168 Proceedings of the Eoyal Society 
the angles of torsion of the single wire and movement of the weight 
are constant. This second method I had at first rejected;, as ex- 
perience had shown me the difficulty of changing the weights 
without affecting the stability of the instrument. I desire now to 
note that I devised, about a year ago, and communicated to different 
men of science, a method of varying the weight, suspended with 
the greatest delicacy, and without jar to the instrument. This 
method consists in suspending from the weight a metallic wire, 
which enters a piece of barometer tube fixed below the instrument : 
by means of a screw entering a cistern below the tube, mercury (or 
another fluid, can be forced into the tube so as to immerse the 
metallic wire in the fluid ; the wire being properly chosen as to 
fineness, or as to its specific gravity compared with that of the 
fluid, and the height of the fluid being read to a thousandth of an 
inch as in the barometer, the weight suspended can be made, by 
turning the cistern-screw, to vary gently and gradually, by as 
minute a quantity as we please ; while the eye is occupied with the 
coincidences of the telescope wire and its images, which indicate 
the constancy of the angles of torsion of the single and double 
wires. 
Although the difference of the specific gravities is considerable, 
I propose to employ iron for the wire and mercury for the fluid. 
It has occurred to me that this method of varying a weight might 
he of use in other branches of research. 
12. On the Diurnal Variation of the Magnetic Declination 
near the Magnetic Equator, and in both Hemispheres. 
By John Allan Broun, F.R.S., late Director of the Obser- 
vatory of His Highness the Maharajah of Travancore, 
G.O.S.I., at Trevandrum. 
