322 Mr. J. R. Jones on Instruments for measuring [Feb. 24, 



influenced by any unsteadiness of the instrument, gimbals (diag. 1. fig. 1, 

 and diag. 2. fig. 1), gg, are employed, as in the case of the mariner's 

 compass ; and underneath the card c c (diag. 1. fig. 1), to which the 

 magnet is attached, is a copper wire (w iv) permanently fixed at right 

 angles to the magnet m m, along which an electric current is made to 

 pass, and thus has no tendency to deflect the magnet from the magnetic 

 meridian. 



Attached to one end of this copper wire (w iv) on the upperside, and 

 passing through the card, is a fine platinum wire, pp, about 3 inches 

 long, and tapering to a mere fibre at the top, and the wire is carefully 

 counterpoised at the other end. The pivot on which the magnet is 

 poised is of brass (b) and pointed with platinum (p"), and is in connexion 

 with the sea through the pivots of the gimbals, gg. A double casing of 

 thick sheet-copper (ABCD) is strengthened by stays (i i i) calculated to 

 stand the great deep-sea pressures ; and the air-chamber between the 

 inside and outside casing renders the whole instrument buoyant, and is 

 intended to take off all the strain from the line when being hauled up. 

 e e is a section of an ebonite disk, the plan of which is shown in diag. 2. 

 fig. 2; and fig. 1 is a section of the instrument taken through the 

 dotted line BbDd, showing the ebonite disk, gimbals, card, and magnet 

 (m m), with the copper wire (w w) affixed at right angles to it. Let into 

 the periphery of the disk (ee) are sixteen pieces of brass, insulated from 

 one another by the ebonite, but placed in such close proximity to each 

 other that the fibrous end of the platinum wire (pp) must always be in 

 contact with the under surface of one of them. Sixteen insulated wires 

 enter the top of the instrument in the form of a cable, which divides into 

 two parts (h h), eight wires on either side. 



These are conducted through two holes (h', h') in the ebonite disk (e e) to 

 the lower part of the instrument, where they pass through a second hole 

 in the centre of the concave bottom of the compass-chamber e" e", which 

 renders the compartment e'eee" watertight, and to the underside of 

 which the thermometer to be afterwards described is attached. The 

 ends of the wires are finally connected to the sixteen terminals on the 

 back of the thermometer. Before, however, passing through the holes 

 (h', h') short leading-wires are soldered to each and connected to the upper 

 surfaces of the sixteen brass contacts on the ebonite disk (e e). These 

 contacts pass through the disk and project a little beyond its lower sur- 

 face, where they are grooved as in fig. 1, oo, in order to prevent the end 

 of the platinum wire pp, which is so adjusted as to pass immediately 

 under the centre of the groove, from trespassing beyond the line of con- 

 tacts, through any unsteadiness of the instrument. These brass contacts 

 are amalgamated on the under surface to render the contact more 

 decided ; and the top of the platinum wire pp must be so fine and 

 flexible as to reduce the friction to a minimum, while at the same time it 

 ensures perfect contact. A stuffing-box, s s\ with hemp and wax pre- 



