 35' 12, 35-2(5), and 35-12. 



The correction for declination values by the compass attachment of circle 35 resulting 

 from various comparisons with standardized magnetometers is: for values through June, 

 1908, where mark was read with peep sights, +7'.5, and for subsequent work +8'; for 

 values where mark was read with telescope, 9'. 2. 



Dover circle 71. Type (c); the property of the Hongkong Observatory; loaned the Department 

 for some of the earlier work in China. The corrections adopted from comparisons at 

 Hongkong in March, 191 1, with circle 206 standardized are: needle No. 3, 1'.5; No. 4, 

 o'.o; No. 7, I'.y; No. 8, 1'.6. The tabular designation is 71-3478. 



Dover circle 122. Type (a); the property of the Massachusetts Institute of Technology; loaned 

 the Department during 1905 for work in Labrador and Newfoundland. The corrections 

 adopted from standard intercomparisons are: No. i, +o'.9; No. 2, +i'.i. The tabular 

 designation is 122-12. 



Dover circle 142. Type {a) ; was used by Professors Beattie and Morrison for work in Africa in 

 1910, and belongs to orie of them. Intercomparisons were obtained at the observatories, 

 Kew, England, and Helwan, Egypt, the relations of whose standards to the Department 

 standard had been previously determined. The following corrections as based on all 

 available data were adopted : 



The tabular designation is 142- 1235. 

 Lloyd-Creak Dover circle i6g. Type {b)\ the property of the Department. It was used in the 

 work of the Galilee in 1905, during her first cruise in the Pacific Ocean. Needles I and 2 

 were used throughout the land work of this cruise. The corrections adopted from all the 

 land intercomparisons were: needle No. i, I'.i ; No. 2, s'.O. In view of the experience 

 had with this circle and with No. 168, it was returned for modification to the maker in 

 June, 1906; additional dip and intensity needles were supplied, two deflection distances 

 and a telescope were provided and the pivots of needles and jewel bearings improved. 

 The modified instrument was returned to the Galilee in December, 1906, and used through- 

 out her entire third cruise in the Pacific Ocean during 1907 and 1908. The results show 

 No. 169 to be a very superior instrument, observations made with it at land stations being 

 comparable with those obtained with good land circles. For the work of 1907 and 1908 

 the corrections adopted from least square reductions of data obtained from land circle 

 comparisons with standardized land dip circles are given by the following formulae: 

 Needle No. i FM = 3'. 9 + 5'. 5 cos I + i'.2 sin / 

 Needle No. 2 FM = 4 .6 + 6 .0 cos / + i .5 sin / 

 or, for the mean value by the two needles. 



Mean of Needles i and 2 FAI = 4'.3 + 5'.4 cos I + i'.4 sin I 

 The tabular designation is 169- 12. 



The adopted corrections for declination values by the compass attachment of circle 169 are: 

 for the 1905 work 2'. 5; for subsequent work 2' when the mark readings are made 

 with the peep sights, and +3' when the mark is read with the telescope. 



