abstracts: terrestrial magnetism 207 



netometer for astronomical work and the determination of magnetic 

 declination and horizontal intensity; in general principles along usual 

 lines but differing greatly in details. The second type is a universal 

 magnetometer by means of which the three magnetic elements may be 

 determined as well as the astronomical elements. 



The magnet and suspension systems of both types are of like design. 

 The magnets are true cylinders encased in gold-plated brass cylindrical 

 sheaths to prevent rusting and to provide means of mounting the 

 collimating optical system in order that the magnets may be left as 

 perfect cylinders. The suspension systems are comparatively free from 

 complex or intricate parts, all devices for clamping the phosphor-bronze 

 ribbon used for suspension being such that they are not removable and 

 may be manipulated with great ease. For the determination of the 

 plane of no torsion there is attached permanently to the stirrup a grad- 

 uated circle which may be brought into the focus of the magnetometer 

 telescope (previously focused on a distant mark) by interposing an 

 auxiliary lens; the graduations of the torsion circle are in divisions of 

 two degrees so that the plane of detorsion may be rapidly determined 

 within a fraction of a degree. 



The theodolite magnetometer weighs with its case but 11 kilograms, 

 which is about half that of the older forms of corresponding instrument 

 used by this Department. 



The universal magnetometer is completely assembled in one unit, 

 comprising astronomical telescope and magnetometer for determina- 

 tion of magnetic declination and horizontal intensity, and dip circle 

 for the determination of inclination and total intensity, and is there- 

 fore always ready for immediate use upon mounting on its tripod. The 

 advantage of being able to proceed with the observations for any of 

 the elements, without first assembling a number of parts, is evident 

 and of peculiar value in taking advantage of every available opportunity 

 for work. The weight of this instrument without its packing case is 

 1\ kilograms; the case weighs about 6 kilograms so that the total weight 

 is only about 13 kilograms. 



The results of comparisons with the new instruments, both at the office 

 and under field conditions, have clearly shown that the degree of pre- 

 cision obtainable with them is of the same order as thai to be had by 

 types of standard instruments previously used by the Department and 

 other magnetic organizations, or, with care, an absolute accuracy of 

 0.5 minute in declination, 0.0002H in horizontal intensity, and about 

 one-half minute in inclination. •'• - v - ' • 



