228 
MR. HEARN ON DISCREPANCIES OBSERVED 
pendicular to the rod, would be exceeding small, and might possibly by some con- 
trivance be altogether counteracted. Under such an arrangement the only effective 
force would be that of gravitation. I would also do away with the planks, support 
the masses on square blocks of wood, and transport them from the positive and 
negative positions by means of wooden tram-roads parallel to the torsion box, so that 
all motion round an axis might be avoided. 
It has also struck me that by suspending the rod by means of a single silk thread 
having little or no power of torsion, and by having very delicate hydrometers at each 
end of the torsion rod, the stems of which are attached to very fine hairs or silken 
threads passing over small fixed pulleys in the horizontal plane of the torsion rod, the 
horizontal portions of the hairs or threads being affixed to the ends of the torsion 
rod, the experiments might be rendered purely statical. Supposing the hydrometers 
just floating in their position of equilibrium when the rod is in its zero position, and 
those hydrometers on contrary sides of the torsion rod opposed to the masses, it is 
clear that when the rod moves towards the masses, and raises the hydrometers 
above their position of equilibrium, that the tensions of the threads would increase, 
and vary as the angle through which the torsion rod has moved. These forces of 
tension would ultimately be in equilibrium with the gravitating action, and by ob- 
serving this position of equilibrium the force of tension and therefore that of gra- 
vitation would become known. There may be practical objections to this arrange- 
ment, but I am of opinion that by using proper substances for the stems, See. of the 
hydrometers, and proper care in the manufacture of the small pulleys, they might be 
overcome. 
After my views were fully matured I had some correspondence with Sir John 
Herschel, to whom I in part detailed them, and with the kindness and urbanity 
which so eminently distinguish him, he undertook to lay my communication before 
the Royal Society: in one of his letters he thus expresses himself. 
“Very many years ago, immediately after the publication of a joint paper by Mr. 
Babbage and myself on the magnetic action of revolving copper discs, &c. on mag- 
nets, a course of experiments suggested itself to me which, for want of a proper locale 
where I could establish an apparatus of considerable dimension and great delicacy 
out of the way of currents of air, I did not execute, a thing I now much regret. 
“ The plan of these experiments was to 
attach to the two arms of a long torsion ba- 
lance two magnets, a stronger and a some- 
what weaker; thus A being the weaker and 
B the stronger, A horizontal and B so in- 
clined as precisely to counteract and destroy 
the directive power of A (by forming ‘neutral couples’), the line AB being in the 
magnetic meridian, which would always be practicable as the superior power of B 
