8 Land Magnetic Observations, 1911-13 



attachments for dip-circle microscopes, vertical circle, reading lenses, clamps, etc. The 

 graduated vertical-circle and counterpoise plate are mounted in the magnet house at the 

 end of the telescope bearing and behind a piano-parallel glass window, somewhat larger 

 in diameter than the circle. The counterpoise proper, consisting of a cylindrical brass 

 weight, is attached to an arm moving with the upper works which carry the magnet house 

 and theodolite. 



The vertical circle is 101 mm. in diameter and graduated at 30-minute intervals, the 

 least count on the two verniers being one minute of arc, but estimations being easily and 

 definitelj'' made for one-half minute. It is fixed in position, the verniers moving with the 

 telescope. 



The magnet house is of well-seasoned mahogany, carefully finished, polished, and rein- 

 forced as necessary to prevent any warping. Plate 2, Fig. 2, shows the interior of the house 

 when the instrument is used for dip observations. 



The magnetometer telescope, of magnifying power 8, is supported in two wj'es secured 

 to the base in such a way that its line of sight is central. As will be seen in Fig. 2, Plate 

 2, the wyes are, for economy in packing-space, quite near the object end of the telescope, 

 which issuitably counterweighted. (Inthefigure, the telescope isshown in a vertical position, 

 as it is when not in use; when observing for declination, or when making the oscillation 

 observations, the telescope is, of course, turned into a horizontal position.) The counter- 

 weight serves the additional purpose of making possible the elimination of the objectionable 

 features of the hood arrangement in the theodolite-magnetometer. The end surface of 

 the counterweight is part of a sphere of large radius ; there is a spherical-surfaced depression 

 of like size and radius in the outer face of the end of the house ; the depression is lined with 

 velvet and so made that when the telescope is horizontal, or nearly so, the two surfaces 

 make a snug fit, thus preventing any currents of air entering the house. The window and 

 illumination-device at the south end of the house are somewhat modified over those of the 

 theodolite-magnetometer. The piano-parallel glass for keeping out currents of air, the 

 iris diaphi-agm, and the mirror and reflector are assembled together, the whole being 

 attached to the box and swinging on a hinge, so that when mark readings are desired the 

 clamping pin may be removed and the system thrown over to prevent interference; when 

 observations are being made it is of course put back into position and clamped. In order 

 to have the center of the magnet house free for the dip work, the stirrup as shown in Fig. 2, 

 Plate 2, is clamped in one corner of the house, and the suspension head is sufficiently 

 elevated to make the bronze ribbon nearly taut, and so out of the way of the needle. The 

 detail of the clamping arrangement by knee lever and spring, in the lower left-hand inside 

 corner of the house, will be seen from the figure. 



The supports for the agates of the dip circle, used for the needle bearings, are attached 

 securely to the base independently of the house. They are made of shape similar to the 

 telescope standard in order that there may be as little interference of parts as possible in 

 sighting on the needle ends. The needle lifter is operated with the aid of an eccentric and 

 lever, by a milled head on the south end of the house. The dip needles were made by 

 Dover, and are of the usual land pattern, except as regards length, being somewhat longer 

 in order that the ends may project beyond the vertical circle. The sighting microscopes 

 are a little unusual in that the object lenses are mounted inside of the magnet house and 

 fixed permanently to the counterpoise plate so as to move with the verniers, while the 

 remainder of the optical arrangements are mounted outside ; this arrangement has proved 

 satisfactory. It should be noted that no matter what position the needle may take it is 

 always possible to read the upper end; by observations made on either side, it is a com- 

 paratively easy matter to supply the data for the parts of the circle where the lower end is 

 obscured bj' interference of the supports. 



Attachments for total-intensity observations, according to Lloyd's statical method, 

 were provided only in the case of universal magnetometer No. 14, the deflecting needle being 



