Instrumental Equipment of Parties 11 



microscopes; the smallest graduation of the vertical circle is 10 minutes of arc and 

 its diameter 15.5 cm. Suitable commutator arrangements are made and connec- 

 tions provided for wiring to the galvanometer; the latter may be made very sensitive 

 by the use and adjustment of control magnets. The diameter of the horizontal 

 circle is about 24 cm. and the graduation to whole degrees, while the least count of 

 the vernier is 5 minutes of arc and settings may be made by estimation to one-half 

 of this amount. Suitable levels are provided for the purpose of determining the 

 relation of the axis of the coil when vertical to the line of sight of the microscopes, 

 which are fixed in position. For a specimen showing the method of observation 

 see page 41. In the case of No. 2 the vertical circle is protected entirely by a 

 cover, while in No. 48 it is freely exposed ; this modification was made by reason of 

 the rapid tarnishing of the graduation which results after short use of the instrument 

 when the graduation is exposed. 



Accessories. 



Each observer, besides being supplied with some form of the magnetic instru- 

 ments already enumerated, is furnished with the following accessories: (a) a half- 

 second pocket chronometer and three high-grade watches; (b) non-magnetic 

 observing tent with cover, tripod stubs, and other appurtenances; and (c) mis- 

 cellaneous accessories such as pocket compass, extra thermometers, aneroid, boiling 

 point apparatus, measuring tape, camera, forms, etc. 



The observing tent is of the pyramid type with two beams for spreading the 

 canvas near the top, and one center pole which is in two pieces held together by a 

 brass sleeve. The total height of the tent is 2.8 meters, the sides at the bottom 

 when set up about 2.6 meters, while at the spreaders where the height above the 

 ground is about 2 meters, the sides are reduced to 1 meter. To facilitate sightings 

 on marks and for illumination purposes, the tents are provided on two sides with 

 openings; they are set up with these open sides placed approximately in the magnetic 

 meridian. In order that the center pole may not interfere with the instrument 

 during observations, it is placed somewhat eccentrically as regards position of the 

 tripod. Plate 8, Fig. 1, gives the general appearance of the tent when set up. 

 Occasionally light-weight non-magnetic wall tents have been used, but on the whole 

 the tent just described has been found preferable and is generally used, being 

 considerably lighter and much more easily and quickly set up than the wall tent. 



The additional camp, packing, and other temporary equipment which may be 

 necessary is usually obtained locally by the observer according to his specific 

 needs. To insure freedom from possible disturbing influences, the observer is in 

 no case permitted to make use of a combined living and observing tent. 



The items listed under (c) hardly call for description except perhaps in the 

 case of the boiling-point apparatus used on some of the expeditions. The pattern is 

 essentially that described and figured in the British Antarctic Mmiual, 1901, on 

 page 94. It consists of a suitable copper boiler with a glass steam chamber, the 

 thermometer being inserted in the latter and read entirely in the steam. An 

 alcohol lamp is used for heating the water. The mercurial thermometers provided 



