NO. 15.] APPARATUS AND METHODS. 57 



pricks"; in this way the distances covered by the boat-model in equal inter- 

 vals of time, were directly obtained. 



For getting a sharply defined layer of fresh water above salt water, 

 the following simple method was used (Fig. 6, PI. I). A plane disc d of tin- 

 plate, cleaned with acid, is suspended horizontally from one end of a lever I; 

 the other end of the lever is properly loaded, so that the disc, when lowered 

 on to the surface, adheres to the water which is raised a little above its free 

 level. The fresh water, running from a siphon s upon the disc, flows softly 

 out over the salt-water, and the disc is maintained on the upper surface by 

 the lever. With two such discs, 25 cm. in diameter, a sharply defined fresh- 

 water layer of 5 cm. thickness could be formed in less than one hour, even 

 if the salt water had a specific gravity of 1*01 only. 



The specific gravity of the water layers was measured with an Aderman 

 hydrometer. These determinations were not made with as much care as they 

 might have been; in particular I did not pay much attention to the changes 

 of temperature of the surface-water. The differences of density of salt- and 

 fresh water are for this reason, in most cases smaller than the values actu- 

 ally given (see section D of this chapter). 



It was necessary to have the surface of the water freed from dust. If 

 this was not done, the dust was gathered up in front of the stem of the boat 

 and caused resistance. The dust was skimmed off by a strip of thick blotting- 

 paper, supported by a metal plate and reaching over the whole breadth of 

 the tank (Fig. 7, PI. I). It was moved slowly from one end of the tank to the 

 other, by means of a sort of sleigh, and as the paper protruded only one or 

 two mm. into the water, it had no disturbing effect upon the surface of sepa- 

 ration of the water-layers. After the dust was skimmed off and collected on 

 the blotting-paper, this was washed under the water-tap and moved once more 

 from one end of the tank to the other; there it was left standing, while the 

 experiments were being made. 



To make the different water-layers visible, two methods were employed. 

 In most of the experiments, the salt-water (or the fresh-water) was blackened 

 with liquid Chinese ink 1 , before the water-layers were prepared. In salt-water 

 of 4 % salinity or less, good Chinese ink keeps floating for several days; in 



Fuchsine would be very good if it did not diffuse so quickly from one water-layer 

 into another. Chinese ink, of course,, does not diffuse at all. 



8 



