ON A NEW CURRENT-METER. 167 



experiments were also made from the ice. A tripod is made of 

 three stakes, in which a block or meter-wheel is hung over the 

 hole in the ice, and the line is taken from the winch through 

 this, and fastened to the apparatus. Before the latter is let 

 down, care is taken that both the pendulum and the magnetic 

 needle are free and in order. Care must also be taken at the 

 water-surface to see that the compass-box is quite filled with 

 water, so that there are no air-bubbles to disturb the needle. 

 The position of the sliding weight on the pendulum-rod is noted 

 every time it is changed, and is given in centimetres below a cer- 

 tain point on the pendulum. When the apparatus is lowered to 

 the depth desired, the winch is made fast, so that the apparatus 

 may become perfectly still, and the magnetic needle may assume 

 its final position. This does not require any great length of 

 time. For a depth of less than 10 metres, half a minute is suffi- 

 cient in all cases, and for depths down to 50 metres, 1 minute. 

 After this time, the messenger is sent down, and the apparatus 

 is hauled up and read off. 



With practice, one current-observation can be taken in a 

 minute, in the smallest depths and as long as everything acts 

 properly. At a depth of 50 or 60 metres, about 5 minutes are 

 required for each observation. 



The sensitiveness of the pendulum must be changed according 

 to the force of the currents; the sliding weight is raised or 

 lowered, or is augmented with the additional weight, so that the 

 displacements of the pendulum are convenient. 



The first task in the working up of the observations is to 

 make them all capable of comparison among themselves, and to 

 reduce them to a certain unity. The physical quantity which 

 the instrument directly gives, is, with a trifling error, the pressure 

 of the current against the bob and the wings of the pendulum. 

 By the calculation of this quantity, all the observation-results 

 become commensurable. We may disregard the small differences 



