1876.] Currents and Temperature in the Ocean. 329 



Euston Road, London, and which are most suitable and trustworthy. 

 When it is required to make an observation, a sinker is put on to the 

 detaching-apparatus, and the instrument lowered to the required depth, 

 where it remains steadily in a perpendicular position in the direction o£ 

 the current, and the wheel commences to rotate, more quickly or slowly 

 according to the velocity of the current influencing it, carrying round on 

 its axis the soft iron armature (a a) which revolves in front of the poles 

 of the magnet (m m), causing induced currents to circulate through the 

 coils, which change in direction at every half-revolution. In order that 

 these induced currents may circulate in the same direction through both 

 coils of wire b b, the bobbins are wound, one right-handed, and the other 

 left-handed. 



These reverse currents are transmitted, on depressing the key (Jc), 

 through the insulated wire of cable, key, galvanometer, counter, to 

 earth E, the other leading-wire being also connected through the instru- 

 ment to earth, as before mentioned. Thus a succession of reverse cur- 

 rents traverse the counter (fig. 3) so long as the contact-key (Jc) con- 

 tinues to complete the circuit, and both the counter and the wheel in 

 mid ocean keep perfect time together. The counter may be adjusted to 

 zero, or the position of the hands noted ; and when the instrument is 

 at the desired depth, the contact-key is held down and the counter begins 

 to indicate the revolutions of the wheel for one minute, or for whatever 

 period of time may be required. Thus by finding the number of revo- 

 lutions produced in a given time by a current flowing with a certain 

 known velocity, data may be obtained for finding the velocities of other 

 streams or deep-sea currents, a constant being allowed for friction, 

 which must be calculated. On reaching the bottom the weights are 

 detached, and the instrument, whose weight has been previously adjusted 

 to enable it to float and remain in a perpendicular position, offers little 

 strain to the cable when being hauled up. 



As the pressure is equally great on the inside and outside of this 

 instrument, it is not necessary to have so strong a casing as the other 

 instrument requires. 



Instead of only two poles, as in the diagram, four poles may be used, 

 the principle being the same as adopted by Sir Charles Wheatstone in 

 his extremely beautiful ABC instrument, in order to get rid of the hob- 

 bling motion peculiar to other magneto-electric machines. By this 

 arrangement the stronger current induced in the coils as the armature 

 passes off one pole, compensates for the weaker induced current which 

 circulates through the coils when the armature approaches the other 

 pole, and thus makes the reverse currents much more regular. The 

 coils must be wound so that the currents circulate in the same direction. 

 If this method is adopted there will be four reverse currents for every 

 revolution of the armature instead of two, as before mentioned, and the 

 number indicated on the counter will have to be divided by two. 



vol. xxtt. 2 c 



