ON THE KEW OBSERVATORY. S59 



hitherto been, by substituting one of the smaller pulleys, i. e. No. 9 (s^, Plate 

 IV., Report for 1849), diameter 0*'t64 inch, for that before used, i. e. No. 12, 

 diameter 0"614 inch, and by making the interval at the upper ends of the 

 wire(s) less than this quantity. A still smaller pulley (i. e. No. 8) had been 

 tried with parallel wires, and found to be too small (thus used) to allow the 

 magnet to be placed at right angles to the magnetic meridian. The adjust- 

 ments were made by ^leans of the torsion apparatus (S), and by using the 

 image of the slit in the moveable shield (6'), projected upon ground glass at 

 the mouth-piece (E). 



On September 2, a very unsatisfactory mechanical agitation of the magnet 

 was evinced by the photographic curve ; and on the 5th some additional 

 precautions were taken to prevent the interference of air-currents with its 

 natural motions. 



On September 7, the instrument having still exhibited symptoms of dis- 

 turbance, evidently not magnetic, was rendered less sensitive, by placing the 

 upper ends of the wire (s^) at an interval of 0*51 inch from each other. The 

 angle of torsion was thus reduced by 21°, 19° more than it had been before 

 August 31. 



On September 19 and 20, new adjustments, &c. were made. The angle 

 of torsion was fixed at 52° 25', and the arc value of 1 inch, or 50 divisions 

 of the ordinate scale (fig. 2, Plate XXI.), was found to be 109'*G ; or the arc 

 value, in parts of the whole horizontal force of the ordinate for j-Vth of an 

 ^ inch, was =0'00049. On this occasion " a scale was drawn on gelatine paper, 

 one division being made equal to ^^j''^ °f ^"^ inch, and placed beside the 

 ground surface of glass in the camera sliding-frame" (H). 

 On the 26th, a little adjustment of the shields took place. 

 In October it was worked with some regularity. [A specimen of October 

 16 is reserved.] 



On the 5th it was found that the time of the passage of a given point on 

 the Daguerreotype plate over the aperture of the mouth (at E) was about 2 

 minutes*. 



On November 1, it was minutely inspected by Colonel Sabine and some 

 further adjustments were made. 



On the 15th, Dr. Faraday visited the Observatory and recommended the 

 use of the very broad damper alluded to at p. 351, ante. 



On December 14, experiments were made in Dr. Lloyd's manner for de- 

 termining its temperature corrections ; when eight observations at the mean 

 temperature of 55°'3 gave 0*000312, and eight observations at 76°'7 gave 

 0-000344. 



On January 10, 1851, the new broad damper was affixed. 

 Between January 20 and March 20 it was worked pretty constantly. 

 New adjustments and determinations of its coefficients were made on the 

 latter day, when the magnifying power of the lenses was found to be 3*46 

 times (by means of the apparatus described at p. 352, ante), and the distance 

 from the axis of motion of the magnet, &c., to the moveable shield (b^) was 

 ascertained (by direct measurement) to be 9'08 inches ; consequently the arc 

 value of 1 inch of the ordinate scale was, as stated, 109'-4 (p. 351, ante). 



At the end of March the screens were attached at the object-end of the 

 camera, and a tliernioraeter (by Adie) was inserted in the magnet case (V). 

 The temperature of the air of the magnet case without these screens was 



* Which is a very much longer period than that formerly occupied hy such transit. 



