ECHO-SOUNDING AND H YDROGRAPHICAL STUDIES 



31 



takes the pointer exactly 12 seconds to go round, and each time it passes 

 zero it marks a dot on the paper, a sound impulse being dispatched to- 

 wards the bottom at the same time. The marking of the paper is caused 

 by an electric current flowing at the moment of transmission and recep- 

 tion from the pointer through the paper to the paper roller. The paper is 

 kept moist, and as it is impregnated with potassium iodide and starch, a 

 mark is left at the point where it is pierced by the electric current. By means 

 of the transparent scale fitted to the apparatus, the depth is read at brief 

 intervals and recorded on the paper direct. In later supplementary read- 

 ings of the echo curve a loose scale is used which is rather shorter, corres- 

 ponding to the contraction of the paper as it dries. But as paper does not 

 always act uniformly, even when of the same make, and furthermore is 

 affected by the climate, the original readings are always to be regarded 

 as the correct ones, later readings being adjusted accordingly. 



The iodine marks on the paper will gradually evaporate and fade; 

 consequently, it is necessary to go over the curves with a fine pencil if 

 they are to be preserved. 



It is a complicating factor in echo-sounding that the velocity of sound 

 varies with the nature of the water, being greater with greater warmth, 

 salinity, and pressure. Readings must therefore be corrected in accor- 

 dance with tables provided for various regions. 



The installation functioned perfectly throughout the expedition; and as 



Time and depth are recorded 

 on the echo-sounder's paper 

 every quarter of an hour. An 

 electric current passes through 

 the flex to the pencil, and the 

 writing medium is iodine. 



