atus used. 



410 HYDROGRAPHICAL SURVEYING [chap. xix. 



OBSERVATIONS ON UNDER-CURRENTS. 



Though not in the ordinary run of surveying operations, a 

 shght description of the method of discovering the direction 

 and approximate rate of under-currents may be useful. 



To ascertain these satisfactorily, special gear is necessary. 

 General The general principle is to expose a large surface to the action 

 iinup e, ^^ ^j^^ under-current, and to support this in the water by a 

 floating buoy which will present as small a surface as possible 

 to the action of the surface stream. 



The experiments must be carried on from boats, and 

 therefore the gear must be as light as possible, for easy 

 handling. 



A series of observations on the under-currents in the Bos- 

 porus and Dardanelles resulted in the author's adopting the 

 following :* 

 Appar- A light, flat wooden board, 6 feet square, v/ith a wing 2 feet 



in length, at right angles to the rest of the frame, was used as 

 the submerged drag (Fig. 84). 



To the extremities of the wing the sling, a a, was made fast, 

 and to this sling the supporting line to the buoy was bent, 

 at such a point as kept the surface of the drag vertical when 

 the strain came on. 



It weighed 70 pounds in air, and took 120 pounds of lead to 

 sink it satisfactorily. These leads were made fast with a little 

 drift, and another line, c, was bent, both to them and to the 

 lifting sling, h h, so that the weight of the leads could be taken 

 off the drag, when pulled up to the surface, before finally hoisting 

 it into the boat. 



An iron buoy, 1 foot in diameter and 5 feet long, supported 

 this structure well when the surface current was not very 

 strong, and only presented an area of less than 1 square foot 

 to pull tlirough the water. 



When the surface current was swift, other buoys had to be 

 added, attached in line to the upper end of the first, for under 

 these circumstances the single buoy was dragged under water, 

 and its motion could not be followed. Several disappeared 

 in tliis way, some to reappear when the apparatus got into 

 slacker water, some for good and all, 



* " Observation of Currents in Dardanelles and Bosporous." 



