58 EKMAN. ON DEAD-WATER. [NORW. POL. EXP. 



water of 10 °/o salinity it settles out in streaks and sinks a good deal in the 

 course of two or three hours. About 20 cm. 3 of it, make an area of one m. 2 

 of water almost impenetrably dark. A white screen s, illuminated by a lamp 

 I, was placed behind the tank (Fig. 9, PI. I), and against this screen even 

 very small waves could be observed at the boundary between the blackened 

 and the clear water. 



In this way, only a silhouette of the waves is obtained. To get a better 

 idea of their true shape, they must be observed from above or from below 

 by reflected light, so as to appear in relief. For this purpose, the salt water 

 was carefully filtered free from all solid particles, and the fresh-water was 

 made milk white. After several trials the most convenient substance for this 

 purpose was found to be silver chloride as precipitated from a solution of 3 

 gr. silver nitrate in 20 liters of water 1 . When this fluid was spread out in 

 a layer upon the salt water, the boundary, observed from below, appeared 

 quite opaque and of a pure milk-white colour, and so it remained tolerably 

 sharp for one or two hours. The greater the salinity of the water below, the 

 longer the boundary remained sharp. Fig. 10, PI. I, shows how the experi- 

 ments were arranged; no screen was used, and the boundary was lighted 

 directly by the lamp. As there was some difficulty in obtaining good "relief- 

 photographs", and the necessary preparations required a good deal of time, 

 as a rule only "silhouette-photographs" were taken. 



In photographing the waves — in either of the two above ways — a 

 flash-lamp (f, Fig. 10, PI. 1) burning a mixture of 3 parts magnesium powder, 

 2 parts potassium chlorate, and 1 part antimony sulphide, was employed. 



When an experiment was to be begun, the boat was kept at its starting 

 place by a hook h (Fig. 2, PI. I), the towing weights and starting weight (if 

 such was being used) were hung on, and the platform p Fig. 4 was placed 

 in an appropriate position. The Grenet's elements were connected up, the 

 pendulum and the telegraphic tape were put in motion, and when everything 

 was in order, the boat was unhooked by the action of the electromagnet e 



Milk, or milk and water, does not float very long above salt water. The salt diffuses 

 into the milk; and milk being heavier than fresh water, salt milk is heavier than salt 

 water and consequently sinks. New quantities of milk come into contact with the 

 water, become salt, and sink in their turn; and after a little while, the milk is seen 

 to fall through the salt water as a shower of small vortex-rings. The same thing takes 

 place in the case of all precipitates in fresh water, but at different velocities. 



