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



A part of the double casing of the instrument is also capable of being 

 removed by means of flanges and screws, in order to afford access to 

 the compass-box and thermometer, care being taken to readjust the 

 portion removed so as to render it perfectly water-tight after all the 

 internal adjustments have been looked to. The instrument must be 

 made smaller should experience prove the present size too bulky ; it will, 

 however, be necessary to have a sufficiently long compass-needle to 

 enable it to overcome the resistance due to the friction of the end of the 

 platinum wire, which presses against the under surfaces of the contacts 

 (o o), as above mentioned. 



Instrument for measuring on board Ship the Velocities of Deep-sea 



Currents. 



Diagram 3. fig. 1 represents a full-sized section of this instrument ; 

 and here, again, the chief characteristic is its simplicity. 'In outward ap- 

 pearance this instrument is somewhat similar to the previous one, and 

 like it is made to place itself in the direction of the current by means of 

 a vane F. 



oc z is a section of a wheel with six rays, of which fig. 2 is a full-sized 

 plan, the section representing the part ocyz, and being made through 

 the centre of the wheel, as shown by the dotted line. 



One half only of this wheel (a? w z) is exposed to the free action of the 

 water at once (cf . fig. 2). The wheel revolves on its axis ( W W), which 

 is carefully pivoted on jewels in the small brass cups, cc, fixed on the 

 centres of the disks (d d, B D). „ 



The upper end of this axis carries a soft armature (a a), which rotates 

 with the wheel immediately under the poles of a large horseshoe magnet 

 (m m). Around the poles of the magnet are two bobbins of insulated 

 wire (b 6), wound in opposite directions, and in connexion with them are 

 the two leading- wires, one of which is put to earth, and the other passes 

 up the core of the J cable to the registering-instrument on board. Be- 

 tween the poles of the magnet and the armature (a a) is a disk of ebonite 

 or boxwood or other non-conductor (B D), strengthened round the edge 

 by a rim of brass, and sufficiently thick to prevent its warping, which is 

 intended to render the compartme t(BCD) water-tight and protect the 

 magnet and coils. 



The reason for not employing a sheet of copper or other metal is to 

 avoid the magnetism of rotation discovered by Arago, or the reaction of 

 currents induced in the copper plate by the armature revolving, as ex- 

 plained by Faraday; this reaction, though probably small and insignificant, 

 is better eliminated, as the tendency is to improve the motion of the 

 armature. The wooden disk {d d') keeps the magnet firmly in its right 

 place, and a stuffing-box (s' s') prevents sea- water from entering the com- 

 partment, which is also filled with paraffin wax as an additional security. 

 The registering-instrument on board is an ordinary double-current counter 

 (fig. 3), similar to those made by the British Telegraph Manufactory, 



