352 REPORT— 1851. 



An improved mode of attaching the diaphragm plate O, figs. 1 & 2, to the 

 slab X has been adopted, whereby the horizontal distance between the two 

 shields 6* and o' is now very easily adjusted. 



A screen has been (occasionally) interposed between the lamp and the 

 camera. 



A new long-tubed thermometer (by Gary) has been affixed to the magnet 

 case. 



The sliding-frame H has been treated in the same manner as that of the 

 declination magnetograph (vide p. 350, ante). 



The temperature corrections of this magnet were, on December 14, 1850, 

 at 50°'4.= 0-000283 and at 7l°-5=0-000319. 



The time of vibration (with its shield arm l^ and all other appendages) in 

 a horizontal plane was, — March 31, at a temperature of 52°, =17'9 seconds, 

 and in a vertical plane at 53°, =20 seconds ; but its time of vertical vibration 

 has been found several times altered since that day. 



The arc value of 1 inch or 50 divisions of the ordinate scale is 76'23. 



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

 (e') is 1^ minute {vide p. 361, post.). 



Apparatus used in ascertaining arc values of the ordinates of the curves 

 and errors from distortion, &c. has been made. It is of a very simple kind, 

 but may be said to consist of four parts. 



The first is a plate of transparent glass, about 3| inches long and fths of 

 an inch broad, supported by a pair of pillars, &c., and having a scale etched 

 upon it divided to j^V''^ of an inch. 



The second is a plate of glass, ground to semi-transparency of similar 

 dimensions, fitted into a frame and similarly divided. 



In making use of these two to ascertain the magnifying power of the lenses, 

 the transparent scale is placed with its undivided face in contact with the 

 fixed shield (o*) of all the plates of magnetographs, its scale occupying the 

 locus of the slit itself in the moveable shield (6') (if the magnet, &c. were in 

 position), and the semi-transparent plate is placed, with its undivided face, in 

 contact with the mouth-piece (E), its scale occupying the locus of the Image 

 of the slit in the moveable shield (the thickness of both plates being properly 

 adapted to the purpose). 



The lenses (of G) are then adjusted to bring the image of the first-men- 

 tioned scale into focus as nearly as possible upon the second scale, so that 

 the image of the first scale and the second scale itself can be conjointly and 

 scrupulously examined by the microscope {/'') and the corresponding read- 

 ings noted. 



The third part is merely a long scale, &c. which is employed to measure 

 the exact distance between the common axis of motion (of the magnet, the 

 shield-arm, Sec.) and the moveable shield. 



The fourth is (applicable to vertical force instruments only) for measu- 

 ring the radius from the knife-edge (at b^) to that part of the slit in the 

 moveable shield which produces a corresponding spot of light at the mouth 

 (e*) upon the second glass plate. 



one of the above-mentioned two corbels. The new short corbel is a substitute for the short 

 pier ?•••■, fig. 1, Plate I., Report for 1850, but this substitution has been here found unne- 

 cessary {vide p. 561, post.). The original plan of supporting this kind of instrument, i. e. the 

 method described in the Report for 1850, where the short pier and the pillars are used con- 

 jointly, and where no pier sustains the extremity of the slab X, is, however, far preferable 

 perhaps. 



