74 RECORD AND RESULTS OF 



For the instrumental constants, see determinations further on. Wiirdemann's 

 prismatic azimuth compass reads from south through east to 360° ; the other small 

 compass reads from north to west. 



The magnetic observations were commenced by Mr. A. Sonntag; after his death, 

 in December, 1860, the care of the magnetic determinations devolved upon Mr. 

 H. G. RadclifF, who was assisted by Messrs. C. C. Starr and G. F. Knorr, and also 

 by the commander oi the expedition. 



The instrumental constants necessary for deducing the results for horizontal force 

 and for scale value of the differential observations were made by me m Washington 

 in June, 1862. 



The geographical positions and chronometer corrections required in the discussion 

 wiU be taken from the preceding astronomical paper (Part I of the scientific con- 

 tributions by the expedition) without further special reference. 



DIFFERENTIAL OBSERVATIONS AT PORT FOULKE. 



These observations were made at the observatory (of which a general description 

 has already been given) ; Dr. Hayes wrote to me the following note respecting the 

 mounting of the instrument. " The magnetometer was mounted in the centre of 

 the room upon a stand made of two kegs whose heads being removed, and the ends 

 carefully fitted together, were filled with beans and water. These were of course 

 soon frozen into a solid mass, and the lower keg being placed upon the solid rock 

 through a hole cut in the floor, the support for' the instrument was as firm as pos- 

 sible. No stove or other artificial means of warmth was at any time used." 



Diurnal Variation of the Magnetic Declination. — For the purpose of investigating 

 the diurnal march of the horizontal needle, hourly observations were recorded on 

 15 days, at Port Foulke, between November 26, 1860, and March 4, 1861. As 

 the diurnal excursions of the magnet frequently exceed the range of the scale 

 fastened to the telescope, the horizontal circle had to be shifted in order to bring 

 the direction of the magnet at all times within central range of the telescopic 

 scale ; the record consists therefore of readings o^ the azimuth circle and of readings 

 of the reflected scale. The observers are indicated by their initials, R. for E.adcliff', 

 K. for Knorr, and S. for Starr. 



The instrument having been properly adjusted, the following readings were 

 taken : — 



