586 REPORT— 1897. 



secular variations of the magnetic needle, widely held, and to advocate another 

 vrorking theory which seems to harmonise the system of magnetic observations in 

 a suitable manner. 



7. Observations at Toronto with Magnet Watch Integrator. 

 By Professor Frank H. Bigelow. 



8. The Yerkes Observatory. By George E. Hale, Director. 



The author gave an account of the buildings and instruments of the Yerkes 

 Observatory, with a statement regarding the first observations made with the 

 40-inch telescope. 



9. The Effects of Tension and Quality of the Metal upon the Changes in 

 Length produced in Iron Wires by Magnetisation. By B. B. Brackett. 



10. On the Susceptibility of Diamagnetic and Weakly Magnetic 

 Substances. By A. P. Wills. 



In the paper the author describes in detail a new method, applicable in the 

 experimental study of the magnetic properties of those substances in which the 

 coefficient of magnetic susceptibility is very small, and either positive or negative. 



The method is based upon the property which all bodies have to a greater or 

 less degree — namely, that they experience a mechanical force when placed in a 

 non-homogeneous magnetic field. This force acts to impel the body towards 

 stronger or weaker parts of the field, according as the body is magnetic or 

 diamagnetic. 



By means of a large electromagnet a practically uniform field is obtained, 

 at least sufficiently uniform to suit the purpose to which it is put. The magnet 

 is so designed that the pole pieces face each other. They are prismatic in form, 

 and the surfaces are about 1| x 8 cm., and there is a space of about 1|^ cm. between 

 them. The long edges of the pole pieces are placed horizontally. The body to be 

 investigated is made in the form of a thin slab. The dimensions of the slab are 

 about i X 4| X 8 cm. It is suspended, by means of a long wire, from one end of 

 the beam of a delicate balance, and with the 4^ cm. edges horizontal and parallel 

 to the pole faces and the 8 cm. edges vertical. The vertical direction is called Z ; 

 the horizontal direction parallel to the pole faces Y, and that perpendicular to 

 the plane of these two X. The lower Z face of the slab is placed in the horizontal 

 plane of symmetry of the pole pieces. 



The conditions of symmetry show that there will be no mechanical force 

 acting upon the slab save in the Z direction. The balance serves to determine 

 this force, which is called P. Theoretical considerations show 



P = ''4'''(H=-H,=), 



where A is the area of one of the Z surfaces, H is the strength of field at lower 

 Z surface, Hq that at upper Z surface, k the coefficient of susceptibility, defined by 





47r\ 



where juj is permeability of air, and n„ that of slab. Hq- in comparison with H'^ is 

 found in practice to be negligible. If fi^ is put equal to unity, then 



2P 

 ""AH^" 



H is determined by measuring the force exerted by the field upon a conductor of 

 known length when placed in the field at the proper position, and through which 



