NO. 6.] INTRODUCTION. INSTRUMENTS. IX 



A small altazimuth by Olsen. The circles have diameters of 10 cm. 

 and are graduated to half degrees, two opposite verniers giving the single 

 minutes. The relative movement of circles and reading-apparatus is the same 

 as in the larger instrument. A fixed level, placed parallel to the vertical 

 circle, was read off on measuring altitudes, but its position was not such 

 that it can be considered as an alidade-level. It is divided from the middle 

 in parts of 0'.8. The telescope, whose axis is broken by a reflecting prism, 

 has an aperture of 2 cm, a focal length of 16 cm., and a magnifying power 

 of 12. The wires in the focus consist of one horizontal and two vertical 

 lines (about 4' apart) engraved on glass. 



This instrument was not much used on board but followed Mr. Nansen 

 on his sledge expedition. 



A sextant by Hechelmann, of the usual construction, giving the angles 

 to 10". Usually the altitudes were measured from the ice as a natural 

 horizon or over a basin of mercury as an artificial horizon. On some few 

 occasions a glass horizon by Negretti & Zambra was used; the level which 

 was read off in its two positions, when placed in the vertical plane of the 

 celestial object, was divided to 2'.6. There was also another glass horizon 

 with aluminium mounting, by Porter, which was only used 1893 September 

 28 and October 2; its level was divided to 3'.9. Occasionally a trough of 

 tar or a pool of water on the ice was used as an artificial horizon. 



The astronomical telescope was almost invariably used. 



A small pocket sextant by Gary, London, was used by Mr. Nansen on 

 his sledge expedition. The limb, of radius 4.5 cm., is divided to half degrees, 

 the vernier giving single minutes. The instrument is made of aluminium 

 which did not, however, prove to be a good metal for this purpose, the 

 screws becoming immoveable from oxidation. 



Several compasses, among them an azimuth-compass by Hechelmann 

 with 8 small needles suspended by silk wires. The card, divided to degrees, 

 has a diameter of 21 cm. The reading of the card was always made both 

 ways, the eye being held in the plane through the vertical and the horizontal 

 wire of the diopter-apparatus, either before the thread or before the slit. 



