NO. 15.] EMPIRICAL LAWS OF RESISTANCE. 135 



big stern-posts. The screw is able to develop a somewhat greater force, 

 when the vessel has her speed diminished by a hindrance, but I cannot from 

 the information which I have been able to get, say to what extent. 



The next question concerns the depth and spec, gravity of the water- 

 layers. Exact measurements of the water-layers were not made. But it is 

 recorded that the water at the surface was almost fresh, whereas through 

 the bottom-cock of the engine-room perfectly salt water was obtained. The 

 specific gravity of the surface-water was measured. Its greatest values while 

 the Fram was in dead-water on Aug. 29th was nearly 1-008, and it was 1-005 

 on the two other days 1 . To judge from the measurements on other occasions, 

 the water at 5—10 metres' depth has probably had a specific gravity of 1-023 

 or l - 024. The difference of spec, gravity between the surface-layer and the 

 sea-water below should then have been as much as - 016 during the whole 

 time on Aug. 29th, and 0019 on the other days. Prof. Nansen suggests 

 that the water-layers might have been analogous to those measured on July 

 11th 1894 (I. c. pp. 249—250), and that the light surface-layer might have had 

 a thickness of about 25 m. The densities in situ were according to the 

 measurements taken on the day: at 0"05 m. 1*001, at 0*12 m. 1*001, at 1 m. 

 1*001, at 2 m. 1*002, at 3 m. 1-023, at 5 m. 1024, and at 10 m. 1-024. 

 These measurements made in an analogous case, prove that the boundary 

 between the two water-layers might well have been rather sharp. 



There was only one direct observation on the thickness of the surface- 

 layer: the salt-water was on a level with the bottom-cock. This was 3*5 m. 

 below the constructed water-line, and as the ship lay at the time perhaps 

 half a metre deeper, it would seem that the fresher surface-layer in any case 

 cannot have been more than 4 m. thick. It must however be remembered 

 that the water was taken through the bottom-cock, while the ship was in 

 motion, and consequently there were waves in the boundary between the 

 salt and fresh water. The bottom-cock is situated 5 or 6 m. from the stern; 

 and the photographs on PI. XVI— XVII show that the salt-water owing to 

 wave-motion may have been there raised probably one metre or more above 

 its mean level. The surface-layer might consequently have been as much 

 as 5 m. thick or even a little more. 



1 See Fridtjof Nansen, On the Oceanography of the Nortli Polar Rasin (Vol. Ill of these 

 reports) pp. 158—159. 



