ON THE KEW OBSERVATORY. 351 



with a view of adapting it to the course of experimental trials, as well as its 

 wooden supports and other wooden appliances, and the use of a mirror (as 

 above) would permit. It was well known to Col. Sabine and to other gen- 

 tlemen of the Kew Committee, &c., before those trials began, that although 

 a long series of good curves on paper had been produced in 1 846, and some 

 exhibited at the Royal Society in 1847, yet no expectation of accuracy ap- 

 proaching to that obtained by the otlier two magnetographs, as regards di- 

 stinctness of the curves, &c,, was ever entertained. It may be also remarked 

 that its locality is a passage room, and is used for several other purposes. 



The arc value of 1 inch or 50 divisions of the ordinate scale for this in- 

 strument is, when corrected by the torsion coefficient, 28'71 {vide p. 362, 

 post.'). 



The time of transit of a given point of the photographic surface over the 

 mouth at (c') is about 5\ minutes (as formerly ascertained). 



The Horizontal-Force Magnetograph, placed on the .stone brackets solidly 

 attached to the great quadrant wall (and descx-ibed in the Report for 1849, 

 &c.), has undergone the following alterations : — 



A damper, 3 inches broad, ^ inch thick, and having its interior horizontal 

 surfaces about 1 inch apart, has been substituted for the former damper (6''), 

 about f inch broad, with its interior surfaces about I5 inch apart. This va- 

 riation was kindly suggested by Dr. Faraday on a visit to Kew, and has 

 proved (as he anticipated) a very profitable one. The magnet, which always 

 performed well, has been rendered remarkably steady by this improvement. 



A new thermometer (by Gary), the tube of which passes through a stopper 

 in the top of the case, has been affixed. 



Screens have been placed between the lamp and the cases for protection 

 of the magnet against the h<|at of the flame. 



The sliding-frame (H) has been treated in exactly the same manner as 

 that of the declination magnetograph. 



The arc value of] inch of the ordinate scale is 109*4. 



The diameter of the pulley (at «") is 0-464 in. 



The distance of the ends of the wire (at S) is 0*413 in., and the angle of 

 torsion 64°"45. 



The temperature corrections on December 14 were, at 55°*3 = 0'000312, 

 and at 76°-2=0-000344. 



The time of passage of a point of the photographic surface over the mouth 

 e^ is about 1^ minute {vide p. 360, post.). 



A new Vertical-force Magnetograph has been constructed and fixed upon 

 the three corbels (already mentioned) on the quadrant wall. Its magnet is 

 nearly perpendicular to the plane of the astronomical meridian, the locality 

 not, at present, permitting a more favourable disposition. 



This instrument is, in most respects, similar to the vertical-force magne- 

 tograph which was sent to the Toronto Observatory, and described in the 

 Report for 1850. 



The following little variations and additions have been made : — 



The four brass pillars (Q, fig. 1, Plate JI. of that Report) have been tem- 

 porarily removed, the intermediate corbel sustaining the magnet, &c. con- 

 tained in the case V *. 



* The slab X rests upon the two corbels which are used instead of the piers P^ and P", 

 fig. 1, Plates I. & IV., Report for 1849 ; and the slab R, fig. 1, Plates I. & II., Report for 

 1850, was at first supported by the strong bolts, nuts, &c. of the pillars Q, only very near to 



