Descriptions of Instruments 9 



mounted on suitable supports attached to the telescope, which is at right angles to the line 

 of sight of the microscopes. Magnetometers 19, 20, 21, and 22 of this type were not thus 

 provided. For, as the result of field experience with No. 14, the thickness of the web of the 

 standard supports was increased in these instruments ; this made the distance available 

 for the use of the total-intensity method too great for the dip needles of the size that could 

 be used. Magnetometer 14 is supplied with two pairs of dip needles and two pairs of 

 intensity needles. Nos. 19, 20, 21, and 22 have each two pairs of dip needles. 



Plate 2, Fig. 3, shows the instrument mounted on the tripod, and with the deflection 

 bar inserted for deflection observations. 



Magnetometer 14 is equipped with a tripod having legs of the telescoping pattern; 

 this style of tripod, unless carried in the hand, is not entirely suitable for packing, as the 

 numerous screws and nuts are quite likely to be bent during transportation. Magneto- 

 meters 19, 20, 21, and 22 accordingly have been equipped with tripods somewhat similar in 

 pattern to that usually supplied with dip circles manufactured by Dover. This type is not 

 much heavier than the telescoping pattern and, because of its simpler construction, is a 

 superior tripod for general transportation. 



The inside dimensions of the instrument cases for magnetometers of the type 4(6), 

 except for slight variations with individual instruments, are about as follows: length 48 cm., 

 width 25 cm., depth 21 cm.; the outside dmiensions are about 2 cm. more each way. The 

 weight of the instrument with all appurtenances, but exclusive of the case, is about 7 

 kilograms; the case weighs about 6 kilograms, so that the total weight of instrument and 

 case is about 13 kilograms. The weight of the tripod and deflection bar is about 4 kilograms. 



COMBINED MAGNETOMETER AND EARTH INDUCTOR. 



Owing to the difficulties generally experienced in the satisfactory determination of 

 the corrections of dip needles on adopted standard and the possible changes in these 

 corrections with geographical variation in magnetic intensity and inclination and with wear 

 of pivots of the needle, it was found desirable to improve further the universal magnet- 

 ometer of the Department of Terrestrial Magnetism by substituting for the dip circle an 

 earth inductor. The comparisons made with earth inductors by the observers of the 

 Department in various regions of the globe have been far more satisfactory than the dip- 

 circle comparisons. Accordingly an instrument combining the earth inductor with the 

 magnetometer was designed and constructed in the shop of the Department. The instru- 

 ment consists essentially of four parts: (a) the base, (b) the magnetometer attachment, 

 (c) the earth-inductor and theodolite attachment, and (d) the galvanometer. The effort 

 has been to secure a compact, portable instrument, with a minimum number of loose 

 accessories, for rapid field work of high precision. 



The base (see Plate 3, Fig. 1) is of the usual double-center tyiae. The horizontal 

 circle is 102 mm. in diameter, with graduations at 30-minute intervals and with two 

 verniers, the least count of each vernier being one minute of arc; the graduation is such, 

 however, that estimations to one-quarter minute of arc can readily and definitely be made. 

 The base is supported by three foot-screws. 



The magnetometer attachment (see Plate 3, Fig. 1), including magnet system and 

 suspension, is similar to that of the theodolite-magnetometer of type 1(6) described in 

 Volume I. A small improvement over the type there described has been made by the 

 elimination of the hood connection between the magnetometer telescope and the magnet 

 house. The hood has been replaced by an attachment with concave, plush-lined spherical 

 surface attached to the magnet house, and against which an adjustable cap with corre- 

 sponding convex spherical surface mounted at the object end of the magnetometer tele- 

 scope is in contact when the telescope is horizontal. This permits free vertical motion of 

 the telescope when making sightings on marks slightly elevated or depressed and, at the 



