222 report — 1859. 



a wooden cross-piece which fits into a large earthenware dish, and is capable 

 of being roughly levelled by means of screws which support the dish. 



It is raised about an inch above the bottom of the dish. A solution con- 

 sisting of — 



Distilled water 8 ozs. 



Acetic acid 1 drm. 



Old exciting bath 4 drms. (or 1 drm. of solution of 



nitrate of silver 100 grs. to 1 oz.) 

 Gallic acid solution .... 1| drm. (see art. 48) 



is poured upon the plate, and the exposed sheets floated side by side upon it. 

 The time required for this operation varies from two hours in summer to six 

 or eight hours in winter. 



Note — In dull weather, the sensitive paper above described may be used 

 with advantage for printing copies of the curves — requiring an exposure of 

 only a few seconds to diffused daylight. 



Section IV. On the Method of ascertaining the Instrumental 

 Coefficients, tabulating from the Curves, etc. 



1. Declination Magnetograph. 



In this instrument the distance between the centre of the mirror and the 

 registering cylinder is 6'5 feet, and consequently a change in the position of 

 the dot of light on the sensitive paper, equal to one inch, denotes a change 

 of 22'*18 in the position of the magnet. 



The mirrors are so arranged that the moveable dot is north of the fixed 

 dot on the cylinder (see Plate 3. fig. 1) ; an increase of declination therefore 

 will bring the two dots nearer together, while a decrease of declination will 

 have the opposite effect. 



Should the suspension thread be without torsion altogether, or should its 

 torsion remain constant, the same distance between the two dots of light will 

 always denote the same absolute declination ; so that if by any means we 

 know the absolute declination corresponding to a given distance between the 

 dots, we shall be able to tell what it is for any other distance, or, in fact, for 

 any moment of time. 



The comparability with one another of the various tracings afforded by the 

 instrument, depends on the constancy of the torsion ; should this vary, the 

 curves are no longer absolutely comparable. Great attention should therefore 

 be paid to secure, if not an entire absence of torsion, at least a constancy in 

 the little that remains. 



The thread should be well freed from torsion when the magnet is suspended : 

 by slightly rubbing it with bees-wax, or by some other similar process, it 

 should be rendered less susceptible to hygrometric influences, and a dish of 

 chloride of calcium should be kept under the glass shade to absorb all moisture. 

 When the magnet is in perfect adjustment, there can be no objection to seal 

 the shade to the marble slab all round with bees-wax, at least if an ordinary 

 loosely fitting shade be used. 



Besides all this, it is necessary to make at least every month at some spot 

 free from the influence of iron, observations of absolute declination, noting 

 the precise moment at which each observation is made. The distance be- 

 tween the two dots of light, that is to say between the curve and the base- 

 line of the declination magnetograph, at the moments of observation, wil 



