Stresses in a Magqneto-static Field. 399 
The advantages claimed for this form of vertical-force 
magnetograph are :— 
1. The elimination of the knife-edge, and hence the absence 
of irregularities introduced by the presence of dust &c. on the 
supporting plane. Also the needle does not move in azimuth 
owing to tremors of the supports. 
2. The number of materials and separate parts entering into 
the composition of the movable system is small, and, with the 
exception of the steel of the magnets, they are unoxidizable 
and unaffected by impurities in the air. 
3. The method of compensating for temperature is simple, 
and does not involve a complicated counterpoise likely to get 
displaced. 
4. The moment of inertia of the movable system is small 
although the magnetic moment is considerable. 
5. The mirror cannot be distorted by its mounting, nor is 
there any likelihood of the position of the mirror with refer- 
ence to the magnets being variable with the temperature. 
A second instrument has been constructed according to 
this design, the cost being defrayed by the Government Grant 
Committee of the Royal Society ; and it is proposed to set 
up this instrument in the new Magnetic Observatory belonging 
to the National Physical Laboratory. 

XLVIII. On Stresses in a Magneto-static Field. By GroreE 
W. Watker, J0.A., A.R.C.Sc., Fellow of Trinity College, 
Cambridge, Lecturer on Physics in the University of Glasgow*. 


HE usual methods of analysis lead to a system of stress 
in a magneto-static tield which is exactly analogous to 
the Maxwellian system of stress in an electrostatic field. The 
stresses may thus be spoken of as stresses of the electrical 
type. 
In his work entitled ‘Aberration and the Electromagnetic 
Field, p. 76, Mr. G. T. Walker obtains a system of stress in 
a magneto-static field which differs from the stresses of elec- 
trical type in the superposition of a hydrostatic pressure 
, - & =} H?. Iam not concerned here with the analysis 
OT Fo 
by which this result is obtained; but on page 79 et seg. 
Mr. Walker discusses an experiment due to Quincke, which 
he considers crucial between the two views. 
The experiment was as follows:—A thermometer-shaped 
yessel with a capillary tube attached was filled with a solution 
* Communicated by the Physical Society: read February 26, 1904. 
2K 2 
