114 MR. RONALDS ON PHOTOGRAPHIC SELF-REGISTERING INSTRUMENTS. 
a silken skein of the same kind as that used for suspending the Greenwich declina- 
tion magnet. The damper surrounding the magnet is of mahogany coated (by 
means of the electrotype process) with pure copper, and the mode of suspension is 
essentially similar to that of Coulomb. The length of the skein from the magnet to 
the point of suspension is nine feet, and the detorsion plate is supported by braced 
frame-work, fixed firmly by brass bolts upon the two pillars which formerly carried 
the Kew transit instrument. The interior and exterior cases containing the magnet 
are coated entirely with gold paper, as are those of the Greenwich magnets. 
A light conical brass tube projecting about six inches beyond the north end of the 
magnet, is affixed to the lower side of the stirrup which carries the magnet. Into this 
end slides (for adjustments) a small cylindrical tube, and from the extremity of the 
latter descends, vertically, a very small blackened wire (called the index) which 
passing through slits (long enough for its free motion always) cut through the 
bottoms of the cases, takes the place of the above-described electrometer, &c. in a 
lucernal microscope below. 
The lucernal microscope is in this instance much longer than before, in order that 
the motion of the index may be considerably magnified in its image, without sensible 
error of aberration, &c. The long case (now T vertical) with its sliding frame, &c., 
have the usual form and dimensions ; and the clock, with brass pendulum, weight, 
& c., is placed at a proper distance from the magnet. 
I have taken great pains to prevent internal and external currents of air, but the 
magnet is far from being as steady as the Greenwich magnets, particularly during 
strong westerly and north-westerly winds. If it were as little subject to mechanical 
disturbances as the Greenwich magnets, 1 feel quite sure that the photographs would 
present as sharp outlines as those of the barometer, electrometer, &c. 
Concerning the impressions which have been submitted to the Astronomer Royal’s 
inspection, and compared with the readings of the Greenwich declinometer, I am 
permitted kindly by Mr. Airy to say, that the agreement of those results with such 
readings is highly satisfactory. 
Postscript. — January 7, 1847* 
Since writing the above I have felt strongly impressed with the great advantage 
which would result in these kind of registrations of the barometer, if that instrument 
could be rendered accurately self-corrective for temperature ; and I have been much 
occupied with a project of this kind (promising very fairly). It consists in applying 
a solid metallic thermometer in such manner as to cause the whole barometer to 
descend exactly as much as increments of temperature cause the mercury to ascend. 
