744 DR E. M. WEDDERBURN AND MR A. W. YOUNG ON 



and it was desired to have an instrument to measure these. The requirements 

 of such an instrument were that it should measure (l) the horizontal component, 

 (2) the vertical component, (3) the direction, and (4) the strength of the current. 

 As previous experience had shown that currents in lakes varied rapidly in direction 

 and in strength, it was thought best to have an instrument which would record these 

 four factors at any point of time, in preference to an instrument which would give 

 an average over a considerable time. It was accordingly decided to measure the 

 strength of the current by the angle through which it caused a propeller to revolve, 

 the propeller being kept in tension by a spring similar to the balance spring of a 

 watch. The propeller was mounted in gymbals and was therefore free to move 

 in any direction, and was kept directed towards the current by means of two sets 

 of vanes at right angles to each other — one set of vanes being acted on by the 

 vertical component of the current and the other by the horizontal component. 

 When an observation was to be made, the current-meter was suspended from the 

 boat at the desired depth by a bifilar suspension, so that the zero position of the 

 vanes was known. By a series of trigger arrangements released by a messenger 

 weight, the vanes and the propeller were clamped in the position which the current 

 caused them to take up, and when the instrument was brought to the surface the 

 directions and strength of the current could be read off from scales attached to 

 the instrument. 



Numerous experiments were made to determine the most suitable form of vanes, 

 and ultimately the type adopted consisted of two vanes at right angles to each 

 other, forming a cross, with a third vane at right angles to these at some distance 

 from the propeller. One of the vanes was attached to the propeller so as to be in the 

 same plane as the axis of the propeller, and the centre portion of this vane was 

 cut away. Subsidiary vanes were also attached to that portion of the gymbals 

 which was free to move about a vertical axis, and these made the instrument more 

 sensitive to the horizontal component of the currents. A fixed propeller with its 

 vanes inclined in the opposite direction to those of the pivoted propeller was placed 

 alongside the latter to counterbalance the torsion which would otherwise have been 

 exerted on the instrument by currents. 



The instrument was carefully adjusted by means of counterpoise weights so that 

 when immersed in water the vanes would remain in whatever position they were 

 placed. Wooden floats were attached to the movable parts of the instruments to 

 reduce their weight in water and the consequent friction on the bearings. The 

 gymbals were set in phosphor-bronze ball bearings. The bearings of the propeller 

 were phosphor-bronze points on agates. The spring which kept the propeller in 

 tension consisted' of a steel clock mainspring heavily silvered to prevent rust. A 

 general idea of the instrument may be gathered from the photograph forming fig. 2. 



The calibration of the instrument was, through the courtesy of Professor Hudson 

 Beare, carried out in the Engineering Laboratory of the University of Edinburgh 



