362 REPORT — 1851. 



The daily Daguerreotype impressions have been almost equal in sharpness 

 to those of the horizontal-force instrument. But there still exists some im- 

 perfection (probably in the knife-edge) requiring attention. 



Declination Magnetograph. 



In March 1851, some of the improvements, described at p. 350, ante, on 

 this old instrument {vide PI. XI. Phil. Trans. Pt. I. for 1847), were exe- 

 cuted here. 



On the 26th it was carefully examined. The wedges were tightened and 

 the camera (A), &c. were adjusted, by turning them through a small arc upon 

 the axis of the suspending skein, so as to cause the image of the zero slit in 

 the fixed shield to occupy its proper place at the mouth (E) on the ground 

 glass scale (or on the silver plate or Talbotype paper)*. 



On the 27th, 29th and 31st, further examinations and adjustments were 

 made. The suspending skein seemed to have remained nearly unaltered, 

 since 1846, as to the condition of its fibres : a torsion of 10° only existed 

 (which was eliminated in the computations), but the effect of a twist given 

 to it =90° of the torsion circle displaced the magnet a quantity =44°. The 

 distance from the axis of motion to the moveable shield (in the place of b^) 

 was 18 inches. The magnifying power of the lenses was =6706 times (as 

 ascertained by the means described at p. 352). The arc value of 1 inch of 

 the ordinate scale, when corrected by the torsion coefficient, was 28°*71. 



On April 1 at m. it was put into activity for the " trials," and, in the course 

 of this month, several little improvements (not affecting its indications) were 

 made. 



On the 29th it was found that, in consequence either of the fixed shield 

 having descended or of the moveable shield having risen, light had passed 

 between the edges of them to the photographic plate. The delay of a day 

 was therefore required, in order to remedy the evil, by lengthening the zero 

 slit in the mouth-piece, and lowering the moveable shield a very little. These 

 expedients answered the purpose at that time, but similar faults required se- 

 veral repetitions of the last-mentioned process afterwards ; therefore, 



On June 14, the wooden bearers (X) were raised, and four perpendicular 

 supports of straight-grained deal, resting upon the rafters belo'w, were placed 

 under them. This expedient will not, probably, secure the instrument against 

 errors arising from the use of wood for hearers, fittings, &c., and perhaps the 

 expansion and contraction of the skein. 



It has nevertheless generally presented us with curves, measurable to 

 -i-dth of an inch at least. The exceptions occur in a few instances of 

 magnetic disturbance. The damper (a narrow one composed of wood with 

 a coat of copper electrotyped upon it) seems to have little effect. 



Tabulating and Tracing the Magnetic Curves. 

 Colonel Sabine has in his Report {ante) clearly and ably described the 

 nature and modes of procedure which have been carried out, by Mr. Welsh, 

 relative to these operations ; and the apparatus employed has been detailed 

 at p. 84 of the British Association Report for 1849. 



* In 1846 a similar kind of adjustment was made, which suggested the possibility of adapt- 

 ing apparatus of telescope, &c. to a metallic camera, &c., which might be moved about the 

 axis of the suspending skein in the manner of Colonel Beaufoy's variation transit-box, and 

 thus become a means of procuring self-registered curves of absolute determination, if it were 

 desirable. 



I 



