6 Land Magnetic Observations, 1905-10 



belonging to the Zi-ka-wei and to the Hongkong observatories. Also, for certain 

 work in Africa executed for the Department by Professors Beattie and Morrison, 

 use was made of two instruments of the Kew type, one the property of the London 

 Royal Society, and the other of the Royal Observatory of the Cape of Good Hope. 

 This style of instrument has no theodolite attachment but instead an "azimuth 

 mirror"; since the use of the latter requires an accurate knowledge of the chronom- 

 eter correction to local mean time, a matter presenting difficulties in unsurveyed 

 regions and in exploratory work, separate theodolites were supplied for the astro- 

 nomical portion of the observations. 



Magnetometers 30 and 36, used during some of the earlier field work, are the 

 property of the United States Coast and Geodetic Survey, and are substantially 

 the Magnetic Survey of India pattern,^ but without the vertical circle, except that 

 36 has the dimensions of the magnets so modified that theoretically the second 

 distribution coefficient would be zero. This type is in a general way similar to the 

 Kew pattern, except that a phosphor-bronze ribbon suspension is used in place of 

 the silk fiber, and the short magnet is mounted in a stirrup above and parallel to 

 an aluminum collimator similar in its optical arrangements to the long or deflecting 

 magnet. For reading the lines of collimation of the magnets, instead of having a 

 graduated scale mounted in the magnet, there is a graduated glass diaphragm in 

 the reading telescope, while in the focus of the coUimating lens of the magnet there 

 is simply a piano-parallel glass with two lines engraved at right angles. In deflec- 

 tion work the deflecting magnet is protected against sudden changes of tempera- 

 ture by a wooden housing which, together with its centering arrangements and the 

 magnet, may be shifted from distance to distance. The brass deflection bar is of 

 one piece. This type of instrument is shown on Plate 2, Fig. 2. 



Three magnetometers of the type 3, viz., that of the Magnetic Survey of France, 

 have been used: No. 11, constructed for the Department; No. 21, the property of 

 the United States Coast and Geodetic Survey; and No. 24, the property of the 

 Zi-ka-wei Observatory. A detailed description of this type is given by Mascart 

 in his Traite de Magnetisme Terrestre} This type is very light and has a telescope 

 with vertical circle mounted eccentrically for astronomical observations, the magnet 

 suspended by silk fiber in a brass house being central on the same base used for 

 the astronomical work. The magnets are cylindrical steel bars of the dimensions 

 given in Table 1. The optical arrangements consist of concave mirrors attached 

 to the ends of the magnets, in the foci of which is a platinum scale, the reflection 

 being read by means of the reading telescopes; in the case of the short magnet, by 

 means of brass extension pieces mounted at the ends, the mirrors are placed in the 

 same relative position as in the long magnet. The magnets are very light and with 

 this type of instrument much trouble from variability of the torsion efi^ect of the 

 silk fiber suspension has been experienced. The type is shown on Plate 4, Fig. 1. 



The only universal magnetometer used in the present work was that built 

 after Eschenhagen's design, with some modifications, by Tesdorpf; it has the 



' Fraser, H. A. D., /. c. Also Hazard, D. L., I. c. (59-60.) 



2 Mascart, E. Traite de Magnetisme Terreslre. Paris, 1900. (206-219.) 



