SECTION II. 



MAGNETIC INCLINATION. 



Instrument and ReniarJcs. — The observations for dip were made by Mr. Sonntag 

 by means of a Barrow dip circle received from Prof. Henry, of the Smithsonian 

 Institution, through the courtesy of Col. Sabine. The inclinometer was supplied 

 with Lloyd needles, for determining the total intensity, but unfortunately the com- 

 plete record of these observations could not be recovered ; the absence of the record 

 for determining the constants necessary for_ their reduction being wanted, no use 

 could be made of these observations, even for relative intensity at Saikatle and 

 Marshall Bay, and the partial results given in Appendix XV., vol. II. of the Nar- 

 rative, must, therefore, remain fruitless for the present. There is likewise a defi- 

 ciency in the record of the dip observations at Van Rensselaer Harbor after 

 February 23, 1854; the results, however, are all preserved in the Appendix just 

 mentioned. 



In regard to the index error of the dipping needles, we can only make an 

 approximate comparison. The observations at New York, Avhere the dip has been 

 represented by the formula 



I=:72°.69 — 0.00491 (t— 184.5) + 0.00114(t— 1845)", 

 with a probable error of any single observation^ of ^l3'.3, would apparently pro- 

 duce a correction to needle 1 of — 9', and to needle 2 of — 14', the changes, how- 

 ever, from one station to another in the immediate vicinity of the city are much 

 greater, and these quantities may, therefore, as well indicate local deviation as 

 index error. The polarity of the needles has been reversed at each station, the 

 effect of this operation upon the resulting dip is somewhat irregular, and will be 

 found exhibited in tabular form. 



* See Coast Survey Report of 1856, p. 240. The formula includes dip observations taken between 

 December, 1822, and August, 1855 (exclusive of the observations of the present expedition). 



