48 



Land Magnetic Observations, 1905-10 



Dover circle 171. Type (a); the property of the Department. It was used in 1905 in the West 

 Indies, by the Galilee party for part of the shore work in 1905 and a part of 1906, during 

 1906 in land work in Polynesia, and in 1907 to 1909 in the land work in China and Chinese 

 Turkestan, being returned to the Office in 1910 for supplying new needle pivots and for 

 general overhauling. The corrections adopted as based on a careful consideration of all 

 data available are as follows: mean of needles Nos. i and 2 in 1905 and 1906, for values 

 of dip from 60 north to 60 south, o'.o, and for values of dip between 60 and 80 north 

 and 60 and 80 south, +o'.5; mean of needles Nos. 5 and No. 6 of circle 172 during 1906 

 to October, 1908 for all dips, +i'.o. Subsequent to October, 1908, needles Nos. 5 and 6 

 of circle 172 and No. 7 of circle 178 were used; unfortunately the pivots of all three needles 

 developed rust pits in one or two places. From an examination of the results from com- 

 parisons and from a scrutiny of the field results, the correction adopted for the mean of 

 the three needles is o'.o; on the average, however, the results so obtained appear to be within 

 2' or less of the standard. The tabular designations are 171 12,171 (56), and 171 (567). 

 The correction for declination values by the compass attachment of circle 171, as adopted 

 from all the intercomparison data, is +2'. 5. 



Dover circle I'jz. Type (a); the property of the Department. It has been used over a large 

 portion of North America, from Central America and the West Indies to Canada and 

 Labrador. The corrections adopted for all values of dip for work prior to furnishing new 

 needle pivots and generally overhauling the instrument in the latter part of 1910 are: 

 Needle No. i, +i'.4; needle No. 2, 2'. 9, or for the mean of the two needles, o'.8. 

 During March and April, 1907, the deflection distance was increased about 5 mm. in order 

 that a more extensive use of the instrument could be made in the determination of total 

 intensity by Lloyd's method. The corrections for dips resulting from deflections of 

 needle No. 3 by No. 4 for work prior to March, 1907, are given by the formula: 



FM = +4'. 5 19'. 7 cos / 

 During 1908, for a few stations where very high dips could not be observed in the field work 

 in Canada, the results with needle No. 3 deflected were used for some of the stations, the 

 correction adopted being 1'.2. In 191 1, subsequent to the general repairs above men- 

 tioned, the corrections of the mean value from needles Nos. i, 2, 5, and 6, were from 

 standard earth inductor comparisons, +o'.i for dip 70 north, and s'.o for the dip 60 

 south. The tabular designations are: 172-12 or 172' 123, and 172- 1256. 



The correction for declination values by the compass attachment for the work prior 

 to the general overhauling of 1910 is +2'. 5; subsequently, o'.o. 



Dover circle 177. Type (a); the property of the Department. It has been used extensively in 

 Canada in 1906, Central America in 1907, Persia, Turkey in Asia and Europe, Arabia, 

 Egypt, Asiatic Russia, Austria-Hungary, Germany, and England, in 1908 to 1910, and in 

 Brazil and Peru since 19x0. It has been, perhaps, the most satisfactory dip circle used by 

 the Department. The corrections for the 1906 work in Canada, adopted from the com- 

 parison data then available, were: mean of needles Nos. i and 2, 1'.2; mean of needles 

 Nos. 5 and 6, 1'.2; No. 3 deflected by No. 4, I'.i, and No. 7 deflected by No. 8, 1'.6. 

 The corrections adopted from a careful study of all the available data from comparisons 

 with earth inductors available for observations and from a consideration of needle differ- 

 ences at field stations are as follows: 



The tabular designation is 177- 1256 or 177- 125637. 



The corrections for declination values by compass attachment are: prior to alterations 

 during the first part of 1908, +i'.5; subsequently, -f-o'.6. 



